Estudios geotérmicos con técnicas isotöpicas y geoquimicas ...
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IAEA-TECDOC-641 Estudios geotérmicos con técnicas isotöpicas y geoquimicas en America Latina Actas de una Réunion final de coordination de investigaciones celebrada en San José, Costa Rica, 12-16 de Noviembre de 1990 Geothermal investigations with isotope and geochemical techniques in Latin America Proceedings of a Final Research Co-ordination Meeting held in San José, Costa Rica, 12-16 November 1990 ORGANISMO INTERNACIONAL DE ENERGIA ATOMICA
Transcript of Estudios geotérmicos con técnicas isotöpicas y geoquimicas ...
IAEA-TECDOC-641
Estudios geotérmicoscon técnicas isotöpicas y geoquimicas
en America LatinaActas de una Réunion final de coordination de investigaciones
celebrada en San José, Costa Rica, 12-16 de Noviembre de 1990
Geothermal investigationswith isotope and geochemical techniques
in Latin America
Proceedings of a Final Research Co-ordination Meetingheld in San José, Costa Rica, 12-16 November 1990
ORGANISMO INTERNACIONAL DE ENERGIA ATOMICA
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ESTUDIOS GEOTERMICOS CON TECNICAS ISOTOPICAS Y GEOQUIMICASEN AMERICA LATINA
GEOTHERMAL INVESTIGATIONS WITH ISOTOPE AND GEOCHEMICAL TECHNIQUESIN LATIN AMERICAOIEA, VIENA, 1992IAEA-TECDOC-641ISSN 1011-4289
Impreso por el OIEA in AustriaMarzo de 1992
PREFACIO
Publicamos en el présente volumen las memoriaspresentadas en la ultima Reunion para coordinar lasinvestigaciones del Programa Coordinado de Investigacion(PCI) del OIEA para America Latina sobre empleo de técnicasîsotopicas y geoquimicas en la exploraciön de recursosgeotérmicos. La reunion se célébré en San José de Costa Ricadel 12 al 16 de noviembre de 1990 y se organize» con lacooperaciön del Institute Costarricense de Electricidad, alcual expresamos nuestro profundo agradecimiento.Anteriormente se celebraron reuniones para coordinar lasinvestigaciones con el fin de examinar la labor en curso, endiciembre de 1986 en el Instituto de Asuntos Nucleares deBogota (Colombia) , y en noviembre de 1988 en el InstitutoEcuatoriano de Electrificaciön de Quito (Ecuador).
Cientificos de nueve paises de America Latina(Argentina, Bolivia, Colombia, Costa Rica, Ecuador,Guatemala, Mexico, Peru y Venezuela), que participaron en elPCI, presentaron los resultados de sus estudios sobre elterreno en la reunion de San José. Ademas de éstos,cientificos de otros très paises latinoamericanos conimportantes recursos geotérmicos, que no participaron en elPCI (Chile, El Salvador y Nicaragua), y expertos de Italia,Nueva Zelanda y los Estados Unidos de America, que cooperaronen la ejecuciôn del programa, asistieron a la reunion ypresentaron informes sobre sus actividades.
En términos générales, las investigaciones cientificasrealizadas en el marco del PCI del OIEA constituyen el primerestudio geoquimico sistemâtico de los recursos geotérmicos deAmerica Latina. La reunion de San José permitio obtener unapanorâmica actualizada de las exploraciones de recursosgeotérmicos que se vienen efectuando en la région utilizandola geoquimica e isötopos ambientales, que constituyen tambiénun instrumente geoquimico.
Los resultados estân ahora a la disposicion de lasautoridades nacionales interesadas, que deben decidir acercade la continuaciön de las investigaciones, especialmente enel caso de aquellos campos geotérmicos en los que se hanobtenido resultados particularmente prometedores, habidacuenta de una posible explotaciön con fines de generacion deenergia. La ejecuciôn de la siguiente etapa de lasinvestigaciones, que es mas costosa, y que puede incluirsondeos exploratorios, podria demorar un tanto en aquellospaises que disponen de fuentes de energia mas economicas,taies como petröleo o centrales hidroeléctricas, o cuyoscampos geotérmicos estân ubicados en regiones remotas,escasamente pobladas. Aun asi, consideramos que valiö la penallevar a cabo las investigaciones dado que contribuyeron a laevaluaciön de los posibles recursos energéticos que podrianutilizarse en el futuro, asi como a la capacitaciOn de unnuméro de geoquimicos latinoamericanos en el empleo de lastécnicas isotopicas y geoquimicas.
FOREWORD
We are publishing in this volume the papers presented atthe last Research Co-ordination Meeting of the IAEA Co-ordinatedResearch Programme (CRP) for Latin America on the Use of Isotopeand Geochemical Techniques in Geothermal Exploration. Themeeting was held in San José de Costa Rica from 12 to 16 November1990, and was organized with the co-operation of the InstituteCostarricense de Electricidad, which is warmly acknowledged.Previous Research Co-ordination Meetings to discuss the work inprogress were held in December 1986 at the Institute de AsuntosNucleares in Bogota, Colombia, and in November 1988 at theInstitute Ecuatoriano de Electrificacion in Quito, Ecuador.
Scientists from nine Latin American countries, i.e.Argentina, Bolivia, Colombia, Costa Rica, Ecuador, Guatemala,Mexico, Peru and Venezuela, which took part in the CRP, presentedthe results of their field studies at the San José meeting. Inaddition to these, scientists from three other Latin Americancountries with important geothermal resources, which did not takepart in the CRP, i.e. Chile, El Salvador and Nicaragua, andexperts from Italy, New Zealand and the United States of America,who co-operated in the programme implementation, were present andreported on their activities.
On the whole, the scientific investigations carried outwithin the frame of the IAEA CRP constitute the first systematicgeochemical survey of the geothermal resources of Latin America.The San José meeting provided an updated picture of the currentexplorations of geothermal resources in the region usinggeochemistry and environmental isotopes, which are also ageochemical tool.
The results remain now at the disposal of the interestednational authorities, which have to decide about the continuationof the investigations, especially for those fields where theresults have been particularly promising, in view of a possibleexploitation for power production. The next, more expensivestage of investigation, which may include exploratory drillings,might not take place soon in those countries which dispose ofcheaper sources of energy like oil or hydroelectric plants, orwhere the geothermal fields are located in remote, scarcelypopulated regions. Even so, however, we believe that theinvestigations have been useful and worth being carried out,because they helped to assess potential energy resources whichmay be used in future, and to form and train a number of LatinAmerican geochemists in the use of isotope and geochemicaltechniques.
NOTA EDITORIAL
Al preparar esta publication para la imprenta, el personal correspondiente del OrganismoInternational de Energîa Atômica ha montado y paginado los manuscritos originales facilitados parlos autores y ha procurado una présentation satisfactoria.
Las opiniones expresadas en las mernorias, las declaraciones formuladas y el estilo generaladoptado son propios de los autores citados. Dichas opiniones no reflejan necesariamente las de losGobiernos de los Estados Miembros o las de las organizations bajo cuyos auspicios se han elaboradolos manuscritos.
Las denominaciones concretas de paîses o territorios empleadas en esta publication no implicanjuicio alguno par parte del OIEA sobre la condition juridica de dichos paîses o territorios, de susautoridades e institutions, ni del trazado de sus fronteras.
La mention de determinadas empresas o de sus productos o mar cas comer dales no implicaningun généra de aprobaciôn o recomendaciôn por parte del OIEA.
Corresponde a los autores obtener el permiso necesario para reproducir textos ajenos sujetos aderechos de propiedad intelectual.
EDITORIAL NOTE
In preparing this material for the press, staff of the International Atomic Energy Agency havemounted and paginated the original manuscripts and given some attention to presentation.
The views expressed do not necessarily reflect those of the governments of the Member States ororganizations under whose auspices the manuscripts were produced.
The use in this book of particular designations of countries or territories does not imply anyjudgement by the publisher, the IAEA, as to the legal status of such countries or territories, of theirauthorities and institutions or of the delimitation of their boundaries.
The mention of specific companies or of their products or brand names does not imply anyendorsement or recommendation on the pari of the IAEA.
INDICE
Resumen del Programa coordinado de investigation .................................................. 9Summary of the Co-ordinated Research Programme .................................................. 11
Reservoir characteristics of the vapor dominated geothermal field of Copahue, Neuquén,Argentina, as established by isotopic and geochemical techniques .............................. 13J.L. Sierra, F. D'Amore, H. Panarello, G. Pedro
Isotopic and geochemical study of the Domuyo geothermal field, Neuquén, Argentina ........ 31H. Panarello, J.L. Sierra, F. D'Amore, G. Pedro
Flow patterns at the Tuzgle-Tocomar geothermal system, Salta-Jujuy, Argentina:An isotopic and geochemical approach ................................................................ 57H. Panarello, J.L. Sierra, G. Pedro
Informe geoquimico sobre la zona geotérmica de Laguna Colorada, Bolivia ..................... 77G. Scandiffio, M. Alvarez
Geochemical report on the Empexa geothermal area, Bolivia ....................................... 115G. Scandiffio, W. Cassis
Geochemical report on the Sajama geothermal area, Bolivia ........................................ 141G. Scandiffio, J. Rodriguez
Geochemical and isotopic exploration of the geothermal area of Paipa, Cordillera Oriental,Colombia .................................................................................................... 169R. Bertrami, A. Camacho, L. De Stefanis, T. Medina, G.M. Zuppi
Isotopic composition and origin of thermal and non-thermal waters from theMiravalles geothermal field, Costa Rica .............................................................. 201W.F. Giggenbach, R. Corrales, L. Vaca
Modelo geotérmico preliminar de areas volcânicas del Ecuador, a partir de estudiosquimicos e isotöpicos de manifestaciones termales ................................................. 219E. Almeida, G. Sandoval, C. Panichi, P. Noto, L. Bellucci
Avance de las pruebas de radiotrazado en el campo geotérmico de Ahuachapân,El Salvador ................................................................................................. 237W.J. McCabe, E. May en, P. Hernandez
Isotopic and chemical composition of water and gas discharges from the Zunilgeothermal system, Guatemala .......................................................................... 245W.F. Giggenbach, D. Paniagua de Gudiel, A.R. Roldân Manzo
Investigaciones geoqufmicas realizadas en los campos geotérmicos de Zunily Amatitlân, Guatemala .................................................................................. 279A.R. Roldân Manzo
Caracterfsticas geoqufmicas e isotöpicas de los fluidos producidos por los pozosde Los Humeros, Puebla, Mexico ..................................................................... 307E. Tello Hinojosa
Geochemical report on the Challapalca and Tutupaca geothermal areas, Peru ................... 345G. Scandiffio, D. Verastegui, F. Portilla
Geothermal exploration by geochemical methods of the thermal area El Pilar-Mundo Nuevo,State of Sucre, Venezuela 377F D'Amore, G Gianelh, E Corazza, J Jauregui, P Varela
Origins of acid fluids m geothermal reservoirs 423A H Truesdell
IAEA mterlaboratory comparative geothermal water analysis program 439W F Giggenbach, R L Goguel, WA Humphries
Lista de participantes 457
RESUMEN DEL PROGRAMA COORDINADO DE INVESTIGACION
El PCI para la America Latina sobre el empleo de técnicasisotöpicas y geoquimicas en la exploraciôn de recursosgeotermales se iniciö en 1984. Los recursos financières parala ejecuciôn del programa fueron facilitados por el Gobiernoitaliano, al cual el Organismo desea expresar suagradecimiento. La primera actividad realizada fue elseminario sobre el empleo de técnicas isotöpicas ygeoquimicas en la exploraciôn geotérmica, celebrado en juniode 1984 en Morelia (Mexico) , en las instalaciones de laComisiön Federal de Electricidad, Gerencia de ProyectosGeotermoeléctricos. Durante el seminario, al cual asistieronreprésentantes de las instituciones que posteriormenteparticiparon en el programa, se examinaron los objetivos, lasprincipales lineas de investigaciôn y los campos geotérmicosque se estudiarian durante el PCI.
Los primeros contratos de investigaciôn se adjudicaronhacia finales de 1984. Los trabajos de campo empezaron en1985 y continuaron hasta 1990. Durante la ejecuciôn del PCIse estudiô un numéro considérable de campos geotérmicos, enlos nueve paises participantes. Mas adelante figura unarelaciôn de dichos campos. Las investigaciones efectuadasfueron bastante amplias desde el punto de vista geoquimico,en la mayoria de los casos, pero en algunos otros seencontraban todavia en una etapa de reconocimiento cuandofinalize el PCI; estos Ultimos estudios no son tema delprésente documento, pero, en principio, se puede tener accesoa los datos obtenidos solicitândolos a las institucionesnacionales correspondientes. Si bien las investigacionesutilizando técnicas geoquimicas convencionales ya se habianiniciado en varios de los campos antes de 1985, los métodosisotôpicos se aplicaron por primera vez en todos los casosdurante este PCI.
Debido a la localizaciön de muchos de los camposestudiados en lugares remotos y de gran elevaciôn y a lascondiciones meteorolôgicas adversas durante prolongadospériodes del ano, las investigaciones no avanzaronrâpidamente; esa es la principal razôn de la duraciônanormalmente prolongada del PCI, que solo pudo concluir trasmas de cinco anos de haberse iniciado.
Las zonas geotérmicas investigadas en el transcurso delPCI son las siguientes:Argentina; Volcan Copahue, Volcan Domuyo, Epulafqué, todos enla provincia de Neuquén, zona occidental de Argentina;Tuzgle-Tocomar en el altiplano del norte de Argentina,provincias de Salta y Jujuy;Bolivia: Laguna Colorada, Empexa y Volcan Sajama, en elextremo occidental del altiplano boliviano y en la cordilleraoccidental, cerca de la frontera con Chile;
Colombia; Paipa en la Cordillera Oriental;Colombia-Ecuador ; Narirïo-Tufino-Chiles-Cerro Negro: lo queconstituye un solo campo geotermico compartido por los dospaises;Costa Rica; Volcan Miravalles en la Cordillera de Guanacaste;Ecuador; Chachimbiro, Papallacta, Tungurahua, Chimborazo yCuenca, todos en la Cordillera Central;Guatemala; Lago Amatitlan, Zunil, San Marcos y Tecuamburo, enla Sierra Madré al noroeste de la Ciudad de Guatemala;Mexico; Los Humeros, Estado de Puebla;Peru; Challapalca, Paucarani, Calacoa, Calientes y Tutupaca,Todos en la Cordillera Andina en el sur del Peru;Venezuela; Las Minas, Mundo Nuevo y Aguas Calientes en elEstado de Sucre, nordeste de Venezuela; Merida, Tachira yTrujillo en la region andina en el oeste de Venezuela.
Los isotopos ambientales se utilizaron principalmentepara identificar el origen de los componentes del fluidogeotermico, la mezcla de aguas geotermicas con otras aguas, ylos procesos que ocurren en profundidad que producenfraccionamiento isotopico, como por ejemplo, lasinteracciones agua-roca y las perdidas de vapor. Por primeravez se sugirio la presencia de una hipotetica "aguaandesitica", de una composicion isotopica bastante biendefinida, derivada del agua de mar transportada porsubduccion junto con sedimentos marinos en zonas desubduccion (vease memoria de Giggenbach y otros, sobre elsistema geotermico de Miravalle, Costa Rica).
La geotermometria isotopica se limito a unos pocoscasos, posiblemente debido a la complejidad de lasmediciones; no obstante, los pocos resultados disponibles(Colombia, Mexico y Peru) confirman que el sistema desulfato-agua constituye un buen geotermometro, mientras quelos sistemas basados en componentes fluidos gaseosos(metano-CO2 y metano-hidrogeno) probablemente reflejan elequilibrio isotopico a la temperatura de formacion, que essuperior a la de la zona de almacenamiento geotermico.
Los analisis quimicos de los fluidos geotermicos seutilizaron ampliamente en geotermometria, asi como paia elestudio del origen y la historia geoquimica de loscomponentes fluidos.
Otra actividad que vale la pena mencionar, organizadapoco despues del inicio del PCI, fue la intercalibracion delos analisis quimicos de aguas geotermicas entre loslaboratories participantes en el PCI. Asimismo, laboratoriosde instituciones geotermicas de otros paises pudieron tomarparte en el ejercicio. Los resultados de dichaintercalibracion se dan a conocer en la ultima memoria deeste volumen.
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SUMMARY OF THE CO-ORDINATED RESEARCH PROGRAMME
The CRP for Latin America on the Use of Isotope andGeochemical Techniques in Geothermal Exploration started in1984. The financial resources to implement the programme wereassured by the Italian Government, to which the Agency would liketo express here its gratitude. The first activity carried outwas a Seminar on isotope and geochemical techniques in geothermalexploration, which took place in June 1984 in Morelia, Mexico, atthe premises of the Comision Federal de Electricidad, Gerencia deProyectos Geotermoeléctricos. During the seminar, which wasattended by representatives of the institutions which later tookpart in the programme, the objectives, main research lines, andgeothermal fields to be studied during the CRP were discussed.
The first research contracts were awarded towards the endof 1984. The field work started in 1985 and continued through1990. During the implementation of the CRP a considerable numberof geothermal fields, the list of which is reported below, werestudied in the nine participating countries. The investigationscarried out were geochemically quite comprehensive in most cases,but in some others they were still in a reconnaissance stage whenthe CRP ended: the latter studies are not reported in theseproceedings, but the data obtained are in principle availablefrom the relevant national institutions. While investigationswith conventional geochemical techniques had already started inseveral fields before 1985, isotope methods were applied for thefirst time in all cases during this CRP.
Due to the remoteness and high elevation of many of thefields studied and the adverse meteorological conditions duringlong periods of the year, the investigations could not proceedrapidly: this is the main reason for the unusually long durationof the CRP, which could be concluded only after more than fiveyears after its inception.
The geothermal areas investigated in the course of thisCRP are:
Argentina: Volcan Copahue, Volcan Domuyo, Epulafquén, all in theNeuquén Province, western Argentina; Tuzgle-Tocomar in thealtiplano of northern Argentina, provinces of Salta and Jujuy;
Bolivia; Laguna Colorada, Empexa and Volcan S a jama, on thewestern edge of the Bolivian altiplano and on the WesternCordillera, close to the border with Chile;
Colombia: Paipa in the Cordillera Oriental;
Colombia- Ecuador ; Narino-Tufino-Chiles-Cerro Negro: thisconstitutes a single geothermal field shared between the twocountries;
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Costa Rica: Volcan Miravalles in the Cordillera de Guanacaste;
Ecuador: Chachimbiro, Papallacta, Tungurahua, Chimborazo andCuenca, all in the Central Cordillera;
Guatemala: Lake Amatitlan, ZuniL San Marcos and Tecuamburo, onthé Sierra Madré north-west of Ciudad de Guatemala;
Mexico: Los Humeros, state of Puebla;
Peru: Challapalca, Paucarani, Calacoa, Calientes and Tutupaca,all in the Andean Cordillera in southern Peru;
Venezuela: Las Minas, Mundo Nuevo and Aguas Calientes in thestate of Sucre, north-eastern Venezuela; Mérida, Tâchira andTrujillo in the Andean region in western Venezuela.
Environmental isotopes were mainly applied to identify theorigin of the geothermal fluid components, mixing of geothermalwater with other waters, and processes occurring at depth whichproduce isotopic fractionations, such as water-rock interactionsand vapour losses. The occurrence of a hypothetical "andesiticwater", with a rather well defined isotopic composition, derivingfrom sea water subducted together with marine sediments insubduction zones, was suggested for the first time (see paper byGiggenbach et al. on Miravalle geothermal system, Costa Rica).
Isotopic geothermometry was limited to a few cases,possibly because of the complexity of the measurements:nevertheless, the few results available (Colombia, Mexico, Peru)confirm that the sulphate-water system constitutes a goodgeothermometer, while the systems based on gas fluid components(methane-CCU and methane-hydrogen) probably reflect theisotopic equilibrium at the temperature of formation, which ishigher than that of the geothermal reservoir.
Chemical analyses of geothermal fluids were extensivelyused for geothermometry, and for studying the origin andgeochemical history of the fluid components.
Another activity worth mentioning, organized shortly afterthe beginning of the CRP, was the intercalibration of geothermalwater chemical analyses among the laboratories taking part in theCRP. Also, laboratories of geothermal institutions of othercountries could take part in the exercise. The results of thisintercalibration are reported in the last paper of theseproceedings.
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RESERVOIR CHARACTERISTICS OF THE VAPORDOMINATED GEOTHERMAL FIELD OF COPAHUE,NEUQUEN, ARGENTINA, AS ESTABLISHED BYISOTOPIC AND GEOCHEMICAL TECHNIQUES
J.L. SIERRA*, F. D'AMORE**,H. PANARELLO***, G. PEDRO*
* Ente Provincial de Energîa del Neuquén,Neuquén, Argentina
**Istituto Internazionale per le Ricerche Geotermiche,Consiglio Nazionale delle Ricerche,Pisa, Italy
*** Institute de Geocronologia y Geologfa Isotöpica,Buenos Aires, Argentina
Resumen-Abstract
CARACTERISTICAS DE ALMACENAMIENTO DEL CAMPO GEOTERMICO DOMINADO FOR LAFASE VAPOR DE COPAHUE, NEUQUEN, ARGENTINA, ESTABLECIDAS MEDIANTE TECNICASISOTOPIC AS E GEOQUIMICAS.
El caiapo geotérmico de Copahue-Caviahue, se puede définir comodel tipo "dominado por vapor" con estratificaciôn horizontal y capasconectadas por fracturas y buena permeabilidad vertical dentro delreservorio.
Los isôtopos oxigeno-18 y deuterio, asociados con anâlisis degases y el estudio de la evoluciôn de la relaciôn gas/vapor, permi-tieron demostrar la existencia de por lo menos dos nivelés producti-ves: el was superficial a unos 800-1000 m y el mas profundo a mas de4000 m (todas las medidas respecte de la boca de pozo).
Las temperaturas obtenidas por métodos quimicos e isotôpicos ron-dan los 200° y 250°C para la capa mas superficial y mas profunda res-pectivamente.
Los valores de carbono-13 y las concentraciones de NZ, He y Arcaracterisaron los gases acompanantes como principalmente magmaticos.
RESERVOIR CHARACTERISTICS OF THE VAPOR DOMINATED GEOTHERMAL FIELD OF COPA-HUE, NEUQUEN, ARGENTINA, AS ESTABLISHED BY ISOTOPIC AND GEOCHEMICALTECHNIQUES.
îhe geothenal field of Copahue has been defined has vapor dominated field tiith stratified layers con-nected by fractures »ith good vertical peneability vithin the reservoir.2H and J <0, associated gas analysis and the study of the gas vapor ratio, alloyed to prove the existence of atleast i?o productive layers, the shallower at ca. 800-1000 t and the deeper at sore than 1400 a (both bellon thesell top level).Isotopic and geotheraosetric teeperatures are rounding 260'C and 250'C respectively, HC, H}, àr and Se analysesallotted to hypothesise the origin of geothertal gases.
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l. INTRODUCTION
1.1. LOCATION AND GEOLOGY
The area of Copahue is located in the AW sector of the Neuquénprovince (Argentine Republic) near the border with Chile. The latitudeis ca. 37 deg 50' S. and the longitude about the 71 deg 0.5' N. Itcover a zone on the Andean Cordillera between 1600 and 2300 m a.s.l.
DomuyoCopahue \*fj ";,'
FIG. 1.
The climate is Patagonic, and that of high mountain, depending ofthe altitude, with snow and rain precipitations from autumn to spring,and vinds prevailing from the AW. (Pacific ocean).
The effusive complex Copahue-Caviahue started to develope in thePliocene and its eruptions originated the large lavic mantle of theHualcupen Formation. This episode Mas over with the formation of a 17km average radius calders.
Because of the draining, result of the continous magmatic activi-ty and to the great explotions produced by variations in the dynamic,all the volcanic building was seated. This fact determined that thebottom was constituted by numerous fragmented blocks that originate an
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area of high permeability. Latter a diastrophyc phase allowed the ac-cumulation of lacustrine sediments in the base of the caldera. A fur-ther magmatic evolution originated an effusive focus that producedlavic flows covering the bottom of the system. The location of thiseffusive focus would have been in the nearness of the volcano Copahueand is known as Las Mellizas Formation.
During the Pliocene-Holocene the magmatic activity continued,leading to new effusive points, favoured by the deep dislocations inthe border of the caldera that produced a set of périphérie eruptions.This postcaldera volcanism can be classified into three effusive sta-ges. The last one is represented by the Copahue volcano.
In the area two wells have been drilled: COP-1 and COP-2. Theformer was drilled in March 1982 reaching 1414 m depth and lead to thediscover of a géothermie reservoir producing dry vapor. Two productivelevels have been detected, one between 800-900 m and a second into thewell bottom. The COP-2 was finished in 1986 with a total depth of 1236m and is producing dry vapor of similar characteristics of COP-1 fromthe shallower layer.
2. METHODOLOGY
Information available in the ENTE PROVINCIAL DE ENEBGIA DEL NEU-QUEN (EPEN) i.e. production curves, build ups, pressure and temperatu-re records of COP-1, COP-2 as well as chemical analyses of geothermalmanifestations and wells was recovered.
During three field trips performed in the 1985/1987 period, weresampled and analysed COz, H2S, HZ, CH4, NZ, Ar, He and CO on gaseoussamples (tab. 1 and 2).
pH, alkalinity, majoritary ions, Fez +, Fe3 + , SiO? and Li+, stableisotopes-' deuterium, oxygen-18 and tritium contents were determined onvapor condensâtes, cold and hot waters (tab. 3 and 4). In additioncarbon-13 was measured in CÜ2 coming with gas in wells and selectedtoani fes ta ti ons.
The large number of analysis and the high precision achieved,allowed to distinguish the productive layers, to calculate thephysicochemical conditions within the reservoir, and to reformulatethe geothermal field model. Based on this model, a third explorationwell is being drilled (COP-3) and is intended to reach 1800 m belowtop well level in order to verify the results of this work.
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N° 3: CHtniCfiL flND ISOTOPIC flNflLVSIS OF CONDENSEES ONO COLD ONO HOT UWrERS C>O .
: SAMPLE i
icopH-i s:
ICOPfl-2 ft!
:CÛPfi-3 H!
: COP«-« :
:copfl-5 ;
: copfl-e. ;
: copfl-7 :
: copn-6 i
l f rtpu 1 ^ '
:copfi-i3 ;
: copft-20 ;
:cwi-i !
: ravi -2 !
:CfWI-3 !
;ciwi-i i
H'flF-l !
! WF-2 !
!WF-3 !
LOCRÎION ; r : rvPE: <°o :
L« rUqumit« ; 130 I Y.por
Poro COP I 1 242 I V»poi-
L« riiqum«- ! 8S ! V*por
Ffrrugi no** ' 51 î Liquid
Sul^uro* • î 61 î "
d«-i rut» :si,s:
d* Vi cKy I 3*1 ! "
fto- d» lo* &*Ros ! ! "
/ 1 L » ...
flgu-i d«*l vol c-in î 30 ! "
Cc.p-.hue ! : V.por
flgu* d«- conxuHO ! 13 I Liquid
run..»*:!-! ; 7 : »
n*n-anti «1 Loncopu*! 10 ! "
C«£C*d* Excondzd* I 12 ! "
V»rti»nt* ! 8 !
n»n«ntl»l ! 10 :
n.n.ntl»l ! 11 !
C>O Ch«-nic»l ftn«lylis: flPft-EPEM. Isotop]
!H «.«.1.1 «O :
! : -10,8 :
: 2000 : -9,6 ;
! i -10,6 I
: 2000 : -11,9 :
. 2020 : -11,9 :
: 2020 : -n,9 !
; 2010 ; -12,0 :: 20-40 -11,8 :
: ; -12,2 :
: 2010 : -11,9 l
: : -3,7 :; : -12,8 ;i ; -ii.s :: ; -13,5 :! 1424 ; -12,9 !
! 1670 ; -11,3 :; : -12,5 :: 1670 : -12,9 :
! 1674 : -13,6 :
c fln«lyin: INGEIS.
TRBLE N"
e.V.. > ; <T.u.y ; : ng/l : «g/l : «g/i : «q,a : «.3/i
-84,2 !O,8+/-0,7: ! ! I ! I
-82,7 :o,6«/-o,7i : : : : i
-85,1 :o,o»/-o,6: : : : ; :-84,7 : t,7«/-C,7: 5,9 ! 489 13 : 19 ! S9 I 29
-84,2 :2 .5>/-0,7! 6,6 1 474 12 13 : SI : 30
-84,3 ! I 5,9 I 282 1 < 1 1 23 : 39 ! 13
-84,3 :3,3»/-0,7l 5,9 ! 287 ! 2 ! 12 1 61 : 9,5
-83,3 !2.4»/-0,71 6,9 ! 286 12 ! 12 1 52 1 20
-84,6 :3,6*/-o,7: i ; : : :
:o.9./-0,7! : : : :
-90,2 : : : : : : :
-81,7 !3,9./-o,4: : : : : :
-97,2 !1,3»/-0,4: 6,2 ! : i i 1
-92,5 :4.s*/-o,4i 6,6 ! : : : :
-8i,9 ;3.6»/-o,s: 6,8 ! 21 : -i : 13 : 2,9 : 2,4
-90,1 i f ,8»/-o,7: : : i : ;
-93,5 !3,2*/-0,7: 7,4 : 42 ; ' 1 1 <S : 3.3 ! 1,2
-94,5 ! 1,8«/-0,7! 7,1 ! 30 < 1 ; <S 1 1,8 : 0,9
1: ISOTOPIC HNftLVSIS OF COP-1 RNO COP-2 UELLS.
! F» 1 5i02 ! N« : K ! Li1 «g/1 ! ng/1 ! wg/1 ' Hg/1 ! Mg'l
: , o, i : • : i
. o,iu: 90 : 53 : 23 : o,0b
: o,i8 : 100 38 ; 19 : < u.os
, 0 , 1 i so : 19 : 6,2 :< 0,05; 0,28 ; 71 ; 24 : 7,8 I < 0,05
1 1 1 . >
: : : : i: : ! i !: : : , :
i ; : : :: ; 12 : 2,9 : 0,8 :< o.os
: : : : ;: 0,25 : 20 : 5.3 : 1,7 : <. o.os
0,1 : 20 : 3,6 : 1,7 ! < o,os
COP-1DfirE : s/ei :
REFERENCES: OURIO ;Oxyg.n-13 : -10.5 !o»u-t*i-i ,JM ; -81 :
i/85 :INGEIS :
-9.6 ;-33 :
u/86 :INGEIS :
-8.3 :-78 :
1 1/86 ! 1/87IIRG : INGEIS
-8. .2 : -9.1-76 1
COP-iDOTE I
REFERENCES;QHyg.n-16 :D»ut»r-jLUM ;
3/86 !INGEIS !-10.8 :
-es ;
i 1/86 :INGEIS :
-7.6 ;-si ;
11/86 : 1/87IIRG ; INGEIS
-7.7 ; -lO.l-83 :
3. RESULTS AND DISCUSSION
3.1. METEORIC WATER LINE
As established by Panarello et al., 1988, cold samples CAVI-1-2-3-4, VAF-1 and COPA-4-5-6-7-8-9 fit closely the average world meteoricwater line defined by Craig (1961) i.e.
62H - 8 + 10 o/oo
com-Othervj.se doesn't exist a good correlation between isotopicposition and altitude due mainly to the following:
- Precipitation mainly as snow.- Hest origin (Pacific) of the vapor mass that make the altitude
gradient reverse.- Small altitude differences that do not allow the sampling of
the water at a well established height. (Panarello et al . , opci t)
Vapor condensate samples are shifted in oxygen-13 and define aline that intercepts the m.w 1. in the isotope composition of watersfeeding the system This waters would have infiltrated at highly frac-tured area known as "El Anfiteatro" (fig. 8).
3.2. CALCULATION OF THE PHYSICOCHEMICAL CONDITIONS INTO THERESERVOIR
As it is showed in fig. 2, static pressure and temperature profi-les in the bottom of the COP-1 well (the well then has collapsed) itis possible to differenciate two zones into the reservoir. The former
DePth
in
m0
t
er8
-200 -
-400
-600
-800
-1000 -
-1200 -
PRESSURET
P (bar)
-1400i
ont or iBAKU Qias* nun BM i
90 110 190 210 230130 150 170Temperature ( C)
FIG. 2. COP-1 static pressure and temperature profiles
250
18
between 800-1000 m »ith a temperature between 195'C and 220'C and thesecond at 1400 m, a temperature of about 250"C and static pressure of4 MPa. This is the last information available that is then comparedwith those obtained through chemical and isotopic considerations.
Q (25
20
15
10
5
03/8
In/h) g/t (IN/K« yapor)
1
i
BX\^ ——
P- 8 ata
i i i i i
C
Eb
1/82 1/83 1/84 1/85 1/88 1/87 YEARS
— - Q (tn/h) n g/v (m/Kg vapor)
80
50
40
30
20
10
0
FIG. 3. COP-1: production and gas/vapor ratio.
Q (ta/h) I/Y (IN/Ki vapor)
5 -
3/86
(IN/Kg vapor)
FIG. 4. COP-2: production and gas/vapor ratio.
19
-IÛO .45
HSH = 3 log IH2S/H20 J- tog ( H 2 / H 2 0 >
FIG. 5. FT-HSH diagram (D'Amore and Truesdell, 1985).
In the FT-HSH diagram (fig. 5) (D'Amore & Truesdell, 1985) allgas analyses and gas to vapor ratio available are plotted in order tocalculate temperature (T°C) and vapor fraction (Y) simultaneously.
It should be taken into account that the gas/vapor ratios are notaccurate because oftently are measured in the well without to stabili-ze. In addition the method reflects a very located situation and notthat of the whole reservoir, since it is based in the gas/vapor ratioin the influence area of the well.
20
The advantages are that there is no need of measuring the CO con-centration to calculate the temperature, and the method shows an ap-proximate historical evolution.
For the «ell COP-1 at the beginning of the production (1982) atemperature T= 175°C and a vapor fraction Y= 1 is computed. This factcan be interpreted as a "gas cap" as observed in The Geysers, geother-mal field (Box, 1987). That cap has been consumed, probably, duringthe first months of production.
The results in February, 1985 were: T= 195-200'C, Y= 0.7. Thissituation came as a consequence of the loss of the "gas cap" and thefurther production from a biphasic system. Then the yield of this wellfall down in 40 % and simultaneously a shift towards more positivevalues in deuterium and oxygen-18 was observed, indicating that theshallow reservoir was becoming exhaust.
In November, 1986, when the well was reopened, after to be closedby 10 months, a temperature of 200-205°C and a Y= 0 25 are calculated,an increase in the production was observed. This could be explained asthe contribution to the production of the residual liquid from theprecedent exploitation. The calculated value of temperature doesn'tindicate a deep contribution. This hypothesis is in agreement with themore positive values of oxygen-18. Also must be noticed that the H 2$value is the same in 1982 and 1985 (0.4-0.5 %) .
In April, 1987 an strong increase in the HzS content (1.0 %) wasobserved. The calculated temperature T= 245-250°C, Y= 0 45, and a re-turn to more negative oxygen-18 values correspond to the depletion ofthe residual liquid (the yield also declines) and to the existence ofa deep contribution. The discharge declined because there is no goodconnection between both reservoirs. The gas/vapor ratio increased to2-5 % due to the deep contribution of gas and vapor First migrate theresidual liquid, then the vapor and the deeper gas (fig. 6).
30Q (lu/h)
20
10
2/83 r««Jdaal l
d««p contribution
•xhaarta4/VI
HZ* - 1.0 (COP-2 nlu« 11/6«)T T» wo c
2/88
-11 -10.5 -10 -9.5 -9 -B.5Delta Oxyieu-18 (X.)
-8 -7.5 _™
FIG. 6. COP-1: production vs oxygen-18.
21
For the »ell COP-2 on March, 1986, a value of T= 200°C and Y= 1is obtained (similar to the beginning of COP-1) and is also producingfrom a shallow reservoir
On November, 1986 when the well was reopened and assuming Bg/v-1.6 x 10-3 a temperature T= 235"C an Y= 0.03 is calculated. This va-lues do not correspond to the "gas cap" but to a residual liquid cha-racterized by high oxygen-18 values and a deep contribution of gas andvapor.
In April, 1987 an strong decrease in the discharge was observedA temperature T= 235"C and a vapor fraction Y= 0.35 is computed. Ta-king into account the analytical error the COP-1 and COP-2 show thesame local origin; mainly deep (vapor and gas).
In addition, also in the COP-2, the oxygen-18 value became morenegative, indicating that the residual liquid has been fastly removed
In fig. 7 the evolution of the gas/vapor ratio vs the delta oxy-gen-18 is showed for the COP-1 and COP-2 wells. The evolution for bothfrom the beginning (vapor cap) until November, 1986 (when they werereopened) fits on a straight line (I)
60Ca«/Vnpor (IN/Kg vnpor)
40 -
30 -
20
10 -
3/6«v ' «« T«por cap
.S/W
dwp «xmlrlbuliemof f«« uid Tmpor
rmidiua Uifollproduction \\
____o "/•• ^ o-11 -10.5 -10 -9.5 -9 -8.5
Delta Oxygen-18 (X.)
A COP-1 0 COP-2
FIG 7. Gas/vapor ratio vs oxygen-18.
-8 -7.5
In fig. 8 (o*H vs $1*0 plot) the evolution for the COP-1 well,starting from its drilling until it was reopened November, 198fi isplotted. COP-1 follows a line with a slope of about 200°C and then thevalues became more negatives. The same evolution is observed for theCOP-2 well, but with a slope of 210-220'C, in shorter intervals, indi-cating that the shallower reservoir is smaller
Using the chemical analysis of gas sampled in November, 1986 somephysicochemical parameters of the fluid in the reservoir were computedwith a methodology proposed by D'Amore et al, 1987
22
-60
-70
-80
-90
-100 -
Delta Deuterium (%.)
-110
11/88
-18 -17 -16 -15 -14 -13 -12 -11 -10 -9 -BDelta Oxygen-18 (%.)
'" Las Maquinitas A COP-1 ^ COP-2
FIG. 8. Deuterium vs oxygen-18.
They are: minim T("C), maxim T('C), C02 pressures (PC02), vaporfraction (Y), gas/total water ratio (g) and redox conditions (pO2 andpS2).
In table 5 maxim and minim temperatures calculated through COanalysis are presented. It roust be noticed that all the samples shot?the temperature of the shallower reservoir, excepting that of the COP-2 (T= 240°C) that was already producing from the deeper layer
Table 6 exhibits the maxim values (method without CO to fixedtemperatures). Two temperature values are presented: that without (*)is a assumed one, that with (*) is a maxim value. The value of thegas/total water ratio in. the reservoir has two different significateaccording to the temperature considered. The corresponding to max Trepresent an almost total evaporation into the reservoir, for instanceat the COP-1 is similar to the gas/vapor ratio measured at the welltop.
The maxim temperature calculated for November, 1986 for the COP-1is 200 °C in agreement with that estimated by other methods, as resul-ted from a shallow production. Conversely for the COP-2, Tmax= 270"C isaccounting for the deep contribution. A value of Y- 0.046 (Sl = 0.30)complete the conditions of the deeper reservoir (1400 m).
"Las Maquinitas" and "Las Maquinas", exhibit a Tmax= 208-216°Cindicating that the main contribution is from the shallower reservoir.At the area of the "Termas de Copahue" the value of Tmax- 245°C indi-cate a strong deep con tri bution.
By using the diagram PC02 vs T (fig. 9) for the sampling of No-vember, 1986 for "Las Maquinitas", COP-1 and COP-2 is possible to seethat the points are aligned on the equilibrium straigh line:
Muscovite (and clay minerals) + Calcite + Quartz =K-Feldespar + Epi do ta (Xps- 0.275)
in full agreement with the minerals observed during the drilling above850 m. Assuming the PC02 calculated using the gas/vapor ratio from
23
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10
REFERE N CESo COP l
LAS MAQUINI1AS
LAS M AQUINAS
COP l 11/86
DEEP RESERVOIR P(C02)SIMULATED g/v 1987-2SQOC
1987 and T= 250 °C a reaction of equilibrium where PCO^ would be con-trolled by the equilibria of the alteration minerals Epidota and Pre-nhite is obtained (cross in the diagram)
3.3. HYPOTHESIS ON THE ORIGIN OF THE INERT GASES
The values of 813C obtained in 1985 from Las Maquinas - -10 7o/oo, Las Maquinitas --68 o/oo and COP-1 - -10 9 o/oo range bet-ween -7 and -11 o/oo approximately These values are typical of activevolcanic zones, where could have existed the following interactionduring the magmatic activity
Calcite + Silicates = C02 + Hollastonite (Ca-Silicate)
Epstein and O Neil showed that this reaction produce a carbonisotope fractionation leading to more depleted values in the ramainingcal cite The shifting to more negative values depend on the degree ofprogress of the reaction
If the relative quantities of N2possible to see the following
Ar and He are considered is
1 The Nz/Ar ratio, taking into account all the sampled manifes-tations and wells range from 300 to 1000 The value for the meteoricwater recharging the system is close to 40 Therefore could be saidthat the N2 and the Ar are not from atmospheric origin
2 In the diagram (fig 10) the position for the points corres-ponding to all the samples indicates a strong deep magmatic contribu-tion that can explain the primary origin of the N2
N2/1CO
1 COP I
2 COP a3 LAS MAQUNIJAS
i LAS MAQU NAS
Hex 10
FIG 10 Relative N2, He and Ar contents m geothermal gas discharges
26
4. CONCLUSIONS
Based on isotopic and chemicals considerations and on the produc-tion data, the following conclusion can be established:
- Copahue is a typical vapor dominated geothermal field, wherethe pressure into the reservoir is fixed by a biphasic (vapor/liquid) system (Sierra. 1981).
- This kind of reservoir have been proposed by D'Amore (1979) andconfirmed in deep perforations at the Geysers by Box (1987) andin Santa Lucia Island by Aqua ter. They are characterized asstratified systerns.The reservoir is constituted by several productive layers se-parated by low permeability zones. The productive layers havedifférents physicochemical characteristics and productions. Forthe Copahue field, a first level with the following characte-ristic have been established:
Depth - — ---------- — - 850 to 1000 mTemperature ---------- 200 to 215'C (excluding the primary
gas cap)v — — _ — — __ — — — _ _ _ _ _ n nff y-,-i n *?1 _ _ _ _ _ L / . L / C I O C ' L / . É j
PC02 --- ----------——— 50 kPa
Production data for the COP-1 and COP-2 indicate that the sys-tem has been quickly depleted.
The shallow reservoir would have a low lateral permeabylity,thus the original fluid, near COP-1, would have undergone astrong drop of pressure, gas and vapor loss (lowering of thegas/vapor ratio and the discharge) due to the exploitation.Also shows an isotopic enrichment, mainly in oxygen-18, and arelative decrease of soluble gases.
After the reopening of the COP-1 and COP-2 wells in November,1986 the first fluid produced by the COP-1 was originated in alocal residual liquid (condensate) that migrates laterally to-wards the well. In addition for the COP-2 a deep gas and vaporsource contributed to the production.The local liquid is fastly consumed, and after five months bothCOP-1 and COP-2 fluids exhibits characteristics belonging tothe deep level (below 1400 m) with the following thermodynamicpa rame t ers •'
Temperature ----- —— — - ————— _ _ _ _ 250-260 °CDischarged vapor temperature ----- 200-220°CVapor fraction ----—-——-—------ 0.03-0.05G/V ratio ----------------------- 20-30 L.N./kgT>nn« _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 1 nn~ ?nn I-P^IT \~t\J % — _ _ _ _ _ _ _ — — — ~~ J_ U U & U U J\.JT et
H2S/H2 -------------------------- > i018Q --------------------------- -10.0 0/00
The future COP-3 well would must find a fluid with thischaracteristics typical for a vapor dominated system of highenthalpy. In order to explain the present discharges of COP-1and COP-2 a system with good vertical permeability must beassummed (fig. 11).
27
PHASE I VHPUR ERSg
==J> =l>
i '" • """"" • • ' • " " :FIRSTy +
3=LEUEL T = 3an-2Q5
G ^ L
<F=
V: D.QS -D.2_ r : 3,5 jîm
xxy--vfoxxfi;:xxxx«»: >fixxrex*>y^SECOND LEUEL
L » <J * G
PHASE II y
X^RESIPUBL LIQUID
LEVELV = U dti
's-..
SECDrtD LEytL'_ » U » G
. R?-.T = 2 S D - S B D _ nv = D -Ü3 - Q..Û5
L: LIQUIDG: GRSY: URPDR
FIG. 11. COP-1 well (evolution).
àciaosledgtKstts - Tiis sork has been perforted sithin the fraiesork of tie làSt Coordinated Research Prograne on theapplication of Isotope and Geocheiical Techniques in Geothertsl Exploration in Latin iierica (Research Contract N'39S8/IG), financially supported by the Gorerntent of Italy.
REFERENCES
Bolognas! L. , Noto P. e Nuti S. (1986) Studio chimico ed isotopicodella solfatara di Pozzuoli: ipotesi sull 'origine e sulle tempera-ture profonde dei fluidi. Ftendiconti Soc. It. Min. e Petr. , 41 (2),pp. 281-295.
Box H. T., D'Amore F. and Nuti S. (1987) Chemical and isotopic com-position of the fluid sampled during drilling at The Geysers (Ca,USA). Int. Symp. on Development and Exploitation of Geothermal Be-sources, Cuernavaca, Mexico, oct. 5-10, in press.
D'Amore F. and Truesdell A. H. (1979) Models for steam chemistry atLarderello and The Geysers. Proc. 5th Workshop Res. Eng., StanfordUniversity, pp. 283-293.
28
D'Amore F., Celati B. and Galore C. (1982) Fluid geochemistry appli-cations in reservoir engineering (vapor dominated systems). Proc.8th Workshop Res. Eng., Stanford University, pp. 295-308.
D'Amore F. and Gianelli G. (1984) Mineral assemblages and oxygen andsulphur fugacities in natural water-rock interaction processes. Geo-chim. et Cosmochim. Acta, 48, pp. 847-857.
D'Amore F. and Pruess K. (1985) Correlations between vapor satura-tion, fluid composition and well decline in Larderello. 10th Work-shop Bes. Eng., Stanford University, pp. 113-121.
D'Amore F. and Truesdell A. H. (1985) Calculation of geothermal re-servoir temperatures and steam fractions from gas compositions. GBCSymp. on Geothermal Energy, Kona, Hawaii, 9, pp. 303-310.
D'Amore F., Fancelli R. and Saracco L. (1986) Development of a me-thodology for measuring CO and its geothermometric applications.Part J. Model development. CNR Report to European Communities (Con-tract 0022-1).
D'Amore F. and Truesdell A. H. (1986) A review of solubilities andequilibrium constants for gaseous species of geothermal interest.Appendix to CNR 1987 Report to European Communities (Part II),(Proposal A2/083/I, Contract 0022-1).
D'Amore F. , Fancelli R. , Mussi M. , Saracco L. , Caprai A. , Calvi E. ,Del Chicca G. (1987) Development of a methodology for measuringcarbon monoxide and its geothermometric applications (Part II). CNRReport to European Communities (Proposal A2/083/I, Contract 0022-1).
D'Amore F. (1987) Some geochemical techniques for reservoir tempera-ture computation. Int. Symp. on Development and Exploitation of Geo-thermal Resources, Cuernavaca, Mexico, oct. 5-10, in press.
D'Amore F., Fancelli R. and Saracco L. (1987) Gas geothermometrybased on CO content. Application in Italian geothermal fields. Proc.12th Workshop Res. Eng., Stanford University, in press.
D'Amore F., Sierra J. L., Panarello H. (1988) Informe Avance delContra to de Investigaciôn 01EA N° 3988/IG.
Jurio R. L. (1977) Caracteristicas geoquimicas de los fluidos terma-les de Copahue (Neuquén - Argentina). Principales implicancias geo-térmicas. Apartado de la revista "Mineria", N" 172, 11 pp.
Mas L. C. (1986) Estudios petrogrâficos de minérales de alteraciôn yalgunos parâmetros fisicos sobre rouestras de roca de perforaciones.Informe CNR - Int. School of Geo thermies. IIRG, Pi sa.
Panarello H. 0. , Levin M. , Albero M. C., Sierra J. L. and Gingins M.0. (1986) Isotopic and geochemical study of the vapor dominated geo-thermal field of Copahue (Neuquén, Argentina). Int. Meeting on Geo-thermics and Geothermal Energy, Guaruja, Brasil, 10-14 äug. 1986, 16PP. (A).
Panarello H. O. , Sierra J. L. , Gingins M. O. , Levin M. y Albero M.C. (1986) Estudio geoquimico e isotôpico de los sistemas geotermalesde la Provincia del Neuquén, Repûblica Argentina, primera parte:area Copahue. Informe anual de avance del contrato OIEA N" 3988 IG.(B).
Saracco L. and D'Amore F. (1988) C02B, a computer program forapplying a gas - geothermometer to geothermal systems. Submitted toComputers and Geosciences (Pergamon Press).
29
Secretarîa del COPADE (Provincia del Neuquên) (1982) Consideracionesbrèves sobre antécédentes hidroquîmicos de vertientes, condensadosacuosos y gases no condensables del area geotérmica de Copahue(Neuquên - Argentina). Informe LATINOCONSULT - ESIN S.A., 59 pp.
Secretarîa del COPADE (Provincia del Neuquên), LATINOCONSULT - ELC.Estudio de factibilidad geotérroica en Copahue: primera etapa. Infor-me COP-I-5284.
Sierra J. L., Gingins M. 0. , Panarello H. 0., Levin M., D'Amore F. yGianelli G. (1986) Estudio geoquimico e isotôpico de los fluidosgeotermales de la Provincia del Neuquên. Informe de avance OIEA(Convenio N° 3988/IG), nov. 1986, 15 pp.
Truesdell A. H., Haizlip J. P., Box H. T. and D'Amore F. (1987)Fieldwide chemical and isotopic gradients in steam from The Geysers.Proc. 12th Workshop Res. Eng., Stanford University, in press.
30
ISOTOPIC AND GEOCHEMICAL STUDY OF THE DOMUYOGEOTHERMAL FIELD, NEUQUEN, ARGENTINA
H. PANARELLO*, J.L. SIERRA**,F. D'AMORE***, G. PEDRO**
*Instituto de Geocronologfa y Geologfa Isotopica,Buenos Aires, Argentina
**Ente Provincial de Energfa del Neuquén,Neuquén, Argentina
***Istituto Internazionale per le Ricerche Geotermiche,Consiglio Nazionale delle Ricerche,Pisa, Italy
Resumen-Abstract
ESTUDIO ISOTOPICO Y GEOQUIMICO DEL CAMPO GEOTERMICO DE DOMUYO, NEUQUEN,ARGENTINA.
Se realizaron analisis isotôpicos y anâlîsis quimicos en sôlidosdisueltos y gases con el objeto de establecer un modelo de circulaciôny determinar la temperatura de reservorio.
De acuerdo a 2 os nivelés de tritio registrados y los va lores delos isôtopos estables deuterio y oxigeno-18, la recarga del sistema seproduce a una al tara de ça. 2900 m s.n.m. en la falda oeste del cerroDomo.
El agua muestreada en un geyser, "El Humazo", se reconoce como lamas relacionada al agua primaria, aunque contiene una pequena propor-ciôn de agua meteôrica reciente. La temperatura del reservorio de don-dé proviene "El Humazo", séria de unos 225 °C de acuerdo con el geo-termômetro de Na-K-Ca. Asimismo se puede inférir la existencia de otroreservorio mas superficial con temperaturas de alrededor de 176°C dedonde provendrian las aguas de "Las Olletas" y "Los Tachos", tambiénafectados por procesos de diluciôn y pérdida de vapor.
Finalmente las manifestaciones templadas como "Aguas Calientes" y"Banos de agua caliente", serian el resultado de mezclas entre aguasdel tipo de "Las Olletas" con acuiferos frios superficiales.
La termometria gaseosa y las temperaturas obtenidas a través deconsideraciones termodinâmicas sobre el equilibrio entre flui dos yminérales arro.jan temperaturas entre 220-248 "C para el reservorio masprofundo en concordancia con la temperatura quimica. Asimismo sugierenun valor cercano a cero de la fracciôn de vapor en el reservorio (Y)confirmando el estado liquido del fluido en el reservorio.
ISOTOPIC AND GEOCHEMICAL STUDY OF THE DOMUYO GEOTHERMAL FIELD, NEUQUEN,ARGENTINA.
SnvironËental isotopes, 2S, II0 and }S as sell as cheiical analyses of dissolved solids and gases havebeen perfoned in order to establish a circulation todel and reservoir teiperatureaccording to tritiai levels and isotope content, recharge occurs 2900 i above sea level on the vest slope of theDoio volcano.Hater fro* a geyser, "El Buiazo", sas recognised as the tore related to the pritary deep geothenal fluid, altoughsith a linor proportion of fresh sater.
31
Reservoir temperature as estimates by the Na-K Ca geotheraoaeter on this tamfestation yield a value of 225'CA shallower reservoir mth température of about lïô'C (Ha tf Ca geotheraoaeter) is evidenced in "Las Olletas" and'Los îachos" also affected by dilution and steaa loss processesfinally, teaperate Manifestations as "/tguas Calientes" and "SaSos de tguas Calieates" are the result of sizing betveen Las Olletas type eaters vith fresh eeteoric shallover vatersGas theraotetry and coiputation of fluid and sineral equilibria yield teaperatares betveen 220-248'C for the deepreservoir, in agreement sith sore classical techniques and suggest a value near to zero for the vapor fraction (7)at depth and confire the liquid state of the fluid into the reservoir
1. INTRODUCTION
The Domuyo geothermal area is located at AW of the Neuquen Pro-vince c a 36° 45' S and 70° 47 W, 40 km away from the Andes Cordi-llera
The climate is defined as Semiarid Patagonic in the lower zonesand as Andino high mountain type in the upper areas
The basement is constituted by acid plutonites, metamorphites andJurasic sediments from the Neuquenian Basin The upper part is coveredby tertiary vulcanites (Pliocene) and quaternary lava, the last cor-respond to emissions of domic vulcanism whose magmatic chamber is res-ponsible of a thermic anomaly
Domuyo
Copahue
FIG 1.
32
Great regional faults cross the zone with E-W. N-S and N-M direc-tion. In these faults intercepts, wain thermal manifestations as ther-mal spring and geysers are found.
The study of the zone began in 1982 »hen a geological interpreta-tion of satellitary images and air photography was made. An area of 40km2 was selected at the west of the Domuyo mount. On this area a geo-logical survey was performed. In adittion geochemistry, gravimetry,heat flow measurements, geoelectric and seismic prospections weredone.
2. METHODOLOGY
During 1985 and 1988 two field trips were performed. In the firstpH. conductivity, Na + , K+. Ca2 + , Mg2 + , Li+, Al , Si02, NH3, B, Cl~,SO42-, 32-, F-, 1*0, 2H and 3ff were analysed.
In the second one a more detailed chemistry of water and gases inselected manifestations was made.
The information obtained during the first stage allowed the che-mistry and isotopic classification of water samples, geothermometry,determination of mixture of waters, altitude and area of recharge,extension of the reservoir and to establish a circulation model. Withdata of the second stage saturation indexes and physicochemical condi-tions within the reservoir were modelled.
3. RESULTS AND DISCUSSION
3.1. CHEMICAL COMPOSITION OF THE HATERS
In table 1 results of chemical analyses are presented. Accordingto their temperature and conductivity, water can be classified into 3groups :
Group I: Comprises the samples corresponding to "El Humazo'(EH1), "Las Olletas" (LO) and "Los Tachos" (LTD, they are alkalineclorurated, mainly Na + , K+, with low concentration of Ca2+ and Mg2 + .Conductivity oscilate between 5.38 and 6.42 mS/cm. Emergence tempera-ture is near to the boiling point at these altitude (~93°C).pH round 8.2.
Group II: Cold waters, MAF 3,4, 5, 6, 8, 10, 11. 12, 13 are in-cluded. They are HC03- Na + Caz + . Conductivities are lower than 102uS/cm. Surface temperature is less than 13°C. pH ranges between 4.5and 8.1.
Group III: Aguas Calientes (AC) 4, 5, 6, Banos de Aguas Calientes(BAC 7), LT12 and MAF 1, 5 and 7 These samples present intermediatevalues between group I and II i.e. conductivities between 629 and 4200US/cm, temperatures of 16 to 64°C and pH of 5.9 to 7.9.
Tritium values showed in table 3 confirm the existence of this 3groups being of (0 to 1) T.U. for the first, (1 to 6.5) T.U. the se-cond and (1 to 3.5) T.U. for the third.
33
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36
3.2. LIQUID PHASE THERMOMETRY
Table 6 presents the temperature values obtained by mean of dif-ferent geothermometers Silica, Na/K, Na-K-Ca, Na-K-Ca corrected byMg, Na-Li, Mg~Li, K-Mg and in figure 2 the graphic method of Giggen-bach (1986) is presented
Assuming as more reliable the Na-K-Ca geothermometer, the highesttemperature correspond to EH1, 225,5°C On LO and LT1 similar tempe-ratures of 176"C and 179 °C respectively are obtained AC and BAC 7yield lower values of 150 and 156°C
The triangular diagram of Giggembach (op cit) (fig 2) shows twogroups of samples, aligned on two isotherms EH1 and LT12 lie on theisotherm of 245°C, LO, LT1 and all AC on the isotherm of about 190°C
Sample EH1 fits close to the full equilibrium curve, however itshows a dilution since it is not on the curve, LT12 is more dilutedThe position of LO and LT1 on the 190°C isotherm suggests the reequi-libration of the system to a lower temperature As can be seen in fig2, all the remaining manifestations are more diluted
The silica geothermometers yield lower temperatures for EH1 thanthose obtained by the Na-K-Ca one, they are 191 °C for the quartz equi-librium, without steam loss, and 177°C if steam loss is consideredThus boiling with SiU2 precipitation is evidenced
TABLE N* 6: 6EOTHERHONETERS
BEOTHERHOI1ETERS
Na/K (F)
Na/K (T)
Na-K-Ca
Na-K-Ca («q>
Na-Li
K-tlg
Hq-Li
TQC
TQfi
TCH
AC4
178,9
140,9
177,5
161,6
148,1
131,2
209,9
174,3
163,6
152,4
ACS
187,4
151,0
177,5
168,9
134,6
133,4
198,2
165,4
156,2
142,2
AC6
188,5
152,5
183,4
159,1
138,3
129,8
194,3
178,2
166,7
156,8
BAC7
198,6
164,7
186,6
137,6
145,3
118,8
175,6
172,0
161,6
149,7
LO
173,3
134,2
176,0
172,8
141,0
151,1
242,7
177,5
166,2
156,0
LT1
176,0
137,4
178,7
174,2
138,6
149,8
236,1
181,6
169,5
160,8
LT12
228,2
201,8
211,3
206,1
144,6
168,9
233,1
168,8
159,0
146,0
EH1
232,4
207,1
225,5
225,5
143,0
212,8
297,5
191,2
177,3
171,9
RP
175,9
136,7
159,7
29,7
227,7
101,2
151,1
149,6
143,8
124,3
Na/K (Fl: FOURNIERNa/K ( î): TRUESDELLNa-K-Ca (ttq): CORRECTION BY HAGNESIU«TQC: QUARTZ CONDUCTIVETCH: CHALCEDONY
~ NOTE: TEHPERAÏURES ARE IN 'C
37
Na/1000
a 12
EHI
%-Na"=cNü/10S'o-Mg" = 100 Vct
.<:,• in mg/kg
LO
LTI
20 X <U M 10 70 M / 90"V.-Ma' ',
enbach, G raph i ca l Techniques, 11B6.- MAE 7
MgFIG. 2.
600
400 -
300 -
ZOO -
100
S1O2 (ppm)
0 20 40 00 80 100 120 140 180 180 200 220 240Rnthalpj (Koal/Kf)
FIG. 3. Enthalpy — silica.
m
a
K
H
2
a
"?
-H
1
HLBITE L[1 ^rj^'
HC5.^'"^°LT1?
_^-''
No-nBMT
KHOLINITE
MICRDCLINE
__ .^„„IT,
SVST. Hci,Hea«t ana oC i„q oH;J',tiH: -a 35= ^u
FIG. 4. Activity diagram.
B IQ
38
BORON VS CHLORIDE LITHIUM VS CHLORIDE
20 Boron (ppm) Lithium (ppm)
15 -
10 -
5 -
mi+
+ U> +LT1
? +«+1C6
BlCTfWlï
) 500 1000 1500 ZOChloride (ppm.)
12
10
B
6
4
2
000 (
KBl. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . £T..+. .+........................................... . +. . . . . . . . . . . . .
1C* «"1C8 + +
LT1Z +aicrqk-
B- ' ' * ' '•+' . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2 -H +
) 500 1000 1500 ZOChloride (ppm)
SULFATE VS CHLORIDE SODIUM VS CHLORIDE
zoo
150
100
50
Sulfate (ppm)
8+
LO
les
500 1000Chloride (ppm)
1500
EB1
+
2000
1400
1200
1000 -
BOO
600 -
400 -
200 -
Sodium (ppm)
0-1»
•ll+-
LO
1C»
uiz
500 1000Chlorid* (ppm)
1500 2000
FIG. 5.
-fe-es
1*0
120
100
BO
80
40
20
0«
POTASSIUM VS CHLORIDE
Potuilum (ppm)
m +
-
-m j
to + iLT1Ï ACS +
+ AC« AC* 4-BAC7 + 4.
, +
*- .K -f-
£ * T"k+ ++0 500 1000 1500 2000
Chloride (ppm)
SILICA VS CHLORIDE
Silica (ppm)
m204 BAC7 T_^ + +
"tin«4- ACS
150 1-
100
50 '
0
7S-f
• H- '< B
; +
I+2
0 500 1000 1500 ZOOOChloride (ppm.)
MAGNESIUM VS CHLORIDE
UH an»«i nrn f ppm)«"«u. ""* U>F ^1
4 -1 + 7!
t , -f ACS1 AC« ,
1 -r 4-2 (- + AC* ttl
i + -r +i ">
f m2 «w± f +
0T —————————— i —————————— i —————————— i ——————————0 500 1000 1500 2000
Chloride (ppm)
ENTHALPY VS CHLORIDE
En.th.alpy (Kc«l/kg)
LO läi i
80-
LT12 ACSAC*6 0 k + f
_PBAC7 ACe
•i-40 r
9S 7
20. r 1"^
••'4-2I
Ol ——————————— i ———————————— i ———————————— i ———————————— I0 500 1000 1500 2000
Chlorid« (ppm)
FIG. 6
Enthalpy (Kcal/leg)300 i———————————————— -*
500 1000 1500 2000Chloride (ppm)
FIG. 7 Enthalpy vs chloride
2500 3000
-100Delta Deuterium (%.)
-21 -20 -19 -18 -17 -16Delta Oxyfen-18 (X.)
-15 -13
FIG 8 Deuterium vs oxygen-18
3.3. GAS GEOTHEBMOMETBY
A. METHOD l
Starting with the composition of the sample EH3, the followingreservoir parameters Here determined t, partial pressure of CÛ2(PCOs), vapor fraction (Y) and gas to water ratio
The used methodology is that proposed by D'Amore et al (1987)It is reliable for reservoirs with temperatures over 140°C and takeinto account the gaseous species C02, HzS, CH4, H2 and CO The follo-
41
wing reactions are considered-
H20 = H2 + 1/2 02 H
H3S - H 2 +1/2 S z S
CH4 + 2 H20 = 4 H? + C02 C
CO ? -f- H 2 = CO + H20 CO
1/2 Fe304 + S z - FeS2 + 2/3 02 MP
Taking into account this 5 equation and the equilibrium cons-tants, partial pressures of 82 and 02 can be eliminated and the follo-wing equations to be written:
4 log P(H2) + log P(C02) - log P(CH4) -log AV + 2 log P(H20)
3 log P(H2S) -log P(Hz) -2 log KH - 3 log Ks - 3/2 log KMP + 2 log P(H20)
4 log P(CO) - 3 log P(C02) - log P(CH4) -4 log Kco + log KC - 2 log P(H20)
The partial pressure of every gas specie may be written as afunction of its molar concentration ( n ± ) respect water in the dischar-ge point (d), the molar fraction of the vapor (Y) and the molar coef-ficient of distribution between vapor and liquid -S2 •
log Pi = log (nl/nH20)d - log A, + log P(H2O)
where A, - Y + (1 - Y) for Y > 0
for Y < 0
and log B1 - a + b t where t - temperature ( " C )a, b = constant for each
gaseous specie.
Replacing in the 3 equations above, partial pressures by theirexpression we have:
4 log(H2/CU2) - log(CH4/C02) ~ fi(t) + fi(t,Y) - 4 logP(C02)
3 log(H2S/C02) - log(H2/C02) = f2(t) + f2(t,Y) - 2 logP(CO2)
4 log(CO/CO2) - log(CH4/C02) - f3(t) + f3(t,Y)
fi(t), f2(t) and fs(t) are functions expressing the dependence ofthe equilibrium constants with temperature and fj(t,Y), f2(t,Y) andf y f t , Y) represent the termins A^ that are function of both, temperatu-re and vapor fraction
42
Solving this system for EH3 we obtain:
t - 220'C reservoir temperaturePC02 =300 kPaY = 0.003 vapor fraction
log H2/H20 - - 6Gas/water = 4.7 x 10-*logP(Oz) = - 39.9
B. METHOD 2
In this method is taking into account the system C02. CH4, H2 andCO. H2S is excluded, thus one constant must be assumed; in this caseY=0.
The obtained results for EH3 are the following:
t - 240'Clog P(C02) - - 38.3PCO2 - 300 kPa
In the manifestation at the border of the geothermal field Rincônde las Papas (RP) we obtain:
t - 210"Clog P(02) - - 41.2PCO? - 2 MPa
C. METHOD 3 (D'Amore et al., 1989)
In this method the reservoir temperature is calculated by usingthe simultaneously composition of the va ter and the gas. in the gasthe concentrations of CE4. H2 and CO refered to the CO2 are conside-red.
In the reaction:
CO + 3 H2 = CH4 + H20 (1)
considering Y=0 we have:
log Pj - log n2/nC02 + log B! - log B(CO2) + log P(C021 (?)
Taking into account equations 1 and 2 we arrive to an expressiononly depending on T and P/cost-
In the same »ay is no» possible with aid of computer programscalculate the P(co2) from the water composition, alkalinity and pH atdifferent temperatures. At the equilibrium temperature both pressurescalculated from water and gas must be equals.
With water from EH1 and gas from EH3, it was obtained:
t - 248°CP(co2) =150 kPa
As can be observed the three methods yield values between 220 and248°C in agreement with those obtained by classic thermometry. Theobtained value for Y, virtually equal to zero define the field as li-quid dominated.
43
3 4. ENVIRONMENTAL ISOTOPES RESULTS
A. COLD HATER SAMPLES
Samples representatives of cold water springs £2 1 close to themeteoric water line o^H - 8 <5 1 »0 + 10 o/oo MAE 1U 11, 12 13 and 14exhibits more depleted in 2H and l S0 values indicating a higher alti-tude of the recharge Conversely the more enriched samples correspondto MAF 1 5 and 8
In fig 14 the oxygen- 18 contents of cold va ter springs are plot-ted as a function of altitude An altitude effect of (0 18 ±0 02)o/oo/lOO m is estimated fitting the equation
6160 - (18 ± 2) x 10 •* /fn h CIO 8 + 05) o/oo
with a coefficient of correlation r-0 9
In similar form (fig 15) an altitude effect of (1 3+0 2)o/ooper 100 m is found for deuterium The regression equation is
Ô2H - (13 ± 2) x 10 3 /m
r~ 0 83
(80 98 ± 0 Ob) o/oo
One pole of these lines are determined by samples MAb 1 and 8with high tritium contents representing local recent precipitationConversely the other pole is represented by samples MAF 3 10 11, 13and 14 with lower tritium contents corresponding to spring located atthe west slope of the Domo mount over 2900 m a i l assumed as the recharge area
B HOT HATER SAMPLES
In figures 9A and 9B the isotope content 01S0 and 6 2H of samplesEH LO and LT show to be near of the isotopic equilibrium at the tem-perature of emergence This fact and the Cl' enthalpy diagram allow tocalculate the isotopic composition prior to the steam separation LOLT1 and HI O^H vs Cl ~ and 6 1 *O vs Cl - diagrams (fig 10 and 11),lead to the calculation of the composition of the deep reservoirs HI*and LO*LT*
From the 62H vs 8iaO diagram can be concluded that watei rechar-ged at the Domo mount recharge the reservoirs corresponding to El Hu-mazo ( H I * ) and that of Las Olletas-Los Tachos (LO*LT*) with the majoroxygen-18 shift for HI* AC and BAC manifestations are produced by themixing of waters LO type with local recent meteoric waters
Tritium contents of hot water samples indicate a minor meteoricwater proportion for EH1 and a significative contribution of freshwater to AC and BAC samples
44
250
200
150
100
50
Temperature { C)
810E3 In
10 12 14 16
FIG 9a Oxygen-18 enrichment as a function of the emergence temperature
250
200
150
100
50
Temperatur« ( C)
-10 10 20 3010E3 In
40 50 60 70
FIG 9b Deuterium enrichment as a function of the emergence temperature
45
-100Delta Deuterium (% )
500 1000 1500 2000Chloride (ppm)
3000
FIG 10 Deuterium vs chloride.
Delta Oxygen-IB (X)
500 1000 1500 2000Chloride (ppm)
3000
FIG. 11 Oxygen-18 vs chloride
46
8
7i
6
5
4ir.
Tritium (T.U. +/- 0.6)
12
2'-
1 -
13
11
ice ACE
0
BAC?ÀC4 LO
KH1
4Ht»
0 500 1000 1500Chloride (ppm)
FIG. 12. Tritium vs chloride.
2000 2500 3000
3.5. CIRCULATION MODEL
In figure 13, a circulation model is presented. It consist of acentral area with a fluid HI* feeding the manifestation of El Humazo yLos Tachos and a second system less hot, responsible of Las 01 le tasand Banos de Aguas Calientes (LO*LT*) diluted with cold water in dif-ferent proportion.
All the system is recharged in the neighboring of the Domo mount.
3.6. HYPOTHESIS ON THE ORIGIN OF GASES
13C composition of gases from EH1, EH3 and PP are -7.6, -7.3 and-8.2 o/oo respectively, indicating magmatic or atmospheric origin.This results would be partially confirmed by the study of concentra-tion of non reactive gases (Giggenbach, 1983).
The source of the Ar, N2 and He can be characterized as follows:
1. Recent waters, air saturated, with low He content and N^/Arratio depending on the quantity of gases withdrawn from waters. It canrange between 78 (low proportion of extracted gas) and 38 (all the gasextracted).
2. A deep magma tic contribution with low He content, N 2/He grea-ter than 3000 and Nz/Ar ratio higher than 500.
3. Cortical component typically observed in systems with slowcirculation. They have an He content increasing as a function of theresidence time of the fluid at the crust.
In figure 16 these parameters are presented as a triangular dia-gram.
47
FIG. 13. Circulation model
-13.0Delta Oxygen-18 (%.)
2000 2500Altitude (m)
FIG. 14. Delta oxygen-18 vs altitude.
3000
-95
-100
-105
-110
-115
-120
Delta Deuterium (%.)
1500
'7•U
2000 2500Altitude (m)
FIG. 15. Delta deuterium vs altitude.
3000
49
M - Mantle N^/IÛOC • CruitulASW - Air Saturate W a t e r at JO 'C
COIWUL
Ari - --
FIG 16 Triangular diagram N2-Ar-He for some geothermal fields of the world (D'Amore et al , 1989)
The PP sample shows a clear origin in air saturated groundwaterConversely EH3, with higher values of He is shoving a deeper originEH3 gases can be considered as a mixture between a cortical and a mantie contribution The sample from Copahue exhibits a typical composition of gases coming from the mantle It must be noted that is possi-ble, at the Domuyo area, not far from El Humazo the existence of abasaltic magmatic chamber This chamber would contain gases of similarcharacteristics to those of Copahue The further circulation throughmethamorfic and granitic rocks, could explain the high He concentra-tion making the crustal component became important
3 7 SILICATE SOLUBILITY PRODUCT
The most of problems related to the suitability of a given geo-thermometer on aqueous samples is frequently due to the unknown inte-raction between the host rock and water, and to the value of the pH atdepth Also to the difficulty for reaching the equilibrium in low en-thalpy systems local cooling effects and mixing processes
The use of secondary mineral equilibrium fixing the water chemis-try, could eliminate any unknown related to the estimated temperaturefor the aquifer
It can be achieved by determining the monomeric aluminium (A1+3)and (Al(xy)H)z with x= OH- F~, 504 =, etc and z~ (3 wy) In the process of leaching of the host rock into microfractured system d certainquantity of polimeric aluminium is produced in addition to otheranions and cations Only a part of this polimeric aluminium producesmonomeric aluminium, being the latter a function of the solution com-position and temperature This monomeric aluminium participates in theformation of hydrothermal altered silicates in equilibrium
50
Using adecuate computer programs and field data like pH, tempera-ture and alkalinity as well as laboratory chemical data, it is possi-ble to calculate the composition of the fluid at depth and the satura-tion indexes (S.I.) of several minerals at any temperature (Arnonson &Sigurson, 1982; Worley, 1979).
The S.I. are computed and plotted for a selected group of mine-rals that are compatible with the local minéralogie matrix For eachmineral, the intercept of the line S.I.= 0 vs temperature yield thecalculated equilibrium temperature.
Taking into account the uncertainty in the thermodynamic data,the interval considéra ted as "in equilibrium' for a given mineralfalls into a range of S I. = 0 ± 0.25
In table 7 the value of S.I. for selected alteration minerals ofthe manifestation EH1 are presented. The program was run assuming thesystem descompression occured at 180°C.
fflBLE N° 7: S*tur.»tion ind*n»I of «on* «lt#r.»tion Hin*r*l***l*ct*d t»np*r*tijr»s CfroM U*tch 3 progr»M ford*sco«position t»np«r-atur-* td« 180 °O.EH1 hot spring, DoMuyo.
tllNERRL
1> rig-Chlorit»
2~> Zoisit*
3") Epidot»
4> Pr*hnit»
5") Rlbit«--KF*1 d»sp*r
6> KF«.ld»*p»r--tluscovi t«
7) flrihydrjt«.+ Fluorit»
200
-2. 11
-2.51
-0.96
-0.57
-0.3$
-0. 10
-0.44
t -»c225
-0.96
-1.52
-1.06
0. 16
0.04
-0.05
-0. 19
250
1.20
-0.26
-0.29
1.31
0.48
O.'J«
0.05
275
3.16
1. 17
1.40
2.6-4
0.90
0.90
0.27
t °C
for 5.1 .« 0
236
251
254
220
223
225
245
t - 237 +/- 15 °C Cfor S.I. - 0.05
t - 238 +/- 17 °C Cfor S.I. " 0.0 »/- 0.253
Sup»r**turation with r»»p»ct to Ctlcit*.Ur»d»rs*tur*-fcl on MI th r**p»ct to2 U*ir*kit*f LuoMontit*^ MontNori 11 orri t«
In figure 17 an average temperature of 238+ 17°C is obtainedThe recalculated pH is 6.8 for a PC02= 120 kPa, for each temperaturevalue, the silica concentration corresponding to quartz saturation wasassumed. The obtained results are in agreement with those calculatedby the triangular diagram (Giggenbach op. cit ), classical geothermo-meters and gas geothermometers.
The computed pH- 6.8 is lower than such measured at the field=8.2. It can be explained as a descompression phenomena with C02 loss.
In figure 19 the pH= 6.8 at a temperature of 240°C is consistentwith the K-feldespar and K-mica-quartz and the salinity.
In table 8 are presented the values of S I. for Pinçon de lasPapas (RP) manifestation and in figure 18 a temperature of 166 ±17'C for the mentioned sample is obtained.
51
_4 _
-2 -
150 200 225 250
Temperature ( C)
+ Zolstte # Kpldote
X JUb-KFeld <>
175
k«-chlor.
Prhenlte
FIG. 17. Saturation indexes at selected temperatures,El Humazo (EH1).
275
SI
(1) Qwt*(t) Cttuomtta
(3)— &l(4) UrwoorlU
(8)UfCblmiU(UUU)(8)
_<
-30 25 50 75 100 125 150 175 200 225 250 275
Temperature ( C)' (D + (2) * 0)D (4) X (6) 0 (6)
FIG. 18. Saturation indexes vs temperature,Rincön de las Papas (RP).
52
8Solution pH
m(N«+K)-0 01
. BT«r«i«r<il(O.OOB)T« KopU(O.OOe)
m(N«+K)-0.1
«Hl DOUUTO-1988(0.060)
»«lr«lc«l(0 044)
•Js Bro«dl«n4«(0 034)
lfoblo(0 1)
ahob«ra(0 07)C«rro Prl«lo(0.2B)
(12)
150 200 250
Température ( C)
FIG. 19. pH vs température.
300 350
TR8LE N° 8ï Saturation ind»H*s fot~ SOM*»•l»ct»d t»np»r»tur»*.RP hot spring, Dowuyo.
MT MP*DCII
15 Ou*i-tr
25 2 Muscowit»--3 L*UMontit*
35 Husco^i t*-- 3 fllfc.it*
*O tluscoyi'fc* —-Zoisi t»
55 Musco^i t*—+ n^Chlorit»
65 riuffcovi t*~- 2 f ldul^r i»
100
0.41
-
-
-
-
125
0. 165
2. -120
0.45
3.65
-1.32
1.36
t ->cISO
-0.01
1.5-1
0.6S
1.82
0.05
0.95
175
-
-0.27
-0.25
0.10
2.97
0. 15
200
-0.37
-1.94
-1.13
-1.52
-
-0.52
t °C
for S.I .- 0
148
171
i&e
177
119
181
t - 16& +/- 14 &C <for S.I. - 0.05t • 166 +/- 17 °C Cfor S.I. - 0.0 */- 0.255SuperA^tur *tl on HI irh r»*p*c^ tUnd»rs«t'jr»tlon MI th r**p»ct to! Epldot», Prehnlt», U«ir»kit», flnhidrit*.
53
4, CONCLUSIONS
The important number of hot and cold springs and their relativelyhigh discharge, allowed to get a great number of data to formulate anhydrologies! model
By mean of chemical data, isotope analysis and enthalpy data »aspossible to calculate the chemical and isotopic composition of theparent water as well as the recharge altitude
The methodology based in the chemical composition of gases andsaturation indexes have led to a better and more accurate knowledge ofthe geothermal field
The Domuyo geothermal field have a Hater dominated reservoir Thefield has a central zone between El Huma^o v Los Tachos having theparent water the following characteristics
Ô2H (o/oo) -115Oi*0 (o/oo) -13 Ut CO 220-248Y O 03PC02 (kPa) 150-300Cl (ppm) 2500
A second zone with a shallower reservoir comprising a more extentarea with a similar isotopic composition a temperature of about 170°C and a chloride concentration of ca 2000 ppm
In addition a third zone (Rincon de las Papas) exhibit stronglybicarbonated waters, typical of the boundary of geothermal aiea^ Themore reliable temperature is that obtained by using the S I166 ± 17°C Gas geothermometers and classic geothermometers yieldhigher and erratic values
Acknoiledgeients - fhis vork has been perforied mtbia the fraievork of the Hlk Coordinated Research Prograste on theapplication of Isotope and Geocheaical techniques ID Geotberial Exploration in Latin tterica (Research Contract K'3988/IG), financially supported by the Governtent of Italy
REFERENCES
Cramer S D (1982) The solubility of methane carbone dioxide andoxygen in brines from 0 to 300°C U S Bureau of mines Report of in-vestigation N° 8706
D Amore F , Celati R and Cal ore C (1982) Fluid geochemistry applica-tions in reservoir engineering (vapor-dominated systems) Proc 8thWorshop Geothermal Res Eng , Stanford University, pp 295-307
D Amore F and Celati R (1983) Methodology for calculating steam qua-lity in geothermal reservoirs Geothermics, 12 pp 129-140
D Amore F and Gianelli G (1984) Mineral assemblages and oxygen andsulfur fugacities in natural water-rock interaction processes Geo-chim Cosmochim Acta 48, pp 857-857
D Amore F and Truesdell A H (1985) Calculation of geothermal reser-voir temperatures and steam fractions from gas compositions G R CSymp on Geothermal Energy Kona, Hawaii, Transactions, 9, Part 1, pp305-310
54
D'Amore F. (1987) Some geothermal techniques for reservoir temperaturecomputation. Istituto Internazionale Ricerche Geotermiche, Pisa, Ita-lia. International Symposium on the Development and Explotation ofGeothermal Resources. Cuernavaca. Morelos, Mexico.
D'Amore F., Fancelli fi. and Caboi K. (1987) Observations on the appli-cation of chemical geothermometers to some hydrothermal systems inSardinia. Geothermics, 16, N° 3 in press. 3994/IG, 55 pp.
D'Amore F. and Panichi C. (1987) Geochemistry in geothermal explora-tion. Applied Geothermics, Ed. by M. Economides and P. Ungemach, pp.69-89.
D'Amore F., Nuti S., Fancelli R., Michard J. . and Paces T. (1989) Re-cent methamorphic and hydrothermal fluids in Variscan structures inEurope. Proc. 6th In term. Symp. Hater-Rock Interation, Malvern, UK, inpress.
D'Amore F. and Truesdell A. H. (1989) A review of solubilities andequilibrium constants of gaseous species of geothermal interest.Sciences Géologiques.
EPEN-INGEIS (1987). Informe de avance Convenio OIE A 3988/IG.
Gianelli G., Passerini P. , Troisi C. and Zan L. (1989) Geothermal deepexploration in Djibouti: Stratigraphy, authigenic mineral assemblageand temperature data of the drilled wells. Structural data from theAsal rift. Geothermics. In press.
Giggembach W. F. (1980) Geothermal gas equilibria. Geochim.Cosmochim.Acta 44, N° 12, pp. 2021-2032.
Giggembach H. F. (1986) Graphical techniques for the evaluation ofwater/rock equilibration conditions by use of Na, K, Mg and Ca con-tents of discharge waters. Proc. 8th New Zealand Geothermal Horshop,PP. 37-43.
Gingins, Mario (1986) Estudio Geoquimico-Isotopico de los Fluidos Geo-termales de la Provincia del Neuquén (Convenio OIEA 3988/IG). EnteProvincial de Energia del Neuquén, Neuquén,Argentina.
Helgenson H. C., Delany J. M., Nesbitt H. ». and Bird D. K. (1978)Summary and critique of the thermodynamic properties of rock-formingminerals. Amer. Journal of Science, Vol. 278, A, pp. 729-804.
Henley R. M . , Truesdell A. H. and Barton P. B. , with a contribution ofWhitney J. A. (1984) Fluid Mineral Equilibria in Hidrothermal systems.Reviews in Economic Geology, 1. Published by the Society of EconomicGeologists.
JICA-COPADE (1982-1984) Proyecto de Desarrollo Geotérmico en la ZonaNorte de la Provincia del Neuquén-Domuyo.
Michard G., Ouzounion G. , Fouillac C. and Sarazin G. (1979) Contrôledes concentrations d'aluminum dissous dans les eaux thermales. Geo-chim.Cosmochim. Acta 43. pp. 147-156.
Michard G. and Roekens E. (1983) Modeling of the chemical compositionof alkaline hot waters. Geothermics, 12, pp. 161-169.
Panarello H. , Albero M. and Levin M. (1987) Informe preliminar de losresultados isotôpicos del Area de Domuyo. Convenio OIEA 3988/IG. Ins-titute de Geocronologîa y Geologia Isotôpica, Buenos Aires, Argentina.
55
Sierra J.L and Pedro G. (1987) Estudio Geoquimico e Isotopico de losFluîdos Geotermales de la Piovincia del Neuquen-Domuyo (Convenio OIEA3988/IG) Ente Provincial de Energôa del Neuquen, Neuquen, Argentina
Wolery T J (1979) Calculation of chemical equilibrium betweenaqueous solution and minerals the EQ3/6 software package LaurenceLivermore Laboratory
56
FLOW PATTERNS AT THE TUZGLE-TOCOMAR GEOTHERMALSYSTEM, SALTA-JUJUY, ARGENTINAAn isotopic and geochemical approach
H. PANARELLOInstitute de Geocronologia y Geologfa Isotöpica,Buenos Aires
J.L. SIERRA, G. PEDROEnte Provincial de Energia del Neuquén,Neuquén
Argentina
Resumen—Abstract
MODELO DE FLUJO EN EL CAMPO GEOTERMICO DE TUZGLE-TOCOMAR, SALTA-JUJUY,ARGENTINA: ESTUDIO ISOTOPICO Y GEOCHIMICO.
En este trabajo se han ernpleado técnicas isotôpicas (tritio.deuterio y oxigeno-181 asi como anâlisis quimicos de aniones y ca-tiones mayoritarios y minoritarios y anâlîsis de gases para establecerlas condiciones hidrogeolôgicas y las tempera taras de reservorio.
Los contenidos en isotopos estables exhiben una buena correlaciôncon la altitud y permiten diferenciar las areas posibles de recarga.
Debido a las condiciones climâticas altamente rigurosas del am-biente Puneno, como gran ampli tad térraica, baja humedad y près ion at-mosférica y escasa precipitaciôn, se producen importantes fracciona-mientos cinéticos fundamentalmente en oxigeno-18.
La termometria guîmica de sôlidos disueltos y gases, definen elarea como un sistema de baja a media entalpia con temperaturas de re-servorio del orden de 90 a 140°C.
FLOW PATTERNS AT THE TUZGLE-TOCOMAR GEOTHERMAL SYSTEM, SALTA-JUJUY, ARGEN-TINA: AN ISOTOPIC AND GEOCHEMICAL APPROACH.
Environiental isotopes '#, *ff and J'O, aajor and linor anions aad cations, and gas analysis have beenlade in order to establish hydrological conditions and reservoir tetperatures.Isotope contents of gaters, 2S, ISO shon good correlation sith altitude and alloy to differentiate the possiblerecharge areas.Due to the rigorous feather conditions prevailing at the high altitud in the Puna environtent i.e. great theraicamplitude, los precipitation, los huaidity and ataospheric pressure, gross kinetic fractionation aainly in oiygen-IS has been found.Chetical of dissolved solids and gas geotheraoieters define the area as a lov to lediui entalpy mth reservoirsteiperatures range fron 30 to 140'C.Circulation of cold and bot water shots a pattern vhere are recreated tost of cases of stable isotope fractiona-tion.
57
l. INTRODUCTION
The area under study is located in the border zone of theprovinces of Sal ta and Jujuy at the AW of the Argentine Republic Thegeographic coordinates are aproximately 24 S and 66 30 H The areacovered is ca 400 km2, where the thermal manifestations of El Tuzgle,Tocomar and Pompeya are located The higher picks, correspond to theTuzgle Volcano (5500 m) and to the Colorado creek (5220 m) The zoneis extremely arid and the basin have no drainage
The climate is particular, since that the average ctltitLide of thefield is more than 4500 m a s l Températures are very low all theyear long, showing a daily amplitude as big as 36°C
The main town is San Antonio de los Cobres, if ff m a s l distant25 Km from the geothermal field A 4 yeat record of the meteorologicalconditions for this town account for the following
Average annual temperature ti 8°CAverage annual atmospheric pressure 649 hPaHighest temperature 27°CLowest, temperature 16"CAverage annual precipitation 1Ü3 mmFreezing frequence 223 day/year
In the géothermie area studied 800 m above, the climatic conditions are by far more rigurous Hinds prevailing fiom H-NW and W-SWhave a velocity between 2-20 m/s, blowing during 10 to 18 hours perday
ifrom the geological point of view the region presents metamorphicand cristaline rocks from the Proterozoic Eruptive rock with greatsignificance have been originated in four principal phases, i eDacitic eruption of the Upper Miocene (old dacites), eruption ofandésites during the early to medium Pliocene, early Pleistocenedacites (new dacites) and finally, eruption of basalts in the Boloce-ne All this metamorphic cristaline sedimentary and eruptive rockswere affected by tectonic movements Tertiary diastrofism of greatintensity, have masked the less important effects of other precedentdiastrophic cycles
2. METHODOLOGY
The samples for this work were collected at an area of about 400km2 comprising all the thermal manifestation of Tuzgle, Toc^nar andPompeya (map 1) on October, 1989 and January 1990 9 cold waters and17 hot water samples 8 corresponding to hi Tuzgle, 6 to Tocomar, 3 toPompeya and 7 samples from creeks
pH and conductivity were determinated at field Fe, Mn, Al Na +,K+, Ca2 + , Mg2 + , Li + . As. NH3 , S = , S04
::. C03=. HCO3~, Cl~, F- BandSiC>2 at the Administracion Provincial del Agua (APA) and tritium,deuterium and oxygen-18 at the Institute de Geocronologia y GeologiaIsotöpica (INGEIS) (Tables 1, 2, 3 and 5)
58
Toeomar
TOP 3 (4262. I SO) »r? «TOFt(4324.13B)
MAP 1
59
TRBLE N° 1: CHErilCHL RNftLVSIS OF UfiTER SRHPLES FROH FUZGLE.
T C°O
coNDUcr.pH
HC03
Cl
501
F
Na
K
C*
Hg
Si 02
B
L»
R x
F»
lin
M2S
NH3
i ruc-i: 19: soo o: 6.1: 370
: 1110: us: 0.70i 900
: 13l: 101: is! 72
: 68: 13: 10; o. 17: 2.20: 0.22; i.io
: ruc-2: 56: 6630
: 6.2; 111: 2120; 101; 0.72
; 1230
: 111i 123
: 21: 71; ei: 17: 12: 0.22; 2.00; 0.22; 1.30
: ruc-3; 18.5l 3220
: 6.7; 110! 808
; 9i; 0.37: sso: 62: 13; 7.9: 72; 32; 7.10; 5.20
: i. 80l 0.57
; <o. i: 0.16
: ruc-1; 15l 3950
: 6.1: 100: mo; 89: 0.37! 690
: 75; 13; 11: 75: 11; 9.20; 3.90
: o. 10; 0.68
1 <0. 1
: 0.58
: TUC-S: 12: 6710
: 6. i126
l 2220
: 80: 0,10: 1260
; 137; lös; 22; 75: 83: 17: 13: 0.32: i.iol 0.29
: i.oo
: ruc-6: 39i 6170
: 6.0: 118: 20001 7l
: 0.15: mo; 123: 98; ig: 73i 72
: is: sl 0.23
; 1.30: <o. i: 0.92
: ruc-7: 11! 6300
: 6.1: 132: 2000: 63l 0.62
; 1120: 129; 88; 22: 75: 76: 16; 7.20! 0.32
: 1.10; <o. i: i.oo
: Tuc-8 :: 13 :: 592 :; 6.1 ;: 190 :; 73 :i 36 :: <0.20 :: 13 ;: 9.3 :; 6i :; 11 :; 10 :: 2.5 ;: 0.30 :; 0.08 :: <o.i :: <o.os :: :; ;
TU flr-
10
1250
7.9
311
1238
80
0.12
750
91
95
16
11
10
7.10
i: TU flr-2!
: 12 ;: 108 :: 6.1 :; 36 :: 7 . 1 :: 5.9 :; <o.20 ;: 9.5 :: 2.6 ;: 6.5 :: 2.2 i
! !
: o.si ;: 0.01 :: <o.02 :
•• '•
; ;
ruF-2 ; ruF-3 : TUF-I8 : ;
183 ; 329 : 122
6.7 l 7.6 ! 7.0
182 : 113 : so18 : 11 : 6.133 : 32 ; 11
<0.20 : <0.20 ! <0.20
31 I 12 i 7.2
6.3 1 2.3 ; 2.2
51 ! 39 ! 11
11 : 11 : 2.8; ;
1.7 ; 0.63 ; 0.13
0.17 : 0.01 : 0.030.08 : <o.02 : <o.o2
: '•; ;
' '
Corfr»n-fcr-»ti on* »r » in
TflBLE N° 2: CHEniCRL HNRLVSI5 OF UHTER SRMPLES FROH TOCOHRR.
T C°C>CONDUCT .
pHHC03ClS04F
N*KC*
ngSi028
LiR*F*n r,H2SNH3
! TOC-1 i; 35 :: 3680 :: 6.7 ;: 725 :: 808 :: 192 :: 1.40 :! 740 :: 65 :: 7.3 i: 4.2 :; 67 :: 42 :: 10 :: o.64 :: 0.12 :: <o.os :: 0.35 ;: 8.40 :
TOC-2 !45 i3640 !6.5 ;730 !828 i185 :
2.80 :730 ;64 :s.e :4.2 :67 :43 :10 :0.87 i
o.ii :<o.os ;0.67 !8.00 :
TOC-3 :62 :3640 !6.8 i740 !828 !187 i
2.40 !740 !66 :12 :3.7 :69 i44 ;10 :
0.79 :0.18 ;<o.os :0.55 :8.80 I
TOC-4 :57 :3640 !6.6 ;720 :828 :183 :1.50 i740 :66 :8.6 :3.7 ;66 :44 :10 i
0.64 :<o.i :<.o.os :0.67 :
8.80 !
TOC-5 :55 :3640 :6.6 :720 :828 :166 i
2.50 ;725 :
69 :12 :3.6 :6i :44 ;10 :0.50 :0.29 :<o.os :i.io :7.60 :
TOC-66033006.8700727176
2.30650615.85.655389
0.730.16<0.050.42
7.60
!TO flr-1!; 17 :; 1734 :: 7. s ;; 352 :; 339 ;: 104 :; i . 70 :; 342 :: 30 :i 17 :: 6.4 !: :: 20 :; 4.50 i: 0.44 !: :: i• '• '
PO flr-2!is :1190 :?.i :247 !218 !199 i1.20 1215 120 :28 :5.2 !
:13 :
2.80 :0.23 :
:'•'•'•
TO flr-3!14 !4970 !6.2 !965 !1162 :229 !i.oo :1125 :98 :4.4 :5.1 :
:64is :
1. 10 :i!;;
TO fir-4!0.5 !1013 :8.2 :321 :154 :52 :1.40 :175 :is :22 :18 :
:12 :
2.90 :2.30 !
:;:i
TOF-l ii104 :7.3 :44 :6. i ;5.9 :<o.20 :5.5 :2.4 :11 :1.8 :6.7 !
0.33 :0.03 :<o.o2 :0.13 :<o.os :<0.1 !
!
TOF-3 !11 !420 !8. l :197 :14 ;47 ;0.25 :27 ;4.2 :24 :24 :
:0.90 ;0.24 :<o.o2 ;
:::;
TOF-4 ! TOF-5: 5
222 ! 1047.2 ! 6.576 :11 :30<o.20 :16 ;2.8 :23 :2.9 :
:0.76 l0.07 !0.05 :
i:::
«r» in ng/1.
fflBLE N° 3: CHEniCflL HNfiLVSIS OF UflfER SRHPLES FROhPOHPEVR - niNfi BETTV - SEV
r c<»C3CONDUCT.
pH
HC03
Cl
S01
F
N*
K
C*
rigSi02
B
Li
fll
F»
Hn
H2S
NH3
: pool :so :
: SEID :: 6.5 :: lose :: 136« :: 25: :; 0.70 i: 1200 :; 109 ;: 8.9 :: 21 :; 11 ;: si :: 11 :: 7.30 i
: i.io :: 0.12 ;: 0.16 :! 2.60 :
POC-2 !
35 :6110 !
6.7 :1210 :1262 ;
298 I
0.90 :1350 :
128 :11 :25 ;16 :60 :13 ;
7.80 i
0.30 1
0.22 i
o. 16 :i.eo :
POC-3
52
5670
6.8
1120
1338
251
0.80
1187
111
11
25
11
51
12
9.20
<0.1
0. 15
0.55
1.30
; PO flr-i;; 19 :: 5950 ;: e . i ;: 1077 :; 1161 :: 278 ;: o.80 :: 1325 :
121 i
: 7.7 :: 26 :! 23 :
! 56 I
: 12 ;; 7.10 ;i <o. i :; <o.os :; ;
; ;
ne20
1307
6.7
162
317
31
<0.2
180
23
53
15
61
II
2. 10
0.56
<0.1
<0.05
<0. 1
<0. 1
: SEV; 12: 230
: 7.0: 57: 26: 28: <o.2i 15
1 1.6
: 22: 3.7i 9
: 1.8; <o.o2i <0.02
: <o. ii <0.05
i <0. 1
: <o. i
on* -»r» l n Mg/1 .
FfiBLE N° 1: GRS RNHLVSIS FROH
C02
H2S
N2
02
CH1
H2
H.
ruc-ii98.11
C<0.093
<0.0002C-3
1.11C80.9S3
0. 16C18.733
0.01CO. 285
<0.001CO. 00 13
0.01CO.O351
TUC-2
99.10C<0.093
NDC-3
0.16C68.713
0. 11C31.033
<0.022CO. 253
<0.001CO. 0025
<0.002CO. 01?
ruc-s95.80
«0.093
<0.0002C-3
3.99C87.683
0. 17Cll. 86)
0.0305CO. 11>
0.001CO. 00 13
0.0031co.osn
roc-3
99. 1C<0.093
0.035c-o
7.22C37.373
0. 10C7.573
0.53C5.013
<0.001CO. 0063
0.0012CÛ.O113
TOC-1
99.35C69.323
0. 16<->
0.31C21.993
0. 13C2.013
0.0201C3.593
0.001(0.08793
<0.002fO. 00973
roc-595.93
C<0.093
0.01C-3
3.51C82.25J
0.06Cll. 893
0.3733C5.813
<0.001CO. 0123
0.002CO. 0363
POC-1
91. 18'KO.og1
0.03C-3
5.56C91 .96)
0. 18t?. 573
0.0399CO. 123
<0.001CO. 0005)
O.OÛ2CO. 018)
POC-2
91.11C<0.093
0 . 005C-3
a. 61C93.S83
0.20C5. 753
0.011CO. 313
<0.001C<0.001)
0.0072C<0.063
62
TOBLE N° 5: ISOTOPIC ftNflLVSIS OF THERflflL fiND COLD UfiTERS.
! SflrtPLEJ
: Tuc-i: TUC-2: TUC-3: TUC-I! TUC-S: TUC-6: TUC-7!TU ftr-l: TU ftr-2: Toc-i: TOC-2: TOC-3: TOC-I: TOC-5: TOC-6!TO fir-lI: TO flr-2: TO fir-3!TO flr-1IPOC-1: POC-2: poc-3!PO flr-1: TUC-SÎTUF-2ÎTUF-3i TUF-1: MBJSEVi TOF-1! TOF-3! TOF-1: TOF-5
: LOCflTION !flLTITUDE!OKVGEN-l!3 ÏDEUTERIUH ! TRITIUMï ! H
: Turgl* !! Turgl* !! Turgl* !! Turgl« !! Turgl* !! Turgl* !! Turgl* !! Turgl v !! Turgl* !! TocoHar !î TocoMar !! TocoHar !! Tocowar- '
! Toconar !! Toconar !! TocoMar !! TocoHar !! TocoMar !
! TocoHar i! PoHp*ya !! Ponp*u.a !I PoHp#y» t
! Po«p.ya ii Turgl* !
! Turgl* !! Turgl* !! Turgl* !î Mi n* B*fc-ty !
:s*y :\ TocoM*r- !
• Tocoh**r- !
! TocoHar !! TocoHar !
a . s . 1 . .' (1190 !1190 :1190 ;1190 :1220 :1220 :1190 i1218 :1218 :1230 :1230 :1230 :1250 :1250 :1250 :
:::;
3838 i3836 :3838 !3838 !1160 !1160 :1261 :1700 :
:1025 !1701 ;1262 !1321 :1380 !
:X.3«X-0.2!-9.6 :-9.2 :
-10.2 ;-9.9 :-9.1 :-9.2 :-8.9 :-9.0 :-9.5 !
-lo.i :-10.3 ;-10.2 i-10.2 i-10.3 :-lo.o :-9.1 !-8.1 :-8.2 :-7.0 i
-lo.i :-10.2 :-10.6 :-9.1 :-9.8 :
-10.0 i-10.2 :-10.1 :-10.1 !-11.7 :-11.7 :-9.3 :-9.5 :
-10.7 :
«.> + /-! : CTU>»/-0.6-76 :-76 i 0.0-77 ! 0.0-77 !-77 ! 0.0-76 !-77 ! 0.0-72 :-77 :-81 :-81 :-81 :-si :-86 !-79 :-79 :-79 !-79 :-79 :-81 :-82 :-83 :-79 :-76 : 0.0
-76 : 0.6-75 ! 1.5-79 : 2.1-83 : 0.0-83 i 0.0-83 ! 2.1-76 : o.o-77 : 0.1-77 : 0.0
63
3. RESULTS AND DISCUSSION
3.1. PHYSICOCHEMICAL CHARACTERISTIC OF THE HATERS
3.1.1. TÜZGLE
There is chemical similitude between hot water samples TUC-1. 2,3, 4, b, 6, 7 snd the TU Ax - 1 creek obtained down stream the thermalmanifestations They are alkaline clorurated and can be classifiedaccording the salt content into two subgroups:
I. Conductivity between 5000 and more than 6700 uS/cm. sam-ples TUC-1 ,' 2, 5, 6 and 7
11. Conductivity between 3200 and 4250 uS/cm. samples TUG 3,4 and TU Ar-1
pH of samples belonging the subgroup I is slighty lower (6.0-6.2)than those of the subgroup II (6.4-7 9) The temperature of all hotwaters oscilate between 39 and 56°C. The other set of samples are ofcold water TUF 2, 3. 4 and the TUC-8. All they are alkalinebicarbonated, pH between 6.1 and 7.6 and conductivities between 329and 592 uS/cm, with exception of TUF-4 (122 uS/cm)
Finally the sample of the TU Ar-2 creek sampled upstreamthermal manifestations is bicarbonated aIka1ine-terrea1 alkaline.pH is 6.4 and the conductivity 108 uS/cm.
Concerning TUC-8, it was sampled and classified as thermal water,but latter due to its physicochemical caracteristics, was consideredlike a cold water with superficial circulation.
3.1.2. TOCOMAB
Hot waters at Tocomar TOC-1, 2, 3, 4 and 6 constitute an homoge-neous group from the chemical point, of view. They are sodium clorura-ted. Temperatures oscilate between 35 and 62°C and conductivites between 3300 and 3700 uS/cm. Their pH range from 6.b tc 6.8. Creeks sam-ples TO Ar-1, 2, 3 and 4 are the result of a mixing among thermal andcold waters.TO Ar-3 is still more concentrated than the thermal manifestations Itcan be explained as result of the process of evaporation, asdemostrated by the iso topic content of the samples (see forward)
Cold water samples TOF-1, 3, 4 and 5 are bicarbonated-terrealalkaline. Their conductivity oscilates between 5600 and 6450 uS/cmTemperatures range 35 to 52°C Sample of the creek PO Ar-1 is morealkaline (pH= 8.1), 5950 uS/cm and a temperature of 19°C
In addition two sampled waters at Mina Betty and Sey, have thefollowing characteristics:
Mina Betty: alkaline clorurated, high calcium and magnesium, pH~6.7, temperature 20°C and a conductivity of 1.3 uS/cm.
Sey: Ca -Mg-Bicarbona ted, pH- 7, T- 12"C and a conductivity of 230fi S/cm.
3.2. ISOTOPE HYDROLOGY
3.2.1. METEORIC LINE AND MIXING LINES.
With data of rain samples taken at Mina Providencia at similaraltitude and climatic conditions the local meteoric water line isobtained. Points fit on a straight line whose parameters are slightly
64
différents to those of the world meteoric water line defined bj- Craig(1961) They are
Ô2H- 7 8 Ô1&U + 13 o/oo
Samples falling closer this meteoric line are TOF-1 and SEY (figt> ) The remaining samples are located under the m l mainly due toevaporation phenomena after precipitation They fit lines with slopesof 3-6 characteristics of very dry zones
-6Delta Oxygen-18 (% )
4D
10
11
12
2D I
n(y-o oa)
3-t2-8
200 400 600 800 1000Chloride (ppm)
TOF 4- TOC * POC D TOAR
3D
1200
x TUF
1400
FIG 1 Chlonde-oxygen-18
Delta Deuterium (% )
-75
-77
-79
-81
-83
-85
-87
'à 28• XX
4 2n__ru
i ErmporvtloQ, (Y--0 08)
-ffZ-3-4
-f
200 400 600 BOO 1000 1200Chloride (ppm)
TOF + TOC * POC O TOAR < TUF
1400
FIG 2 Chloride-deuterium
65
-8.3 i———
-fl.8
-9 3 L',»2
-9.8 r-x'
-10 3,
-10.8
-11 3
-11.8
Delta Oxygen-18 (%.)
500
TOF
1000 1500 2000Chloride (ppm)
X TUF + TUC n TUAR
2500
FIG. 3. Chloride-oxygen-18.
Delta Deuterium (%.)
-72 0 -
-84.0500 1000 1500 2000
Chloride (ppm)
' TOF x TUF + TUC n TUAR
2500
FIG. 4. Chloride-deuterium.
66
Delta Deuterium (% )
-65 -
-12 -115 -11 -105 -10 -95Delta Oxygen-18 (% )
-9 -8 5
TOF X TUF WORLD METEORIC WATER
FIG. 5. Oxygen-18-<Jeuterium
Samples of hot Hater from El Tuzgle (fig 6), have a trendtowards more positive values of S^^O, however the limit between coldand hot waters is not sharp No oxygen-18 shift is observed due to therelative low reservoir temperature and to the kind of rock of matrixmainly siliceous
Samples at Tocomar and Pompeya (fig 7) are différents toof El Tuzgle with an apparent oxygen-18 shift
those
In the 6180 vs Cl - and O^H vs Cl- plots (fig I and 2)corresponding to the Tocomar-Pompeya zone, two différents groups onefor each zone is evidenced There are no evidences of mixing between affeothermal fluid and shallow waters, neither in Tocomar nor in Pompe-ya Only samples at the Tocomar creek and their tributaries allow tosee clearly that TO Ar-1 is the sum of the contributions of TO Ar-23, 4 and cold waters (fig 2) but they are separated from the sourcethermal manifestation due to a strong evaporation process This factis also evidenced in the Ô^H, ô i *0 plot (fig 7)
At the Tuzgle area a mixing line between a geothermal fluid asTUC 2, 6, 5 and 7 with shallow cold water (isotopically similar tothat of the creek) is evidenced Concerning samples of the TU Ar-2creek, this would correspond to a shallow circulation, while TU Ar-1to a mixing of TU Ar-2 type waters with a geothermal fluid The slopeof the mixing line is higher, since is repeated the phenomena descri-bed in Tocomar due to the evaporation of the creek With dashed linethe evolution due to this phenomena is indicated
67
Delta Deuterium (% )
-59 0
-640
--69 0
-740
-79 0
-84 0
-69 0-13
- -L _
-12 5
aX * «
• + Vx 3 4
-12 -115 -11 -105 -10DelU Oxygen-18 (7. )
lD
3 3-«
TOF TUF TUC
9 5
TUAR
B 5
FIG 6 Oxygen-18-deuterium
Delta Deuterium (% )
-60 0 -
-65 0 -
IJ D
*n
-85 0
-900' ——— L ——— L - ' —— ' *• ' i -t -i _i J — -i ——-13-125-12-115-11-105-10-95 - 9 - 8 5 - 8 - 7 5 - 7 6 5
Delta Oxygen- 18 (X.)
• TOF + TOC POC p TOAR > TUf
FIG 7 Oxygen-18-deuterium
J ^. 2 RECHARGE AREAS
In fiff 8 the 62H is plotted a fa in the dltitude of zechazge Ifsamples corresponding to Mina Betty and El S?y ?2~e not considered (thealtitude of the recharge is not veil known) the following equation j sobtained by linear regression a
62H- 1 47 x 10 2 h - 11 3 o/oo
It means a variation of about 1 b o/oo per 1 OU m altitude
68
Recharge altitude (m x 1000)
5.4 -
-88 -88 84 -82 -80 -78Delta Deuterium (%.)
• TOP x TUF
FIG. 8. Deuterium-altitude.
76 -74 -72 -70
In the case of the oxygen-18 vs altitude plot (fig. 9) also sam-ples TUF-4, TOF-3 and 4 have been excepted due to the evaporation thataffects mainly the oxygen-18 content.The resulting equation is:
Oi8O= - 0.30 x 10-2 h + 2.9 o/oo
With a variation of 0.3U ô 180/100 m.
As result of this correlations the following altitude of rechargewere determined:
62HTuzgle --------- 4400 mTocomar -------- 4900 mMina Betty ----- 4800 mSey ------------ 4800 m
01804500 m5000 m
Recharge altitude (m x 1000)
5.4 -
5.2
-12.5 -12 -11.5 -11 -10.5 -10 -9.5Delta Orygen-lB (%.)
• TOF x TUF
FIG. 9. Oxygen-18-altitude.
-9 -8.5 -8
69
Tritium (T.U. +/- 0.6)
3 r
2
l •
TOF-4TOr-3
TUF-4-rOF-l
TUF-3
TUF-2
TUC-8
ffrTUC-3 TUC-7
TUC-5
500 1000 1500Chloride (ppm)
2000 2500
FIG. 10. Chloride-tritium.
The altitude of the sampling point of Nina Betty suggest thatwater emerge from a point distant to the recharge zone The tritiumcontent T- O T.U. confirm the hypothesis. The same consideration isvalid for El Sey.
In the map 2, the probable recharge zone in accordance with theisotope data are located. In map 3 the possible recharge areas havinginto account the hidrology are indicated
3.3. AQÜEOÖS PHASE THERMOMETRY
In table 6 maximum, minimum and average reservoir temperatures ascomputed by différents geothermometers in liquid phase are showed. Thefollowing conclusions may be outlined:
- The Na-K geothermometers yield excesively high temperaturesbecause the equilibrium Na-Feldespar -K-Feldespar is not achieved.These thermometers are reliable only to temperatures over 150°C, notprobable in this case.
The Na-K-Ca is not applicable too because although more com-plete than the Na-K. also requière equilibrium between feldespars.
- Temperatures obtained from K-Mg geothermometer may be conside-red as reasonables. The K-Mg couple equilibrates faster and at lowertemperatures than the couple Na-K In addition the measured activitiesare not in correspondence with a high temperature system, beingcorrect the interpretation of such equilibrium.
- Temperatures yielded by the Na-Li geothermometer are ratherhigh. This fact is due to their calibration in granitic systems.
70
AREA 06 RKARGA PROBABLE PARA AGUA CALI6NTE DEL
AREA DE RKAR&A PROBABLE PARA ZONAS DE ' TOCOMAR r POMPEYA'
MAP 2
71
MLY PERMEABLE
MEDIANAKNTE
POCO PERMEABLE
MpfRMEAatE
FA A ERJPT A3ASAMEN 0 FCRMACON «CO C*
FORM AC ON PJNCC"i!SC^A
, BASAL O
FORMAL ON RINCHERA
ANCCSinS QACITAS Y SLBuPUPO P
FORMAC^^ CAUCH^PI
F RMAC ON PASTOS CHCOS
MAP 3
72
TABLE No. 6. GEOTHERMAL RESERVOIR TEMPERATURE BYCHEMICAL GEOTHERMOMETERS
GEOTERMOMETROSJ AC4
Na/K (F)
NA/K (T)
Na-K-CaNa-K-Ca (Mg)
Na-Li
178.9
140.9
177.5
161.6
148.1iK-Mg 131.2
Mg-Li
TQC
TQA
TCH
209.9
174.3
163.6
152.4
AC5
187.4
151.0
177.5
168.9
134.6
133.4
198.2
165.4
156.2
142.2
AC6
188.5
152.5
183.4
159.1
138.3
129.8
194.3
178.2
166.7
156.8
BAC 7
198.6
164.7
186.6
137.6
145.3
118.8
175.6
172.0
161.6
149.7
LO
173.3
134.2
176.0
172.8
141.0
151.1
242.7
177.5
166.2
156.0
LTI
176.0
137.4
178.7
174.2
138.6
149.8
236.1
181.6
169.5
160.8
LTI2
228.2
201.8
211.3
206.1
144.6
168.9
233.1
168.8
159.0
146.0
EHI
232.4
207.1
225.5
225.5
143.0
212.8
297.5
191.2
177.3
171.9
RP
175.9
136.7
159.7
29.7
227.7
101.2
151.1
149.6
143.8
124.3
Na/K (F): FournierNa/K (T): TruesdellNa-K-Ca (Mg): Correction for MgTQC: Quartz without vapour lossTCH: ChalcedonyThe temperature is expressed in °C.
- Silica geothermometers shows a probable equilibration withchalcedony for El Tuzgle and Tocomar, while that in Pompeya the equi-librium is controlled by the christobalite, if is considered that thewater produced by the well have a temperature similar to that of thereservoir.
3.4. GAS PHASE THERMOMETBY
The sami empirical geothermometer developed by Panichi, D'Amore(1981) is used. Its error is ±15°C and gave reasonable values athigh temperature geothermal manifestations. In addition, gas/waterratio (fas measured on thermal manifestations zones.
Pompeya: The calculated temperature is 71 ± 15°C and the maxi-mum possible is 91°C. The gas chemical composition is typical of lowenthalpy manifestation, and probably connected to the boundaries ofthe geothermal field.
73
Tocomar The geothermometer indicates a temperature of 139 ± 15°C The gas/water ratio decreases from west to east, in agreement withthe reservoir geometry Although, the gas/water ratio is low in allthe area along, in agreement with the calculated température
Tuzgle The calculated temperature are lower (80 °C) than thoseof Tocomar area In this zone, there are interferences with a shallowcold water acuifer The gas/water ratio increase from north to south
4 CONCLOSIONES
Nina Betty The origin of the thermal anomaly is a deep circula-tion, with a heat exchange between rock and water The calculated recharge altitude is 4800 m a s lIt is not completely explained the relatively high value of t80
Sey In this point, the sample has got an isotopic compositionsimilar to TOF-1 The chemical analysis indicates that it was notheated during its circulation The calculated recharge altitude is4800 m a s l , therefore the distance between the infiltration zoneand the discharge point is big
Tocomar Is a medium enthalpy geothermal reservoir, with acalculated deep temperature of 134~143°C and a low gas/water ratiothe fluid conductivity of 3300 3680 uS/cm The calculated rechargealtitude 5000 m a s l There are neither mixed geothermal fluids norshallow water detected
Tuzgle The conditions seems to be similar to those of the Tocomar area a calculated deep temperature of 132-142"C A low gas/waterratio but a higher fluid conductivity (up to 6 7 mS/cm) The rechargealtitude is 4500 m a s l The sampled water is a mixture between reservoir fluid and shallow cold water
Pompeya The sampled water is similar to the well water in thevicinity from a chemical and isotopical point of view In that way ispossible to say, they originate in the same reservoir with atemperature close to that measured at the well head (52"C)
ackaotledgefeats - Part of this sort has been perfoned mthin the fraiesorï of the làSî Coordinated Research Prograiteon tie Application of Isotope and Seochetical Techniques in Geotherial Exploration in Latin Aienca (Research Contract«' im/IG), financial!? supported b? the Coteraient of Italy
REFERENCES
1 Arias J (1979) Areas hidrotermales de las piovincias de Salta yJujuy V Convencion Anual Nacional de la frederacion Argentina deGeologos
2 D Amoie F and Panichi C (1980) Evaluation of deep temperatures ofhydrothermal systems by a new gas geothermometer Geochim cosmo-chim Acts vol 44, pp 549-556
3 D Amore F and Panichi C (1987) Geochemistry in geothermal explo-ration Applied Geothermics, Ed by M Economides and P Ungemach,pp 69-89
74
4. Hidroproyectos S.A. SETEC S. P L -CEP1C S C con la colaboracion deGeologia de Servicios S A (1985) Estudio de la Segunda Fase dePrefactibilidad Geotérmica del Area denowinada Tuzgle, Departamentode Susques.
5 International Atomic Energy Agency (1981) Stable isotope hydrologyDeuterium and oxigen-18 in the water cycle TRS N° 210, pp 103-142, 223-240. 241-271.
6. International Atomic Energy Agency (1983) Isotope techniques in thehydrogeological assessment of potential sites for the disposal ofhigh-level radiactive wastes. TRS N° 228, pp 44-57
7. Vilela C. (1969) Descripciôn geolôgica de la Hoja 6C, San Antoniode los Cobras, Provincias de Salta y Jujuy Direccion General deGeologia y Mineria, bole tin N° 110
75
INFORME GEOQUIMICO SOBRE LA ZONA GEOTERMICADE LACUNA COLORADA, BOLIVIA
G. SCANDIFFIOEnte Nazionale per l'Energia Elettrica,Pisa, Italia
M. ALVAREZUniversidad Mayor de San Simon,Cochabamba, Bolivia
Resumen-Abstract
INFORME GEOQUIMICO SOBRE LA ZONA GEOTERMICA DE LAGUNA COLORADA, BOLIVIA.
La zona geotérmica de Laguna Colorada esta situada en el extremo sur de
Bolivia, cerca de la frontera con Chile y con la Argentina.
Hasta la fecha se han perforado cinco pozos de sondeo: API, SM1, SM2,
SM3 y SM4; salvo el ultimo, todos los demâs son productives.
La composiciôn quiraica e isotopica de los fluidos obtenidos es casi idén-
tica a la del campo geotérmico de El Tatio (Chile), que se encuentra a poco
mas de 20 km de alli.
La verdadera diferencia entre los dos campos consiste en las actividades
de superficie: contrariamente a lo que ocurre en El Tatio, ninguno de los
caudales térmicos de Laguna Colorada muestra una penetraciön importante en la
fase liquida profunda; tan solo unos cuantos manantiales térmicos al borde
del salar de Challviri, a unos 10 km al SE de los pozos, tienen una ligerisimacontaminaciön. Sin embargo, hay varias zonas con entrada de vapor y gas deri-
vada de la separacion en la fase profunda: estanques calientes y fumarolas
forman sistemas irapresionantes que se localizan cerca de los pozos SM1, SM2 y
SM3 (zona de Sol de Mariana), y a 10 km al 0 de los pozos (zonas de Huaylla Ja-
ra y Aguita Brava).
GEOCHEMICAL REPORT ON THE LAGUNA COLORADA GEOTHERMAL AREA, BOLIVIA.
The geothermal area of Laguna Colorada is located at the
southern end of Bolivia, near the border with Chile and Argentina.
To date five boreholes have been drilled: API, SM1, SM2, SM3
and SM4; except the last one all of them are productive.
77
Chemistry and isotopic composition of the produced fluids arealmost identical to those of the geothermal field of El Tatio(Chile), just over 20 km away from there.
The real difference between the two fields regards surfacemanifestations: otherwise than at El Tatio, no thermal dischargeof Laguna Colorada shows a substancial inflow of deep liquid
phase; only a few thermal springs at the edge of Salär deChallviri, some 10 km to the SE of the wells, have a very littlecontamination. Several are instead the zones with an inflow ofsteam and gas derived from phase separation at depth: impressivesystems formed by hot pools and fumaroles occur in theneighbourhood of SM1, SM2 and SM3 wells (Sol de Manana area) , and10 km to the W of the wells (Huaylla Jara and Aguita Brava areas).
1. Lineamiento historico della exploraciön geotermica enBolivia
Como todos los paises de la cordillera Andina, Bolivia poséecondiciones geolögicas muy favorables para la explotaciön defuentes geotérmicas para generar energia. El gobierno de Bolivia,tomando en cuenta tal potencial, promoviö las actividades deexploraciön desde 1974.
Casi todos éstos estudios fueron llevados acabo con cooperaciöninternacional por expertos en Geotermia, en 1976 el gobierno deItalia enviö una firma de consultores Italianos para un estudio dereconocimiento de las areas al oeste de la codillera Andina,realizado por el Programa de las Naciones Unidas para elDesarrollo (U.N.D.P ) dentro el proyecto BOL/71/532.
Posteriormente fue hizo un estudio de prefactibilidad en dosareas seleccionadas en base a los estudios de reconocimiento:Salar de Empexa y Laguna Colorada/Salar de Challviri distantes unade la otra aproximadamente 150 Km.
78
Como una consecuencia de los resultados favorables obtenidos yen base a consideraciones econömicas se escogiö Laguna Coloradacomo area para empezar la factibilidad del proyecto. El estudio defactibilidad fué después ejecutado en el arnbito del ProyectoBOL/84007 [1] de las Naciones Unidas entre los anos 1985 - 1990.
Todas las informaciones de caractér geolögico que se presentanen continuaciön han sido tomadas del informe citato.
1.1. Desarrollo geolögico de la zona en estudio
El area del proyecto forma parte de un arco volcânico de edadmioceno-pleistocénica y esta situada en la parte méridional deBolivia cerca del limite con Chile.
La morfologia es tipicamente volcânica, caracterizada porgrandes extensiones ignimbriticas, que han nivelado las asperezasmorfolögicas, y sucesivamente modelada por fenömenos de érosionglacial.
Las estructuras prédominantes en escala régional tienen dosdirecciones principales: una aproximadamente N-S paralela a lacadena y una NO-SE que muestra un componente transcurrenteizquierdo.
La evoluciön geolögica de la zona puede esquematizarse de lasiguiente manera:
- en el Plioceno, probablemente por encima de plataformasignimbriticas, estaban présentes unos aparatos volcânicos conactividad escencialmente efusiva que emitian productos decomposiciôn dacitica y andesitica;- en el Pleistoceno inferior el ârea examinada fué cubierta
por grandes volumenes de ignimbritas de composiciôn dacitica yandesitica que nivelaron la morfologia preexistente;
después de las erupciones ignimbriticas la actividadvolcânica continua hasta el présente con edificios situados alo largo del eje N-S de la cadena. La actividad es mixta pero
79
prevalece aquella de tipo efusivo de composiciön andesitico-dacitica;- en la parte mas elevada del area son evidentes las huellas
de una glaciaciön post-ignimbritica occurrida probablementehace 35000 y 16000 anos;
- en el periodo post-glacial se mantiene una actividad nco-tectônica evidenciada por fallas activas que afectan a lasplataformas ignimbriticas y que permiten el ascenso de losfluides calientes que alimentan las varias manilestaciönestermales de la zona.Todos los datos geolögicos, vulcanolôgicos y estructurales, y
en particular: 1) la persistencia de la actividad volcânica por unlargo periodo con pocas variaciônes de la composiciön de losproductos emitidos, 2) el gran volümen de las igniinbritas y 3) lascaracteristicas quimicas y petrogrâficas de los productossubactuales similares a las de los productos mas antiguos,sugieren la existencia de câmaras magmaticas volumétricamenteimportantes situadas por debajo del eje volcânico activo, y por lotanto, la persistencia de una importante fuente de calorlocalizada inmediatamente al Oeste de la zona de lasmanifestaciones.
En lo que se refiere al reservorio geotérmico, este seencuentra en formaciones volcânicas profundas como ha sidoconfirmado por el pozo SM1 .
2. Muestreo, determinaciones de campo y de laboratorio
2.1 Generalidades
Los datos que se refieren a las manifestaciones superficialesmuestreadas en 1986 provienen de] Informe Final sobre Exploraciönde Superficie realizado para el proyecto BOL 84/007 [1], y delreporte de la IAEA [2]. También los datos de 1989 relatives a las
80
manifestaciones fueron obtenidos en el ambito del programa de laIAEA, el mismo que permitiö todas las determinaciones isotöpicasde las muestras de los pozos.
Estân actualmente disponibles, ademâs de las informacionessobre las manifestaciones y drenaje superficial, los datos de lospozos API, SM1 y SM2. Los resultados incluyen anâlisis realizadosen el campo o en laboratorio de muestras de vapor condensado,fluido total, liquido en el vertedero, adémas de dos muestras defondo pozo.
El estudio de los datos quimicos de Laguna Colorada tuvo elobjeto de définir de manera preliminar las caracteristicas de losfluidos geotérmicos asi como de proponer algunas hipôtesis sobrela extension del reservorio y los procesos de circulaciôn yprocedencia de los fluidos profundos.
La ubicaciön de los pozos y manifestaciones superficiales esreportada en la fig. 1. La fig. 2 muestra el esquema de aparato decabezal de pozo con los puntos de muestreo.
2.2. Metodologia de muestreo y anâlisis
Los liquides fueron recogidos en 6 distintas muestras paraanalizar los prinipales costituentes y aquellos in traza, aluminomonomerico, silica, isotopos stables ( O y D) y tritio.
Se realizaron en el campo las determinaciones de temperatura,Eh, Ph, condutividad eléctrica, alcalinidad y âcido suifihidrico.
Las muestras de fluidos seleccionados fueron analizadas en loslaboratories de ENEL (Ente Nazionale per l'Energia Elettrica)determinando los siguientes paramètres : pH, ConductividadEléctrica, Alcalinidad, Ça, Mg, K, Li, Rb, Cs, Sr, Fe Total, B, AsTotal, Zn Total, Pb Total, Alcalinidad, Sulfates, Cloruros,Fluoruros, Bromuros, Acido Bôrico, Silice.
81
// ) °*wa, jf^ - V^rX»-A^•v , ' _ ' - - > - ^O-i •~J>~. __>^v*
sfe^Mt^"1"^^
^ass*^ l ^- / 5>V V-" ^ ^<A-^ (" •=•- / / /OV^r h-<««,oJ^t^SHJf^^ -^k i S- ! ^J ^ f / <-^-V4°r^^U ' S^rtpx <V^ J / ^-i~ _,—— ^^>yA K_ v -, I \ cf««d-yiscACM£i«s /KT.,1 \i=^^s^^Ci ^^^ ^ 1 ^^~-Ci'->(/LT!X-—vi S i- I ^>Ro",<riCAMo«'>»;vO'raro^ .aa^fo^ -'KJ XT ,/nv S^l
^ {SM^^A1™ "i1 fHJ? i ^^^^^ fe^^sTir^
^««1^-^«
LODOS CALIENTES
SURGENTES
POZOS
SUPERFICIALES
rs sjT7 -Escala 1 250000
FIG 1 ' MAPA DE IDS PUNTOS DE HUESTREO
82
L E Y E N D A Boca pozoPunto de m u e s t r e o de f lu ido t o t a l y liquido de condensadodespues del separador po r ta t i l
Separador a t m o s f e r i c o
Punto de muestreo de liquide de veriedero
U e r t e d e r o
Fluido t o t a l y gas ( D e s p u e s desepa ra t i on con s e p a r a d o r p o r t a t i f )
à /(5)
Lit |u ido s e p a r a d o a c o n d i c i o / i e s a tmos lencas ( V e r t e c l e r o )
F i g . 2 Esquema de los puntos de muestreo
En lo que se refiere a los componentes isotöpicos fueronanalizados en su totalidad en Viena por la Agenda Internacionalde Energia Atömica a través del programa que se habia mencionadoanteriormente.
En las tablas 1-5 estän reportados los datos de las muestrasde agua consideradas, en las cuales se presentan general-mente desviaciones porcentuales < 10 %.
En la tabla 6 estân reportados los datos de las muestras degases.
Text cont. on p. 93.
83
oo-t-Tabla 1 - Analisis quimico de los pozos AP1.SM1, y SM2(mg/l)
T.M. = ST (vertedero), = FT (fluido total)T.M. = UB (en pozo), = C (condensado)
Muestra
AP1AP1AP1SM1SM1SH1SM2SM2SM2AP1SM1SM2
Muestra
AP1AP1AP1SM1SM1SM1SM2SH2SM2AP1SM1SM2
Fecha Cod.
20/12/88 1507/01/89 96504/01/89 95803/02/89 98015/10/89 292103/02/89 97923/06/89 169723/06/89 169513/10/89 292206/01/89 96303/02/89 97823/06/89 1696
Codigo
15 14965958 1980 32921 0979 01697 01695 32922 39639781696
T. M.
FTSTFTFTUBSTSTFTWBCCC
F
.00
.80
.90
.66
.00
.00
.90
.30
. T.c. pH.c.
°C
20.0 6.4174.5 7.2017.0 5.9518.5 7.32
80.5 0.0083.0 7.3520.0 7.00
14.5 5.4520.5 5.1813.0 5.15
H2S Li
5.10 8.2037.00
10.60 16.8015.70 48.50
12.3014.30 47.5016.70 53.0012.30 45.00
3.601.36 0.286.80 0.0910.20
Con.c.
MS
382017400710018300
175001930018100
15811245
Rb
1.50
2.908.301.80
8.200.410.04
Alc.c. 1
meq/l
0.60.80.80.5
0.50.50.5
0.60.30.2
Cs
3.10 53
5.40 415.90 25.20 11
22
14.90 11.20 7
90.20 3
5
r.l.
°c202020202020202020202020
NH4
.800
.110
.820
.040
.700
.680
.300
.890
.300
.110
.590
.110
pH.l.
6.186.786.896.736.276.756.896.756.706.646.485.57
Sr
0.48
0.993.702.20
3.500.120.02
Con.l.
MS
3500127006780163006910186001770016900157026125272
Ba
0.002
0.0800.3300.500
0.3200.0600.002
Alc.l.
meq/l
0.60.70.40.41.70.40.40.62.80.40.10.1
Zn
0.900
0.0230.0100.220
0.0190.004
Rlc.l. 1
meq/l
0.5 47,0.5 186,0.4 76,0.2 212,1.6 56,0.2 249,0.0 232,0.1 189,2.6 10,0.4 1,0.1 0.0.1 0.
Sb Pb
0.280 0.005
0.270 0.0000.630 0.0000.140 0.030
0.720 0.0000.020 0.0100.012 0.0000.210 0.0000.000 0.000
:a Mg Na K Cl S04
.0 0.140 770.0 128.0 1270.0 80.2
.0 0.075 3620.0 631.0 6500.0 168.0
.6 0.040 1640.0 269.0 2770.0 82.0
.0 0.060 4560.0 707.0 8400.0 30.0
.0 0.160 1390.0 180.0 2340.0 24.6
.0 0.079 5230.0 829.0 9560.0 35.5
.0 0.076 5230.0 925.0 9310.0 35.9
.0 0.050 4130.0 675.0 7830.0 30.7
.3 0.070 296.0 37.5 347.0 119.0
.6 0.006 28.5 4.2 43.3 4.4
.2 0.000 9.4 1.7 12.8 2.0
.0 0.000 3.3 0.0 5.5 0.0
As Fe Al. T Al. M TDS TAN
meq/l
5.80 1.200 0.096 0.000 2840 38.312569 187.0
3.30 0.680 0.060 0.000 5584 80.525.90 0.120 0.320 0.000 15689 247.00.72 4.400 1.500 0.720 536S 66.5
17724 270.017630 277.0
25.90 0.120 0.280 0.000 14596 227.01.20 0.350 2.800 2.400 1255 12.30.16 0.310 102 1.40.06 0.060 45 0.50.05 0.000 29 0.3
HC03
40.537.653.022.5106.824.97.4
27.6170.246.741.138.6
TCAT
meq/l
40.6188.084.7234.070.3268.0270.0213.015.32.00.70.4
Si 02
251.0617.0301.0665.01100.0686.0755.0678.0372.04.62.40.0
018
0/00
-9.69-5.94-8.82-6.39-7.27-5.53-5.55-6.26-6.88
-11.81-11.29-11.25
H3B03
245.0806.0394.0980.0231.01070.01070.0936.048.93.04.54.1
Deut.
o/oo
-83.5-67.8-83.3-79.5-79.6-72.8-74.9-78.1-74.5-99.4
-103.9-102.1
Br I
1.40 0.000.00 0.002.60 0.348.50 0.892.50 0.290.00 0.000.00 0.008.30 0.840.54 0.170.00 0.000.00 0.000.00 0.00
Trit. Prof.
U.T. m.
0.12
0.041000
1240
Tabla 2 - Analisis quimico de todos calientes y fumarolasT.M. = HP (lodos calientes), = F (fumarolas)
Muestn
HAB1HAB2HHJ2HHJ3HSMHSM3AHSM4HSH7HSH8FABFAB3FAB6FHJFHJ1FHJ2FSM1FSM2FSM5FSM6FSHAFSMBFSHDSMOSMO
3 Fecha
15/03/8615/03/8615/03/8616/02/8917/01/8913/03/8613/03/8613/03/8613/03/8615/01/8915/03/8615/03/8616/01/8912/03/8612/03/8613/03/8613/03/8613/03/8613/03/8617/01/8917/01/8917/01/8917/01/8913/03/86
Cod. T. M
867 HP868 HP14 HP996 HP988 HP10 HP11 HP12 HP13 HP999 F1002 F1001 F1000 F15 F16 F17 F18 F19 F20 F1001 F1002 F1003 F1004 F21 F
. T.c.
°c
79.081.082.970.081.553.684.067.273.215.084.584.417.084.684.684.584.584.784.712.020.019.016.0118
pH.c. (
4.583.573.142.443.042.253.386.572.165.10
5.53
4.104.805.004.30
:on.c. A
MS
1670265018303600201040301580841
3140250
108
416
1142
Ic.c. T.I. pH.l. Con. l. Ale. 1. Rlc.
meq/l "C fiS meq/l meq
23 4.09 159023 3.12 268020 3.22 183020 2.04 405020 2.40 2130
2.21
0.8 20 6.12 0.6
0.6 20 6.88 58 0.3
4.5 20 6.62 89 0.6
20 6.58 0.30.3 20 6.10 58 0.30.8 20 6.42 117 0.3
20 4.48 18 0.0
l. Ca
/l
3.48.20.021.644.510.831.01.11.61.4
0.3
0.80.20.10.2
Mg
1.0003.3000.0018.60013.90014.00018.4000.8000.0900.014
0.005
0.0130.0130.0020.005
Na
2.33.40.019.836.418.487.012.91.80.4
0.5
2.30.40.00.2
K
1.42.80.07.916.412.530.521.12.90.0
0.1
0.40.10.10.1
Cl
1.00.60.01.45.50.834.85.40.70.5
0.7
1.60.50.20.3
S04
572.01010.0
0.01530.0941.01740.0732.0276.0600.04.6
4.6
5.93.423.46.3
HC03
0.00.00.00.00.00.00.049.70.065.9
285.6
19.280.583.00.0
S i 02
43.3103.00.0
286.0300.00.00.00.00.00.2
0.1
3.91.71.00.8
H3B03
1.00.70.070.42.99.063.0598.0130.00.0
0.0
0.60.80.00.3
oo(J\
Tabla 2 - ( cont.)
Muestra Codigo H2S NH4 TDS TAN
meq/l
HAB1HAB2HHJ2HHJ3HSMHSM3AHSM4HSM7HSM8FABFAB3FAB6FHJFHJ1FHJ2FSM1FSM2FSM5FSM6FSMAFSMBFSMDSMOSMO
8678661499698810111213999100210011000151617181920100110021003100421
219.00284.000.01089.600109.00129.00125.009. 64054.400
29.10 7.600
15.10 13.600
373203514701935112292679245
40
11.21.0.
31.19.36.16.5.12.0.
0.
9009722954
5
TCAT
meq/l
12.516.60.07.8
11.410.014.61.83.30.50.00.00.8
018
o/oo
0.-0.-0.1.
-2.5.1.
3.-10,-9.
-12.
-8,
.78
.28
.60
.47
.27
.89
.27
.72
.36
.66
.78
.49-9.95-9.-9.
,32,14
-9.30
44.20 0.13029.10 8.2000.48 15.80027.20 0.630
60464236
0.0.0.0.
2252
0.20.50.90.1
-10.-8.
,38,79
-8.82-9.-9.-8.
,05,13,64
Deut. Trit. Altitud
o/oo U.T.
-42.-43.-42.-36.-46.-34.-37.
-37.-80.-77.-87.
-79.-86.-81.-80.-83.-90.-77.-78.-80.-80.-79.
0229 0.422 0.1301
8823 6.5
52986287217
s.n.m.
489048904760476048504850485048504850489048904890476047604760485048504850485048504850485048504850
Tabla 3 - Analisis quimico de las surgentes (mg/l)
Muestra Fecha Cod. T.M. T.c. pH.c. Con.c. Alc.c. T.l. pH.l. Con.l. Alc.l. Rlc.l. Ça Hg Na Cl S04 HC03 SÎ02 H3B03 Br I
LC1LC1LC11LC11LC4LC4LC5LC5SAB7SC1SC1SC11SC11SC2SC21SC21SC4SC4SHJ1SHJ2SHJ3SHJ4SPB1SPB1SPB2SPB3WP1
11/01/89 993 S18/07/86 1 S23/02/89 998 S17/03/86 7 S11/01/90 994 S18/07/86 2 S28/02/89 995 S18/07/86 3 S15/03/86 0869 S10/01/89 989 S18/07/86 0861 S10/01/89 991 S18/07/86 0865 S18/07/86 0862 S10/01/89 992 S18/07/86 0866 S18/07/86 0863 S10/01/89 990 S14/02/89 997 S17/03/86 4 S17/03/86 5 S17/03/86 6 S21/01/89 986 S16/03/86 0870 S16/03/86 0871 S16/03/86 0872 S22/01/89 983 S
°C
22.521.531.030.122.021.121.020.88.240.040.036.536.241.030.529.333.934.530.029.426.727.512.010.912.010.637.0
7.197.717.037.317.617.748.208.107.687.507.797.848.257.697.788.508.257.507.057.227.657.097.486.056.956.607.66
*3773811480
5124478209462641150114048549611804873767016504004401893758636213098374
meq/l
1.11.23.13.21.11.20.70.81.11.61.50.70.81.90.80.91.11.64.34.60.83.10.60.60.40.30.9
°C
202420202024202423202420232420232420202020202021212120
6.506.486.957.236.906.386.906.296.576.677.156.606.717.156.816.487.036.677.097.346.877.236.327.106.676.586.65
us
36239114701670523447903100025311201130484501
37037772767051750915843294988974370
meq/l
0.91.13.13.51.21.20.70.71.41.61.50.70.8
0.90.91.11.14.34.60.83.40.50.50.40.40.9
meq/l
0.91.13.1
1.11.20.70.71.41.41.50.70.8
0.90.91.11.14.2
0.50.50.40.40.9
8.4 3.6007.5 3.80037.1 21.40038.0 24.1009.4 5.5007.9 4.0006.8 4.2006.2 4.20019.0 8.00051.5 6.30054.7 6.70036.4 2.90036.6 2.90051.6 6.80016.8 0.48016.7 0.44029.8 2.90028.9 2.80043.3 24.30038.5 25.0004.8 2.55031.0 19.2007.2 2.0007.0 2.1006.8 1.7005.9 1.60022.2 2.900
63.456.0259.0246.099.466.9187.0172.016.8188.0160.066.860.0190.066.159.0102.0110.034.228.719.028.59.89.05.63.959.8
8.19.028.835.610.99.612.614.49.930.026.04.41.1
31.03.12.814.015.58.711.45.99.01.51.63.12.56.6
59.157.7337.0358.0106.071.2202.0201.03.6
160.0125.03.62.6
180.06.42.659.465.03.21.21.01.03.63.04.12.03.4
30.128.6145.0149.042.632.6104.0104.042.4267.0271.0190.0180.0250.0118.0113.0174.0175.056.450.969.568.213.312.915.012.6126.0
67.171.4189.2194.670.270.243.345.267.197.092.743.347.6112.951.356.165.397.0262.4280.145.8189.233.634.824.419.556.1
80.974.096.9116.080.976.089.882.052.0141.0133.060.770.0139.071.269.0107.0104.0116.0122.092.0116.021.531.847.841.283.5
4.45.120.220.96.55.615.115.11.224.219.91.92.230.02.92.910.810.20.71.61.61.91.11.51.61.26.9
0.00 0.000.00 0.000.27 0.000.00 0.000.00 0.000.00 0.000.00 0.000.18 0.000.00 0.000.17 0.000.14 0.000.00 0.000.00 0.000.00 0.000.00 0.000.00 0.000.00 0.000.00 0.000.00 0.000.00 0.000.00 0.000.00 0.000.00 0.000.00 0.000.00 0.000.00 0.000.00 0.00
oooc Tabla 3 - ( cont.)
Muestra
LC1LC1LC11LC11LC4LC4LC5LC5SAB7SC1SC1SC11senSC2SC21SC21SC4SC4SHJ1SHJ2SHJ3SHJ4SPB1SPB1SPB2SPB3WP1
Cod i go
9931998799429953086998908619910865086299208660863990997U56986087008710872983
F
0.000.420.280.220.000.440.000.340.000.630.640.260.260.660.220.210.520.440.000.000.280.160.000.210.000.000.25
Li
0.210.191.000.120.330.230.680.640.000.870.630.050.030.770.070.030.320.420.030.060.060.060.000.010.010.000.05
Rb
0.000.000.090.030.000.000.000.000.000.170.100.000.000.130.000.000.080.080.000.050.060.060.000.000.000.000.00
Cs NH4 Sr Ba Sb Pb
0.00 0.000 0.00 0.000 0.000 0.0000.00 0.011 0.08 0.000 0.000 0.0000.00 0.000 0.39 0.010 0.000 0.0000.00 0.011 1.50 0.018 0.000 0.0020.00 0.000 0.00 0.000 0.000 0.0000.21 0.011 0.08 0.000 0.000 0.0000.00 0.120 0.00 0.000 0.000 0.0000.00 0.012 0.11 0.000 0.000 0.0000.00 0.050 0.10 0.010 0.000 0.0000.26 0.000 0.34 0.020 0.000 0.0120.33 0.009 0.36 0.020 0.012 0.0000.00 0.000 0.08 0.000 0.000 0.0280.00 0.010 0.08 0.000 0.024 0.0000.33 O.OOÛ 0.34 0.020 0.000 0.0000.00 0.000 0.03 0.020 0.011 0.0000.00 0.008 0.02 0.020 0.032 0.0000.00 0.010 0.20 0.010 0.024 0.0000.00 0.000 0.20 0.000 0.000 0.0000.00 0.000 0.00 0.000 0.000 0.0000.00 0.007 0.14 0.000 0.000 0.0000.00 0.008 0.07 0.000 0.000 0.0000.00 0.008 1.30 0.000 0.000 0.0000.00 0.000 0.00 0.000 0.000 O.OOÛ0.00 0.015 0.04 0.000 0.000 0.0000.00 0.020 0.05 0.000 0.000 0.0000.00 0.010 0.04 0.000 0.000 0.0000.00 0.000 0.09 0.000 0.000 0.000
As
0.160.170.400.500.180.170.380.440.000.400.660.030.020.730.100.050.410.370.030.000.000.000.000.000.000.000.03
Fe
0.0000.0000.0000.0000.0000.0270.0000.0000.0470.0000.0260.0000.0000.0440.0000.0120.0000.0000.0000.0000.0000.0000.0000.2600.0000.0000.000
Al. T
0.000O.OOÛ0.0170.000O.OOÛ0.0350.0000.0000.0590.1500.0000.0280.0000.0000.0180.0400.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.000
TDS
326314
113711854323456666462209688924104039943373235676105495602434669410411090368
TAN
meq/l
3.43.415.616.45.03.88.68.62.1
11.710.74.84.6
12.13.53.46.47.15.65.72.24.50.90.90.80.63.7
T CAT
meq/l
3.73.4
15.815.55.63.99.28.62.6
12.211.05.14.7
12.33.83.56.66.95.95.51.44.61.01.00.80.74.1
018
o/oo
-12.55-12.38-10.48-10.57-12.42-12.48-11.51-11.36-9.09
-11.01-11.01-10.94-10.94
-10.94-10.96
-10.45
-11.35-10.51-11.85-12.06-11.57
-9.70
Deut.
o/oo
-95.9-93.4-76.6-74.6-94.6-92.7-85.8-85.5-67. Z-88.4-88.4-86.6-86.6
-83.8-82.7
-68.8
-75.5-70.5-94.3-91.4-89.7
-79.0
Trit.
U.T.
0.04
0.00
0.007.50
0.00
0.00
0.00
0.25
0.700.000.16
0.20
Altitud
s.n.m.
428042804280428042804280428042804890440044004400440044004400440044004400464046404620458047004700470047004860
pmo<aOJDin
gPJOsinOsininPJts-KlKlPJKlOOOOOOOOsOCDCDOO000OO0OOOOOOOOOOCDOOOOOPJOOCD0(->
000PJOOtoKlPJs4KlKlKlOOOOOOCDOS-OOOO0O0O00OOOCOOOOO0OOOOOOs0PJ•sTOO §PJ£OsO00•<fOtoinsOinKlN.oOCDOOOO00OOOOO0OOOCD0-KlO0OOOCDO0OsOO0000PJO03RO
O00PJsOfs.
f-in0inins*SOin00OOoooooooinoPJoo000ooco0ooinooCDooKlooPJ0PJPJ0o §PJsOOsPJPJsOin0inPJKls4-OOoooooo030Oooo00o00ooooooo0ooCDoooooKlKl0ooois4O
§PJOo'hs-
PJOs00-4PJOsKl03Klin-4KlinKlooN-PJoCDrs-00oooo000oooo03ooooPJ0oo0KlPJ0-4-OPJU
o00PJcoin00in.-PJOs00oooooooo00KlOOooo00ooo0ooooooPJooooooo$CDooomu
o00PJ00inin03Kl«-sOtoSUcosOoooooooo-4-s4-OOOO000o00ooooPJoo0ooooo0•stKl0Klinu
oOsco0inPJts-sOoOssOPJPJoPJPJOsinooS--4-oooo0ooo0o000ooooo0inooCD00oo0000ooooS-m
oos4-o.-PJPJN.$o-oinoooooo-40PJoooooooPJoo-4-Klo0oo0sOPJ0r*.0s.COoKlsuoo-00Osu
o0o0-48o<-ots-oPJ&oo00sOPJ0ooooo0PJ000PJoosOKloOS0ooKlKloooKlsu0ogoo
ooso^in03s*os403PJooooooKloo03PJo0o000oooo03oo0oooo00oo0in00sOPJo&ütn
oo-4o0-O?o(s-sa-sO•J-Klooooooooo(NJo00oooPJooooooCOoooo0o000o0Klo0sOPJoinsO03ou(n
ooKlPJPJioooo«4-ooKoIooooooPJooKlooo0oKlKloKl0r-fs-ooPJ$0PJotn
o0-4-coKlCOCD00KlinKl|s-KlKl000o000oooooo000oPJooKloo0oooCDoooooN-00PJPJoPJPJo
o0ooN-PJ00SUoo1inKlKlKlPJKl0-400PJ00inooooo0PJKl0ooPJooPJoo03ooo0o0oo0KlooPJooPJ
oosOsOin00000oo0oooo0vtPJ00ooooPJo0ooCD00coo0PJKloPJinoKlsOcoootn
0o-4-0-sON.0oCDoooooor*.Klo000oooooo00o0PJooo0ooootoooPJ0oo8;-4-(_)(/ï
03inPJo03inoOsinsOin•J-inooooooooKl00oooo0ooooo0ooooo0ooCDoooooKl00ooos.tn
03ininin0inoooo0ooo00oo0000CDo00oooo0oCDCDoCDinoosO000ooPJ-3tn
oPJooininrs-inKl«--4-PJPJKlPJooooooo000ooo000000oooors.ooco0ooo00sOo0sOo0coPJoinKltn
oCOin00in0in0sOinoo0ooooooo0o00ooooo000ooKlCO0oCDCDoo-0oosO00sOosO-3tn
oors-Klin03•-oOs0soooooooo000o00000o00oooooo00oCDooooo000000osO03Osmû_tn
oof-ooPJo0oooooosOPJ000CDooo00000oooo-4ooinoCD0oo0CD0ooPJo0co0DOr/ï
o0N-N.oCOs.in«~co000oooooooooooo0o00oCDooCD000oinoooPJ0ooooooo00oooÉ3o(\JCÛÛ-<f>
o0•S-0"O0oooooooooo00CD00oCDCDo0000oooo0CDCDooooo000CDooPJN.cooKlmQ_
o$oPJ00oN.oo«-S-COoroooooooooro00CD00o0CDo000oo0oo000oCDooooo00inPJocoo
89
Tabla 4 - Anal is is quimico de aguas supcrficiales (mg/l)T - M . = LA (lagunas), = R (rios), = N (nieve)
Muestra
LC12
RAB
RHJ
RS
SC10
SNOW
Muestra
LC12
RA8
RHJ
RS
SC10
SNOW
Fecha Cod. T. M.
17/03/86 8 LA22/01/89 985 R19/01/89 984 R
21/01/89 987 R18/07/86 0864 LA
/ / N
Codi go F Li
8 0.89 165.0
985 0.00 0.00
984 0.00 0.03987 0.00 0.150864 1.50 2.60
T . c . pH.c. Con.c. A l c . c .
°C MS meq/l
19.2 8.05 31.616.0 6.95 189 0.82.0 8.50 241 3.5
16.5 7.50 288 2.324.0 8.25 31600 2.7
Rb Cs NH4 Sr
12.80 21.30 0.000 0.000.00 0.00 0.200 0.00
0.00 0.00 0.210 0.000.00 0.00 0.130 0.001.60 3.40 0.900 8.80
T. l .
•c
2020202023
Ba
0.0840.0000.0000.0000.000
pH.l . Con. I. A le . I .
MS meq/ 1
8.42 200000 27.26.44 196 0.87.37 439 3.46.79 224 2.37.51 2.6
Zn Sb Pb
0.080 0.020 0.1000.000 0.000 0.0000.000 0.000 0.0000.000 0.000 0.0000.060 0.000 0.000
Rlc. l .
meq/l
0.83.42.32.1
As
76.000.010.030.133.50
Ca Mg Na K
276.0 669.00 41000 3580 5390017.8 6.100 14.3 2.8 233.9 19.800 33.8 9.0 215.2 7.200 30.7 3.0 4
336.0 174.00 5700.0 236.0 8780
Fe ALT TDS TAN T C A T
meq/ 1 meq/ 1
1.230 0.500 128486 2020 1970.00.000 0.000 208 2.0 2.10.000 0.000 467 5.1 5.0
0.000 0.000 267 2.6 2.80.230 0.460 17691 288.0 285.0
Cl S04 HC03
.0 22600 1909.8
.2 55,
.9 64,
.4 7,
.0 1790,
018
o/oo
-0 .51-10.22
-7.68
-12.10-8.04-8.00
.6 45.8
.6 210.5
.0 139.7
.0 162.3
Deut. Tri t
o/oo U. T
-28.1-75.5 0.-54.2 0.-96.1 0.
-68.3-47.4
S i 02 H3B03
93.0 4180.0
62.2 0.6
91.7 0.857.4 1.776.4 410.0
. Al t i tud
s. n. m.
42756 47605 4310
2 434046005000
Br
0.00
0.000.003.60
Tabla S - Analisis quimico de fluidos del Tatio (mg/l)T.M. = FT (fluido total), = S (surgentes)T.M. = HP (lodos calientes), = H (nieve)
Muestra
H28H324H347HC1HC3HC4S227S241S319S56S65SA4SA5SA7SA9SNOWW1U2W3W5
Fecha l.M.
01/11/68 HP01/10/69 HP01/10/69 HP01/11/68 HP01/11/68 HP01/11/68 HP01/11/68 S01/11/68 S01/11/68 S01/11/68 S01/11/68 S01/08/70 S01/08/70 S01/08/70 S01/08/70 S01/11/68 N01/12/69 FT01/06/70 FT01/10/70 FT01/03/71 FT
T.c.
°C
78.078.056.020.086.085.085.085.083.085.086.014.025.012.055.0
211227254212
pH.c.
0.000.000.000.000.000.000.000.000.000.000.000.000.000.000.00
6.806.806.606.70
T.I.
°C
151515151515151515152015151215
15151515
pH.l.
6.703.604.907.106.906.407.407.406.307.600.007.506.708.408.20
7.467.367.657.40
Ale. I.
meq/l
0.50.00.02.70.20.10.70.71.11.41.60.50.10.614.3
0.30.60.10.8
Ca
37372699452728027817123222544828
196197198219
Mg
0.000.000.000.000.000.000.300.000.000.007.200.000.000.000.00
0.810.541.520.80
Na
533228164415
4600432030003060288035652540
3340343025703760
K
18958151052052536715014541464
310431127519
Cl
000000
8220787053705380524053090760
5820602044906690
S04
1803452451422818038262246420565
47334434
HC03
28.10.01.2
164.715.33.7
45.243.964.787.3
31.74.9
37.2860.3
20.734.89.2
48.2
SÎ02
8811287601270
256280149207777643833188
293298173343
H3B03
10709727047267041102397
727727612858
Li Rb Cs
47 6.70 15.8045 6.40 14.9027 4.20 10.3027 2.10 9.9028 2.10 10.000 0.01 0.000 0.00 0.000 0.01 0.004 0.30 0.70
23 3.50 11.3028 4.80 12.3021 1.70 9.5032 5.60 13.10
NH4 TDS
404535392362474236
2.900 15106143819889992710061203592532547
1.400 107962.170 112210.000 82591.500 12527
018
-0.907.506.30-8.60-3.70-3.30-5.50-5.70-6.00-6.10-6.50-8.20-8.80-7.70-8.40-9.70-6.00-6.30-6.80-5.60
Deut.
-44.0-26.0-31.0-55.0-49.0-44.0-69.0-68.0-67.0-68.0-67.0-58.0-53.0-48.0-0.6
-55.0-73.0-78.0-75.0-72.0
Tabla 6 - A n a l i s i s quimico de gases, res = gases residuales1 = Chiodim and Cioni, 1989. 2 Giggenbach, 1989.
Muestra
FSMOFSHAFSMBFSMDFABFHJ
FSMOFSM2FSH3FSM5FSM6FSM9FAB3FAB5F H J 1FHJ2
SM1SM1
Cod
100410011002100309991000
2118
1920
1516
1968a1968b
Fecha
18/01/8917/01/8917/01/8917/01/8915/01/8911/01/89
13/03/8612/03/8613/03/8616/03/8616/03/8616/03/8615/03/8615/03/8612/03/8612/03/86
02/02/8902/02/89
C02%
93.9895.3891 .0896.5994.6395.34
96.0397.3597.4895.0195.9095.3696.4895.4297.6195.57
93.8096 91
H2S%
2.792.172.653.210.690.00
1.171.391.120.731.080.861.091.230.820.83
0 901 32
CH4%
0.831.001 .670.860.590.20
0.360.220.010.750.220.630.280.810 100.11
1 .331.67
H 2%
0.360.130.070.300.580.300.350.390.280.24
N2*
0 . 1 1 02.03 06.17 01 . 76 02.32 03 22 0
2.07 00.91 0.1 22 0.2.22 0.2.23 0.2.82 0.1 .71 0.2.12 0.1 00 0.1.23 0.
6.30 0.0.43 0.
02%
.0000 0
.0000 0
.0000 0
.0000 0
.0000 0
.0000 0
.0081 0
.0035 0.
.0100 0.
.0085 0.
.0091
.0120
.2800 0.
.01500030.0040
0000 0..0000 0.
co%
.00160
.00174
.00062
.00058
.00245
.00078
.00073
.00018
.00012
.00028
.00130
00238.00267
fH20%
99.1798.2198.1698.3797.9298.8698.6297.9097.7697.89
N2% 02% CH4% H2% He% Ar% T PC02 Tres res res res res res °C bar °C
1 1 2
73.2 0.0 13.70 12.30 0.025 1 . 1 1 0 24073.2 0.0 13.30 15.80 0.036 0.411 25074.1 0.0 18.60 6.91 0.029 0.330 22074.7 0.0 12.30 12.80 0.024 0.464 22078.9 17.2 0.62 1.52 0.006 0.000 30080.8 0.0 6.90 14.80 0.036 0.389 240
248 1 .07 290207 4.15 267201 5.30 244211 2.80 256
271 11 .93 310
267262
3. Pozos, reconstruction del fluido profundo y equilibraciôncon el reservorio
Los pozos productives SM1, SM2 y SM3, que no estan ubicadoslejos de las manifestaciones de Sol de Mariana, tienen unaprofundidad variable entre 1200 y 1500 m aproximadamente. Duranteel periodo de perforaciön ha sido reconocidos distintos ninelesproductivos con permeabilitad y temperatura diferentes. Latemperatura maxima encontrada ha sido de 244°C y en las prevuas deproduciôn la entalpia se ha mantenido siempre entre 1050 y 1060KJ/Kg. El pozo API profundo 1600 m, que se encuentra a unos 5 km aSO de los anteriores tiene una temperatura maxima de 260°C y unapermealidad mas reducida. Durante las prevuas la entalpia era, enprincipio, muy elevada, luego se establizö en 1380 KJ/Kg. Estoindica la existencia de fenomenos de ebulliciôn en las cercaniasde la zona de produciôn.
Tenendo en cuenta solamente los pozos SM1 y SM2, de los qualesexiste un numéro suficiente de datos (considerando que elreservorio es del tipo liquido dominante y que el fluido producidoes el resultado de un proceso isoentâlpico) con un balance demasa y energia del sistema puede ser calcolada la composiciontotal del fluido del reservorio y de esta el pH y el estado desaturaciôn del mismo, respecte a un grupo de minérales (Ver fig. 3
Y 4) .Para esta reconstrucciön fueron utilizados los valores medios
de los anâlisis de agua de vertedero, de vapor condensado y de gasseparado muestreados en el periodo enero-junio 1989; en el periodoconsiderado la entalpia del fluido producido no ha sufridovariaciones, manteniendose en el rango 1055 KJ/Kg (244°C).
93
Pozos SW —1 y SM —2 — Fluido de reaervorio
Carbon total:13.07 mmol/Kg
120 160 200Température
240 280 320
FIG. 3 INTERACCION AGUA-ROCA. LOG (SI) Vs TEMPERATURA(CON CONTENIDO DE CARBON TOTAL = 13 mg/1)
Pozos SM~1 y SM—2 — Fluido de reservorio
Ccrbon totol:80 mmol/Kg
160 200Température
FIG. 4 INTERACCION AGUA-ROCA. LOG (SI) Vs TEMPERATURA(CON CONTENIDO DE CARBON TOTAL = 80 mg/1)
94
Debido al tipo de équipe usado durante las pruebas los datosanaliticos disponibles se refieren a 2 tipos de muestras:
a) liquido en la salida del separador atmosférico funcionando a84° C y 0.56 bar (vertedero) , tales muestras ha sido indicadas enla tabla 1 con el codigo ST;
b) fluido total tornado en un punto del equipo que tiene unatemperatura de 123-132°C y presion de 2.2-2.7 bar, tales muestrasha sido indicadas en la tabla 1 con el codigo FT.
Para esto tipo de muestras fueron realizadas correciones,porque como consequencia de fenomenos de canalizaciön en laplanta, la fracciôn de agua muestrada resultaba major de aquellateoricamente possible aquellas condiciones de presion ytemperatura. Los anâlisis de gas (ver tabla 6) se refieren amuestras tomadas en el mismo punto.
El valor de la relaciön gas/vapor se refiere al mâximo medidoen el periodo considerado y es de 1.1 NL/kg.
La reconstrucciön efectuada utilizando un programa de câlculo,estudiado para este objeto, entrega como resultado per i pozzi SM1e SM2 a una temperatura de reservorio de 244°C, un fluido con unvalor de pH = 6.31 y una alcalinidad total de 1.69 meq/1.
La composicion total, (valores en mg/kg), esta representada enla siguiente tabla:
Pozo Condicion pH Ça Mg Na K Cl S04 HC03 H3B03 Si 02 TDS
SM1-, Reservorio 6.31 175 0.056 3807 579 6615 25 802 779 500 13350SM2J
La asociaciön mineralögica résultante a la temperatura dereservorio (Ver fig. 3 y 4), esta de acuerdo con aquellaobservada en las muestras de perforaciön.
Los minérales que llevan Calcio en soluciön (Calcita yAnortita) estân sobresaturados.
95
Si se acepta un modelo mineralögico del reservorio en el cualla calcita es un minéral secundario que se produce de la reacciondel tipo:
Anortita + CCu + H2O --•> CaCC>3 + "arcilla"el echo del desequilibrio con la calcita, para una temperatura
de la soluciön de 244°C, no es consistente.
Para llevar la calcita a un nivel de saturacicn compatible conel modelo es necesario anadir CCu al fluido de reservorio.
La cantidad necesaria résulta ser 80 mmol/kg, con una prcsiôn
parcial de CO2 en reservorio de 7.95 atm.
4. Quimica del aqua
4.1. Clasificaciön quimica de las muestras de agua
Los resultados ana]iticos de los fluidos muestreados durante lainvestigaciön en esta àrea son reportados en las tablas 1, 2, 3,4, 5,. La clasificaciön de las aguas fué cfectuada mediante
diagramas de correlaciön utilizando concentraciones de losconst ituyentes mas fundamentales: Ca, Mg, Na, K, HCO3 , SO4 y Cl.Durante la interpretaciön no fueron consideradas aquellassurgentes muestreadas en el ano 1986, quimicamente eran idc-nticasa las muestreadas en 19B9, (Ver tablas 2, 3, 4).
Fueron utilizadas sin embargo las que tenian distintascaracteristicas para comparar los datos de las muestras, 969 y2921 para el pozo Sol de Mariana! (SM1), de las muestras 1695 y2922 para el pozo Sol de Manana2 (SM2) y la mucstra 958 para el
Pozo Apachetal (API), (Ver tabla 1).En las figuras 5, 6, 7 son reportados los clâsicos diagramas de
Piper. En la figura 5 podemos observar que esta dividida en cuatrocuadrances, en el cuadrante Sud-Este encontramos aguas del tipoAlcalino-Cloruradas. En el cuadrante Nort-Oeste son del tipoAlcalino-Térrea-Bicarbonato, generalmente coligadas a circula-
96
Cl- 100
++(DZ
fPBl
feie
n-. Q)
IQ
- pozos
100 HC03- + S04— 0
FIG. 5 DIAGRAMA DE PIPER CON Cl~ COMO ION SEPARADO.
504 —— 100
+
4-
POZOS
100 HC03-FIG. 6 DIAGRAMA DE PIPER
+ Cl- 0CON 504 COMO ION SEPARADO.
97
Cl- + S04-- 100
+ra
100 HC03- + C03—
FIG. 7 DJAGRAMA DE PIPER.
clones superficiales, finalmente en el cuadrante Nort-Este deltipo Suifato-Calcica.
Como habiamos considerado las muestras colectadas del area en
estudio, 19 de ellas, las que se ubican en el primer cuadrante sondel tipo Alcalina-Clorurada, cuatro en el cuadrante Nort-Este yquedan ocho, ubicadas mas abajo en el cuadrante Nort-Oes'e (ver
fig. 5) .Todo esto mejor se visualiza en las fig. 6 y 7 los diagramas
fueron construidos de modo tal que el ion aislado fuese SO^ en una
de las figuras y Cl en la otra.Por comparaciôn de esos diagramas, très grupos de agua son bien
individualizados, (el primero de los cuales como se vera después
98
puede ser ulteriormente dividido en dos) las caracteriscas deellos se pueden resumir como sigue:
GRUPO « A » conformado por las muestras SHJ1, SJH2, SHJ4, RHJ,SAB7, SPB1, SPB2, SPB3, RS y RAB. Las très primeras son fuentestibias situadas al Nort-Oeste del rio Huaylla Jara (RHJ), emergende lava ignimbritas y son caracterizadas por la temperatura yconductividad variable en los rangos que se indican a continua-ciön: 27.5°C-30°C y 430/iS-520/iS. La muestra SAB7, 8.2°C y 250/iS,es ubicada al Sud- Este de las fumarolas de Aguita Brava, lassurgentes frias SPB1, SPB2 y SPB3 con conductividad deaproximadamente 100/xS ubicadas a 1.5-2 Km del Cerro Lagunitas.Finalmente las muestras RAB y RS aguas superficiales del RioAguita Brava y del Rio Silala, esta ultima esta ubicada muy lejosdel area en estudio. Todas las muestras de agua se puedenconsiderar como una mezcla de porcentajes variables de los trèscomponentes Bicarbonato-Alcalino Térreo, Sulfato-Alcalino Térreo yBicarbonato-Alcalino.
COTOO ya se habia afirmado, el grupo « A » se puede todaviasubdividir, porque las muestras RAB, SAB7, SPB1, SPB2 y SPB3tienen componentes Bicarbonato-Alcalino Térreos levemente masbajos y un componente Sulfato-Alcalino Térreo mayor que SHJ1,SHJ2, SHJ4, RHJ y SPB1.
La muestra del Rio Silala se caracteriza por un elevado compo-nente Bicarbonato-Alcalino.
GRUPO « B » esta formado por las muestras SHJ3, WP1, SC2, SC4,SC11 y SC21, estas tienen conductividad y temperatura en el rangode valores: 158-1130juS y 26.7-41°C.
Este grupo se diferencia del précédente por la falta total delcomponente Bicarbonato-Alcalino Térreo; quimicamente esprédominante el componente Sulfato-Alcalino-Alcalino Térreo conuna contribuciön de Sodio-Bicarbonato. En las muestras SCI, SC2, y
99
SC4 hay también una considerable cantidad de Sodio PotasioCloruro.
La muestra SHJ3 es una fuente tibia de modesta salinidad(158jiS) ubicada cerca de las otras caracterizadas con la mismasigla y discutida anteriormente en el grupo « A », mientras que lamuestra WP1 (30°C y 370/iS) proviene de un pocito profundo de 127m,esta ubicada a un Km al sud del pozo Sol de Manana.
Las otras cinco fuentes éstan ubicadas en la esquina Oeste dela Laguna Salada (area del Salar de Challviri) y se encuentranlocalizadas a un nivel mas alto de la laguna, excluyendo asi unprimer grado de contaminaciön directa.
De las numerosas fuentes, cuyo caudal de agua es de 100 1/seg ycuya conductividad y temperatura disminuyen progresivamente haciael Norte, solo las mas importantes fueron muestreadas.
Es interesante observar los datos de muestreo realizados enLaguna Salada, la temperatura es inversamente proporcional a losporcentajes del componente Suifato-Alcalino y directamenteproporcional al Alcalino-Clorurado, con la sola excepciön de SC11.(Ver fig. 8 y 9)
42
40-
38-
ô 36-
o
O)
32-
30-
28-
26-
SC2
SC 11(1989)
.SCI
SC11'SCII C989)
SC4 (1989)
• SO.
SC21(19891
SC21
20
FIG. 8
25—I—
30—i—
35—i—
40 50N Q - K - S 0 4 %
TEMPERATURA Vs. Na - K - SO,
100
42
40-
38-
36"
I 32
30-
28-
26-
.SCI
SC2
"SC 1(1989)
• SC 11 (1989)
.SC 4 (1989)
SC4
SC 21 (1989)
10 15 20 25N a - K - C l ( % )
30 35 4
FIG. 9 TEMPERATURA Vs. Na - K - Cl
GRUPO « C » Formado de LC1, LC4, LC5, LC11, LC12, SC10 y lasmuestras de los pozos. Este grupo de muestras es del tipoAlcalino-Clorurado se puede dividir en dos subgrupos: el primerocomprende las muestras LC12 (Laguna Colorada), SC10 (Salar deChallviri) y cuatro fuentes tibias LC1, LC4, LC5, LC11 localizadasalrededor de Laguna Colorada. La temperatura y la conductividad deestas ultimas varian respectivamente entre 28.1-30.1°C y 390-1670/zS.
El quimismo consiste principalmente en el componente Alcalino-Clorurado mayor al 50%, en las lagunas puede aumentar mas del 75%.El componente restante puede ser subdividido en Bicarbonato-Alcalino, y Sulfato-Alcalino Térreo. En alguna de estas muestrashay cierta interacciôn con las aguas de Laguna Colorada.
El segundo subgrupo, como ya se ha mencionado, comprendefluidos de pozos profundos gué, a excepciôn de SM2 (2922), tienenun componente Clorurado-Alcalino mayor del 95%. La muestra SM2(2922) que ha sido muestreada del pozo a una profundidad de 1240m, résulta mas compleja presentando una suma total de los doscomponentes Bicarbonato-Alcalino y Sulfato-Alcalino mayor del 15%.
101
4.2. Consideraciones générales de las caracteristicas del agua
La evaluaciön de la composicion quimica de las muestras tomadasa fines del 1986 ponen en evidencia que cada una de ellascorresponde a tipos de circuitos superficiales y ninguna amostrado mezcla con una fase liquida originada por la interacciôna altas temperaturas con un reservorio de naturaleza volcânica.
El calentamiento de las aguas discutidas en el pârrafo anteriorpodria explicarse por el flujo de vapor; en el ârea en estudioexisten numerosas manifestaciones localizadas a diverseskilometros unas de las otras (Sol de Manana, Huaylla Jara y AguitaBrava), originadas por fenömenos de ebulliciön.
La investigaciön de componentes trazas como Amoniaco y AcidoBörico excluyen esta hipôtesis. Por ejemplo las muestras SCI y SC2
(en el ârea de Salar de Challviri) tienen una temperatura de 30°Cmayor a la temperatura estacional promedio. Un flujo de vapor del9% en peso, a 200 "C, es la cantidad requerida para calentarconvectivamente el agua de 10° a 40°C. En ese caso se tiene quetomar en cuenta el contenido de NH3 en las "Steaming pools" y enlas fumarolas respectivamente. Un tal flujo de vapor produciria enSCI y SC2 una concentraciôn de NH3 500 veces mayor a los valoresobservados. Las mismas consideraciones son para el H3BO3, estemismo mecanismo deberia implicar concentraciones deaproximadamente 500 mg/1 contra los 30 mg/l de los actualmenteencontrados. Un proceso de este tipo necesariamente implica unaproporcionalidad directa entre la temperatura y el porcentaje desulfates (oxidaciön del H2S présente en el vapor), la fig. 9muestra que esta proporcionalidad existe solo con el componenteClorurado-Alcalino.
Por otra parte teniendo en cuenta la pequena extension de âreay la cantidad de agua caliente confluiente en la laguna, latemperatura de esos fluidos no puede ser explicada solo por el
102
mécanisme» de conduciön. La disponibilidad de los anâlisis delfluido de pozos a permitido aclarar mejor el origen de las fuentescalientes del Salar de Challviri; si se considéra el diagramatriangular Li, Rb, Cs (ver fig. 10), los puntos representativos deSCI y SC2 resultan extremadamente vecinos a los pozos profundes.
L E Y E N D Ao Muestras de L Colorado
û Muestras del Talio
i i i i i i i l : l l i l i l l l i M ri l i i i l l r M l l l l l i l l l l l i l i
Rb (mg/l) * 4FIG. 10 DIAGRAMA TRIANGULAR Li, Rb, Cs.
La presencia de éstos componentes en agua asi diluida no puedeser atribuida a procesos de interacciôn agua roca superficiales auna baja temperatura, se puede explicar unicamente porcontaminaciön del acuifero en cuestiön por parte del fluidoprofundo.
Un aporte del 3-4 % de un fluido con las mismas caracteristicasde aquellas encontradas en la perforaciôn profunda no solojustificaria el contenido de Cl, Li, Rb, y Cs encontrados en estasfuentes termales, sinö también un incremento en su temperatura de
103
aproximadamente 1CTC. Reconsiderando la Fig. 9 si se extrapola a
cero el components Clorurado-Alcalino, la temperatura teorica delacuifero resultaria aproximadamente 28°C, con un incrementorespecto a la temperatura estacional media de aproximadamente 15 a20°C. Esto es debido probablemente al mecanismo de conducion.
Para concluir también la temperatura de las otras fuentestermales que emergen alrededor de Laguna Colorada y Cerro HuayllaJarita deben ser ligadas a factores conductivos.
Las caracteristicas de las fuentes frias SPB1, SPB2 y SPB3muestreadas a déclive en el cerro Lagunitas son definitivamenteanomalas: estas ultimas, no obstante tienen una temperatura baja ymuestran cantidades de NH4 y H3BO3 mucho mayor de aquellas que sedeberian esperar razonablemente con respecto a los solidos totalesy podrian ser efectivamente contaminadas por un levé flujo devapor. Finalmente en la fig. 10 fueron indicados con triangulospequenos las muestras de pozos y manifestaciones del campo geo-termico del Tatio (Chile), la casi perfecta sobreposicion depuntos representativos, da una vez mas la estrecha similitud delos fluidos geotermicos originados en los dos campos distintos(Sol de Mariana y El Tatio) distantes entre si 20 Km.
5. Geotermometros
5.1. Consideraciones générales
Por lo que se menciono anteriormente no es posible hacerninguna evaluacion basada en la composicion quimica de los puntosde agua superficiales disponibles, porque ninguna de ellas fuereconocida como un fluido de origen profundo; tambien las muestrasSCI y SC2 son contaminadas seguramente a un nivel mas bajo y nopueden ser utilizadas para tal objeto.
En lo que se refiere a fluidos profundos la interpretacion essimplemente hecha usando el diagrama ternario de Giggenbach [3]
104
(ver Fig. 11) basado simultaneamente en dos funcionestermométricas Na/K y K/Mg.
L E Y E N D A
* Agua super f i c ia l
o Muestras de fluido total* Muestras de vertedero
<> Muestras de pozoa Muestras del Tatio
7_ Na/1000On
Curya de fullequi l ibr io
2 x Dilucio'n
10 * Dilucio'n
K/100 0
% Mg
Fig 11- Diagrama de Giggenbach Na, K, Mg
Los puntos representativos de pozos profundes del area Sol deManana se colocan todos sobre la isoterma t Na/K de 280°C., lamayor parte, ya sea muestras tomadas al vertedero (ST) o de fluidototal (FT) , se encuentran a la izquierda de la curva de "fullequilibrium". Una posible explicaciön es que durante la salida delfluido profundo se verifica un fenömeno de precipitaciön quereduce la concentracion inicial de Mg.
Las muestras tomadas en los pozos, SM1(1000 m) y SM2(1240 m)que respecto a la composiciön teörica del reservorio son masdiluidas (aproximadamente 2.2 y 10 veces), en este diagrama secolocan mas alto y en el caso del pozo SM2 también a la derecharespecto a la posicion que deberian tener si fueran diluidas con
105
agua destilada en la proporcion encontrada por los anâlisis. Todoesto nos hace considerar que los pozos presentan distintos nivelésde permeabilidad, caracterizados por fluidos con temperaturas ycomposiciones distintas; las muestras tomadas en condicionesestäticas son una mezcla, con relaciones distintas respecteaquellas muestreadas en el curso de la producciön.
La temperatura obtenida de la concentraciôn de SiO2 en elvertedero, corregida por la pérdida de vapor [4], résulta para SM1y SM2 respectivamente 249° y 258°C. Estos valores son cercanos alos medidos directamente en los pozos, mientras los obtenidos delas funciones geotermométricas iönicas t K/Na y K/Mg indican dehecho temperaturas superiores de 40-50°C.
La hipötesis mas probable es que el fluido producido en el areade Sol de Mariana proviene de horizontes mas profundos de lasfracturas productivas que se encontraron en la perforaciön.
La temperatura en la parte mas profunda del reservorio estadada por la relaciôn Na/K, reequilibrandose lentamente sodio ypotasio, mientras la temperatura en la zona donde se produceactualmente estaria por la concentraciôn de Sicu. La relaciôn K/Mgindica una temperatura mayor que la anterior, esto podriaseexplicar con un fenomeno de ebulliciôn y degasificaciôn en elreservorio.
Esta hipötesis se confirma:- por la presencia fisica de numerosas fumarolas y "hot pools"
en el ârea Sol de Manana cuya extension superficial estanecesariamente ligada a un gran fenomeno de separaciön de fases.
- por calcules teôricos, considerando que la Anortita es elünico minéral présente en el reservorio que proporciona calcio ensoluciôn. A la temperatura de 240°C, con la cantidad de calcioprésente en el reservorio (175 mg/1) la presiôn parcial de C02 que
106
détermina el equilibrio entre la Anortita y la Calcita es de 7.9atm, mientras que la calculada de los datos analiticos de losfluidos producidos en el pozo es de 1.37 atm.
La concentraciôn de SiCu queda en cambio inaltérable y esta deacuerdo con las medidas de temperatura efectuadas.
6. Geoquimica isotôpica
6.1. Reconstruction isotôpica del fluido de reservorio,generalidades y métodos
Como ya indicado anteriormente, durante las pruebas deproducciön no ha sido posible efectuar un muestreo representativodel condensado de los pozos. Esto obviamente, hizo imposible elcalcule de la composicion isotôpica del fluido en el reservorio,por simple balance de masa entre el vertedero y el condensado.Esta problemâtica fué afrontada utilizando el anâlisis del fluidototal (muestras FT) oportunamente corregido como ya hemos indicadoen el capitulo 3. La comparaciôn entre el contenido analitico deCloruro en el fluido total y el resultado teôrico calculado apartir de los cloruros présentes en la muestra del vertederoindican de hecho que las muestras fuéron tomadas a una fracciôn deagua mayor de aquellas teôricas.
Los resultados asi obtenidos fuéron controlados con los valorescalculados a través de la composicion isotôpica en el vertedero(ver fig. 2 punto 2) hipotetizando que el proceso en la producciönsea reconducible a dos mecanismos fundamentales: un fenômeno deseparaciôn de vapor que ocurre entre el reservorio y el boca pozo(asimilable a una continua pérdida de vapor) y a un proceso de"single stage steam loss" lo que opéra entre el boca pozo y elseparador atmosférico.
107
En la siguiente tabla aparecen los resultados obtenidos:METODO 1 METODO 2
POZO <518O 6 D <S180 6 D
SM1
SM2
MEDIA
-7.05
-6.85
-6.95
-81.7
-80.8
-80.9
-6.70
-6.77
-6.73
-77.3
-79.9
-77.6
Considerando la medida de valores calculados se verifica unadiferencia muy reducida, estos valores son ademâs estrechamentecercanos aquellos postulados por Giggenbach [5] para el vecinocampo geotérmico de El Tatio (618O = - 6.9, 5D= - 78).
Fueron escogidos como valores de referenda para el fluido dereservorio 618O = -6.9 y <5D = -78; sobre esta base se elaboraronlas consideraciones relativas a las manifestaciones superficiales.
En el caso del pozo Apachetal (API) no fué posible ap] icarestas metotologias porque el exceso de entalpia medido esta casiseguramente ligado a fenomenos de segregaciön de fase con o sincesiön de calor de parte del reservorio.
Esta circunstancia se traduce en el flujo en el pozo de unacierta cantidad de vapor, cuya composicion con los datosdisponibles no es évaluable. Tal mecanismo no obstante esconsistente con los valores de 18O y D los cuales en Apacheta 1(API) son resultados mucho mas negatives que los correspondientesa las muestras de los pozos SM1 y SM2.
6.1. Fuentes termales y frias
En la figura 12 son reportados los resultados analiticospertinentes a los isötopos, observamos variaciones en el contenidode <S18O y 6D las cuales no son fâcilmente explicables frente a laaltitud. Las variaciones parecen bastante reguläres con una
108
Agua residua despuesde «Single step poilmgenlre 2<0 y 85°C
L E Y E N D AVapor pnmano de aguade reservor'o pnmariadiluida con agua de tipoLW
Agua superficial ( Rios, Lagunas. Nieve)
Surgentes de aguas termales y f r iesVapor pnmaoo de aguadeno dtluid
Muestras de fumarolas corregidas por el fraccionamienten el curso de la condensacidnVapor pnmano de agua
de reservofio primanadiluida con agua de tipoR W Muestras de fumarolas sin correccion
Lodos calientcs Laguna Colorada
Aguas profundes
FIG. 12 DIAGRAMA <S18O Vs 5D EN AGUA.
tendencia hacia de valores mäs negativos en la direcciön N-E (estatendencia puede llegar también al Salar de Empexa).
No es fâcil entender la razön de ese desarrollo probablementeligado a fenömenos metereolögicos (debido a la masa de nubes queoriginan la precipitaciön). Esas ultimas pueden venir predomi-nantemente del Oceano Pacifico.
En particular considerando los datos pertinentes de HuayllaJara y Aguita Brava, nieve del Volcan Michina (N) y el Rio HuayllaJara (RHJ), los puntos muestreados, en absoluto los mas positivos,se unen con el punto D, que séria el agua meteorica localpostulada por Giggenbach para el campo geotermico del Tatio [5].
RAB , SHJ3, WP1, LC11 y de modo menor, SHJ1, SHJ4 y SAB7 tienenuna composicion mas negativa. Por el contenido de deuterio,pudiera ser considerada como una posible recarga de los camposgeotérmicos de Chile y Bolivia. La intersecciön de la paralela aleje de la absisa que pasa por el punto DW (composiciön isotöpicade los pozos) con la recta meteörica da el punto RW (agua derecarga).
Los puntos de agua muestreados al norte de Laguna Colorada RS,LC1, LC4, y aquellas mas al sud SPB1, SPB2 en proximidad al CerroLagunitas, resultan pues en absoluto mas negativas del area enteraen estudio.
Por lo que se refiere a las surgentes en torno a Laguna Saladay la muestra LC5 sus composiciones son intermedias a aquellas dosde los grupos anteriores.
No obstante los puntos de aguas frias, de origen local osublocal, son muy pocas, por lo tanto es dificil dar un cuadro dela distribuciön de los valores isotöpicos, haciendo asi una vezmas, complicada la explicaciön de las variaciones observadas.
La impresiön final es que las variaciones isotöpicas y quimicasno son vinculadas a las anteriores, son mayormente causadas pormecanismos metereolögicos.
110
Resumiendo, las variaciones de 18O y deuterio identifican unalinea local meteörica con la ordinaria pendiente de 8 pero con unaintersecciön menor de 10 probablemente 5. Ciertamente la falta decambios isotöpicos en las fuentes, sugiere que ninguno de esosfluidos reaccionan con el reservorio a alta temperatura.
6.2 Fumarolas, "steamed pools" y aguas profundas
El vapor de las "hots pools" en el lado derecho de la grâfica6O vs 6D del diagrama (ver fig. 12) , esta bien alineado a lolargo de una recta con una pendiente alrededor de 1.6; este valores el misrtio que caracteriza el campo de El Tatio [5].
En la intersecciön de esa linea con la linea de las aguasmeteöricas se obtiene el punto LW (local ground). Este punto, muycerca a los puntos N (nieve del Volcan Michina) y RHJ (Rio HuayllaJara) , puede ser considerado representativo de la composicionisotöpica de aguas freâticas en la zona donde estân présentesestas manifestaciones.
La fumarolas, cuyas composiciones fueron corregidas por causadel fraccionamiento durante la condensaciön, (solamente para lasmuestras tomadas en el area de Huaylla Jara y Sol de Mariana de1986 y para las muestras SMO tomadas en 1989), se encuentran a lolargo de la linea con una pendiente alrededor de 8, extendiéndose3 unidades 518O a la derecha, lejos de la linea meteoricainterceptada a +10.
El punto DW représenta la composicion media del liquido delreservorio que alimenta los pozos de Laguna Colorada, similar alque al présente alimenta el campo fronterizo de El Tatio. La rectaC représenta la composicion de los liquidos residuos que seoriginan en DW por "single stage boiling".
La ausencia de puntos representativos debajo de la linea C estade acuerdo con la falta de muestras representativas del liquidoprofundo en el area investigada.
111
Con la excepciön de FAB6 (que no ha sido corregida por elfraccionamiento durante la condensaciön), todas las fumarolasestân localizadas dentro la region comprendida entre las curvas A,B y B1 como se puede observar en la figura 12.
En las mismas curvas estân también incluidos todos los vaporesque teoricamente se pueden formar:
- por ebulliciön del liquido primario DW a temperaturas masbajas de 240°C por un enfriamiento conductivo (curva A).
- por ebulliciön de un liquido primario DW después de ladiluciôn con diferentes porcentajes de agua de recargarepresentadas por el punto RW (curva B).
- por ebulliciön de un liquido primario DW, sucesiva a ladiluciôn con diferentes porcentajes de agua freâtica localrepresentada por el punto LW (curva B1).
7. Geotermometria de los gases
Durante la producciön de los pozos SM1 y SM2 fueron tomadasmuestras de gas natural con modalidades distintas entre ellas(salida del separador atmosférico y muestreo de fluido bifase).Las muestras mas representativas fueron corregidas por lacontaminaciön de aire y sobre estas fueron aplicadas técnicasgeotermométricas. La composiciôn del gas producido por el pozo SM1esta presentada en la tabla 6; la Fracciön de Vapor calculada porel Flash de 244° a B5°C y la relaciön Gas/Vapor son respectiva-mente O.304 y 1.10 NL/Kg.
Desafortunadamente no existe medidas de H2 para el pozo enexamen, por lo tanto es posible aplicar solo el geotermometro queutiliza el CO de Giggenbach [6]; por lo que se refiere a lasfumarolas puede ser utilizado también el geotermometro deChiodini y Cioni [7], esto ultimo permite de obtener,contemporaneamente, la presiön de CO2 en el reservorio.
112
En la tabla 6 estan representados los resultados por el pozoSM1 (265°C) y las fumarolas.
Estos risultados, a pesar que entre los dos geotermometrosexiste una diferencia costante de 50°C aproximadamente, de todamanera indican que la temperatura y la presiön de CO2 mas elevadasse encuentran en la zona de Aguita Brava muy cercana al VolcanMicina. Este ultimo es, sin lugar a dudas, la fuente de calor masimportante de toda el area estudada.
Como se puede observar en la figura 13, en la que serepresentan los contenidos relatives de N2/ Ar y He, estasmuestras de gas, aun sendo contaminadas por una componentemeteorica, parecen indicar un origen de los gases mas crustal quemagmatica.
No/100IOOO
10He
30
FIG. 13 CONTENIDOS RELATIVOS DE N Ar y He.£1
113
REFERENCIAS
[1] ENEL - Estudio de factibilidad geotérmica en el area deLaguna Colorada, Naciones Unidas/PTCD, proyecto BOL84/007, 1986.
[2] ENEL - Final report of the ENEL technical mission toBolivia (Laguna Colorada), 10-16 March 1986.
[3] Gigghenbach, W.F. - Geothermal solute equilibria.Derivation of Na-K-Mg-Ca geoindicators. Geochimica etCosmochimica Acta, 52, 2749-2765, 1988.
[4] Fournier, R.O. and Potter, R.W. - A revised andexpanded silica quartz geothermometer. GeothermalResearch Council Bullettin, v. 11, 3-9, 1982.
[5] Gigghenbach, W.F. - The isotopic composition of watersfrom El Tatio geothermal field. Geochimica etCosmochimica Acta, V. 42, 979-988, 1978.
[6] Gigghenbach, W.F. and Goguel R.L. - Collection andanalysis of geothermal and volcanic waters and gasdischarges. DSIR Report n. CD2401, 1989.
[7] Chiodini G. and Cioni R. - Gas geobarometry forhydrothermal systems and its application to some Italiangeothermal areas. Applied Geochemistry, v. 4, 465-472,1989.
114
GEOCHEMICAL REPORT ON THE EMPEXAGEOTHERMAL AREA, BOLIVIA
G. SCANDIFFIOEnte Nazionale per 1'Energia Elettrica,Pisa, Italy
W. CASSISEmpresa Nacional de Electricidad,Cochabamba, Bolivia
Resumen-Abstract
INFORME GEOQUIMICO SOBRE LA ZONA GEOTERMICA DE EMPEXA, BOLIVIA.
La zona de Empexa esta situada cerca de la frontera con Chile, en la pro-
vincia de Daniel Campos, Departamento de Potosi, a aproximadamente 68°30' de
longitud y 20°22' de latitud. Se caracteriza la zona por la presencia de un
vasto salar en una cuenca volcànica a una altura de 3 717 m sobre el nivel del
mar. Muchas de las actividades térmicas tienen lugar en los limites de este
salar, pero las mas calientes se encuentran en una zona limitada; la cantidad
y la circulaciön de gases libres son muy reducidas.
Toda la parte sur de la zona es riquisima en azufre y âcido borico.
Existen diversas minas de las que se extraen grandes cantidades de azufre. La
presencia de estas materias y la intensa filtraciön de los depösitos salinos
evaporiticos, causada por el continuo retroceso del salar, hacen muy dificil
caracterizar las muestras de agua y calcular la composicion original del flui-
do termal profundo. La temperatura maxima calculable del depösito a juzgar
por las muestras de superficie, no deberia sobrepasar los 180°C.
GEOCHEMICAL REPORT ON THE EMPEXA GEOTHERMAL AREA, BOLIVIA.
The area of Empexa is situated near the border with Chile in
the province of Daniel Campos, Department of Potosi, at roughly
68°30 ' of longitude and 2 0 ° 2 2 ' of latitude. It is characterized by
the presence of a vast salar in a volcanic basin at an altitude of
3717 m above the sea level. Many of the thermal manifestations are
located at the edges of this salar, but the hottest are found in a
restricted area; the presence and the flowrate of free gases is
very limited.
115
All the south of the area is highly rich in sulphur and boricacid. There are various mines from which high quantities ofsulphur are extracted. These occurrences and the strong leachingof the saline evaporitic deposits, caused by the progressivereceding of the salar, make very diffucult to characterize thewater samples and to estimate the original composition of the deepthermal fluid. The maximum computable temperature in the reservoirfrom surface sample should not exceed 180°C.
1. Sample collection and field determinations
The sampling in the Rio Empexa Valley was carried out over aperiod of four days, from the 17th to the 20th of March 1986. Atotal of 18 samples of hot and cold springs were taken, 2 of freegas, 3 of residual gases collected from two water points and one
fumarole in the sulphur mine Mina conception.The limited amount of time did not allow the carrying out of a
complete survey. Some of the water points recorded in previoussurveys, conducted in 1976 and 1977, were not found [1], thelocation of these points was imprecise and the organization of thefield work did not permit an inventory of all the water pointspresent in this area and the collection of a suitable brine sampleof the salar.
The analysis of the fluids sampled during the 1986 survey areshown in tables 1 (waters) and 2 (gases) . During the elaborationof this report, the data from previous studies on water points notfound at the time of collection, have been also taken intoconsideration.
Samples location (fig- 1) defines an area of about 300 km2 ;the most significative manifestations are located on the southernedge of the salar. The flow rate of the water points at thehighest temperatures (t > 65°C) varies between 0.3 and 4 1/s for a
116
i =.--'-. ^_ CmCX ALTAM1KA Z ~
/°iM-*"?*^ r S1,1 . E«.ne,.w,i"' ~
=? v *'(-X--MM "CAUEJONl *f>t- .
~'SALAR DE EMPEXA
• = Surface water pointsA = Test holes•äfe- = Mina conception (fumarole)
Fig.l - Location map of Empexa area samples.
117
Table 1 - Empexa water samples analysess = spring, r = stream, w = well, b = brine
Sample
E01E02E03E04E05E 06E07E 08E 09E10E11E12E13EHE15E16E17E18E19E20E21E22E23E24E25E26E27E 28E29E30E31E32E33UE1WE2WE4UE5WEGSE135SEUOSEK1
; Date Cod. S. T
17/03/86 0841 S17/03/86 0840 S17/03/86 0839 S07/05/78 S17/03/86 0842 S29/04/78 S17/03/86 0843 S29/04/78 R01/05/78 S29/04/78 S10/05/78 S10/05/78 S20/03/86 0853 S20/03/86 0852 S07/05/78 S12/05/78 S07/05/78 U20/03/86 0854 S18/03/86 0848 S18/03/86 0846 S18/03/86 0847 S19/03/86 0849 S17/03/86 OS44 S18/03/86 0845 S04/05/78 S27/04/78 S19/03/86 0851 S27/04/76 S19/03/86 0850 S02/05/78 S02/05/78 S20/03/86 0855 S26/04/78 W01/01/78 U01/01/79 W01/01/79 W01/01/79 W01/01/79 W01/09/76 135 B01/09/76 140 801/09/76 141 B
. T. f.
°C
11.014.014.013.022.030.035.015.015.013.018.025.025.034.032.075.017.018.016.068.022.069.086.087.045.09.0
11.010.035.015.013.0
11.036.064.029.0
11123.020.020.020.0
pH. f.
7.187.867.256.503.344.304.854.156.805.707.506.706.606.616.801.700.907.044.505.905.705.758.277.675.807.407.086.706.758.207.50
3.807.508.406.906.807.607.007.007.00
Con. f. /
/'S
141520370690
119015200130007550
1510024908910
10500626061608080
495005770067304190
24500
6940655060704910
62159201550
1251960
124080008000540071005400
Uc.f.
meq/l
1.11.81.4
0.70.20.8
12.05.55.69.4
2.1
7.315.94.75.1
7.20.8
0.20.51.12.9
6.32.71.79.11.3
r. l.
•c
2525252525252525252525252323252525242325252325232525232523252524252525252525202020
pH. l.
6.246.606.66
3.50
4.78
6.996.84
6.785.816.736.736.588.728.110.000.006.510.006.500.000.004.220.000.000.000.000.000.007.007.007.00
Con. l. /
ßS
155463372
1160
12700
64507050
67Ï.J4560
26400431005410065806370
6420
1830
210
MC. l. Rlc.l . Ca Mg Na K Cl S04
meq/ 1 meq/ 1
0.8 0.8 12.5 6.00 8.9 5.6 7.4 30.31.0 1.0 62.0 11.60 24.7 6.8 7.6 196.01.7 1.7 40.0 11.80 20.4 9.8 14.4 97.4
56.3 12.80 30.1 12.0 13.0 194.0120.0 13.00 108.0 110.0 156.0 391.0965.0 131.00 2440.0 71.4 4750.0 1620.0930.0 107.00 2350.0 64.0 4580.0 1600.0518.0 56.70 1030.0 40.9 1890.0 1150.0696.0 187.00 2230.0 131.0 4300.0 1410.0186.0 26.30 136.0 6.8 121.0 682.0769.0 103.00 940.0 30.0 2960.0 179.0252.0 196.00 1780.0 89.7 2960.0 907.0
5.4 5.4 390.0 118.00 898.0 74.0 1440.0 1150.05.4 5.4 390.0 130.00 1030.0 84.0 1660.0 1140.0
385.0 161.00 1280.0 108.0 2040.0 1310.0753.0 405.00 6750.0 646.0 13500.0 36.4
1260.0 240.00 9000.0 913.0 18800.0 57.12.0 2.0 171.0 92.00 1110.0 67.0 1910.0 446.00.6 0.6 290.0 48.60 580.0 74.0 1160.0 589.07.0 6.8 83.0 203.00 5100.0 187.0 8100.0 956.0
15.4 15.2 509.0 405.00 8500.0 310.0 13600.0 2340.04.6 4.6 125.0 522.00 11100 412.0 17900.0 2020.04.4 3.8 3.0 0.25 1410.0 53.0 1800.0 338.04.9 4.5 6.6 0.17 1380.0 52.5 1760.0 326.0
7.4 2.80 1320.0 44.8 1700.0 342.0196.0 13.50 338.0 43.3 759.0 226.0
0.8 0.8 52.2 12.60 45 .0 20.8 27.2 202.00.0 0.0 266.0 16.90 486.0 59.2 1040.0 301.00.5 0.5 128.0 8.30 201.0 23.4 459.0 128.0
6.4 2.60 5.0 2.7 4.4 15.480.7 47.00 118.0 23.4 262.0 159.013.6 2.60 11.6 5.8 4.7 69.683.8 12.00 58.4 8.9 8.8 372.0
421.0 133.00 1060.0 121.0 1740.0 1430.0150.0 59.00 1380.0 74.0 1840.0 1150.0261.0 37.50 736.0 101.0 1280.0 672.0
28.1 2.90 1430.0 50.8 1740.0 399.0341.0 21.80 644.0 74.3 1380.0 437.0420.0 3000.0 60000 9200 131000 13400580.0 10000 90000 6100 200000 16300440.0 11000 85000 8500 190000 11000
HC03
0.067.1
107.486.60.00.00.00.0
2013.60.0
45.7732.2335.6341.7573.6
0.00.0
128.10.0
445.4970.2286.887.3
274.6444.8
45.60.0
12.930.563.5
176.30.00.0
380.755.3
103.7536.9
78.1170852074614034
S i 02
51.840.459.660.071.038.020.058.065.053.041.080.2
105.0112.0118.0563.0330.0
82.0105.0174.0146.0164.0244.0278.0400.0
80.078.67.0
118.038.078.072.0
108.0246.0397.0
90.0246.090.1
H3B03
4.52.43.64.07.6
39.638.025.043.525.413.537.841.045.050.2
792.01100.0
38.665.0
177.0280.0288.0120.0115.0164.032.09.0
41.923.03.4
60.93.28.5
54.974.274.2
124.074.1
Br l
0.00 0.000.00 0.000.00 0.00
0.15 0.00c1.40 0.42
0.42 0.310.49 0.31
0.90 0.312.00 0.543.20 0.844.60 1.004.40 0.772.40 0.972.40 0.84
0.00 0.00
0.74 0.42
Table 1 - ( cont.)
Sample
E01E02E03E04E05E 06E07E08E09E10E11E12E13EHE15E16E17E18E19E20E21E22E23E24E25E26E27E 28E29E30E31E32E33UE1WE 2WE 4WE5WECSE135SEKOSE141
Code
084108400839
08420843
08530852
0854084808460847084908440845
08510850
0855
135140141
F Li
0.00 0.000.00 0.000.00 0.00
0.44 0.141.40 4.20
0.75 2.000.76 2.40
0.00 2.200.22 2.700.96 13.401.70 21.400.67 25.002.60 6.002.60 6.?0
0.00 0.01
0.42 1.00
0.00 0.00
190.0260.0440.0
Rb
0.000.000.000.060.32
0.210.26
0.180.400.961.801.800.490.44
0.050.08
0.00
00000
00
0012111
0
0
0
Cs
.00
.00
.00
.00
.35
.00
.00
.00
.24
.30
.40
.60
.10
.40
.00
.22
.00
NH4
0.0000.0140.0070.0100.0180.0560.0680.0110.0060.0203.0170.0200.2000.0050.0050.2880.0000.0610.3000.3000.1000.5040.0560.0430.041O.OOU0.0410.0200.0120.0070.0060.0000.0010.0630.1670.1980.3240.054
Sr Ba Zn Sb As Fe AI. T Al. H
0.10 0.020 0.000 0.000 0.00 0.000 0.000 0.0000.29 0.010 0.000 0.000 0.00 0.000 0.000 0.0000.24 0.020 0.000 0.019 0.00 0.000 0.000 0.0000.36 0.010 0.046 0.020 0.00 0.025 4.200 2.0008.10 0.020 0.043 0.000 0.12 1.600 0.570 0.260
4.60 0.020 0.000 0.000 0.42 0.011 0.050 0.0004.80 0.020 0.000 0.000 0.50 0.010 0.050 0.000
1.60 0.020 0.007 0.000 0.26 0.000 0.000 0.0001.60 0.010 0.018 0.011 1.40 0.000 0.000 0.0002.70 0.210 0.033 0.056 4.70 0.012 0.070 0.0006.60 0.170 0.034 0.016 8.10 0.130 0.000 0.0002.70 0.130 0.230 0.023 4.30 0.180 0.080 0.0001.40 0.040 0.000 0.069 4.40 0.000 0.039 0.0202.60 0.190 0.000 0.070 4.10 0.000 0.039 0.020
0.34 0.020 0.000 .0.000 0.02 0.000 0.048 O.OUO
2.20 0.000 0.000 0.000 1.70 0.000 0.000 0.000
0.04 0.010 2.600 0.000 0.00 0.039 0.032 0.000
TDS TAM
meq/l
127 0.8352 5.4259 4.2384 5.8982 12.5
10055 168.09708 162.04769 77.39096 184.01236 17.65036 88.06315 114.04225 70.14601 76.25462 94.223446 382.031700 531.03919 65.32921 45.015009 256.026138 448.032573 552.03989 59.33937 61.03988 62.41688 26.9448 5.02218 35.81096 16.179 1.5832 13.6186 1.6660 8.05212 85.15125 76.83254 51.84030 66.23064 49.3
234510 4250344040 6320320380 5820
T CAT
meq/l
1.65.34.15.514.6
167.0159.076.4150.017.588.5108.070.177.190.9
381.0497.066.145.6247.0436.0543.062.961.759.226.76.1
37.316.40.813.61.67.9
81.274.350.765.148.7
3110.04920.04840.0
,5180o/oo
-14.82-14.75-14.01-15.30-14.94
-11.41-11.62
-14.73-13.68-12.96-8.45-12.52-13.06-12.77
-15.42-15.06
-9.16
SO Trit. 5180/S04 534S/S04 Alt.
o/oo U. T.
-117-113-108-118-114
-89-87
.2
.6 0.3
.8 0.2
.2
.4
.0
.9
-110.5 0.4-102-98-80-V6-99-100
-113-106
-85
.8
.7
.6
.8 0.3
.3 0.4
.3
.8 0.4
.8
.3
o/oo o/oo m.
425041004100380039003753900371037803760375037503750
9.30 7.60 375037503750375037503750
4.90 8.30 37503750
7.00 8.90 3750-1.20 9.10 3800
3800380038353900387039504010363040004270
Table 2 - Empexa gas samples analyses
TotalC02
E8aFEFE(Aquat. )
96.95.98.
307066
H
041
2S
.00
.00.11
H:
0.0.0.
gases (vol. %)>
000000001
CH4
0.0180.0040.001
N
3,0.0.
2
.60
.30
.20
He(ppm)
3.3415.0
CO(ppm)8.0
38.9
Residual gases (vol. %)
C02 H2S H2 CH4 N2 02+Ar NH3
E6bE8bFE
97.95.92.
501030
<0.05<0.054.20
<0<00
.005
.005 <
.034 <
CO. 005CO. 005CO. 005
2 .4.3.
106030
0.0.0.
582703
-0.060.08
total of roughly 11 l/s. The total flow rate of other less warmsprings, still having temperatures exceeding 30°C, is more than 401/s.
The emission of gases, associated principally with water pointsat the highest temperatures, is very weak; a fumarole (FE) ispresent in the Mina Conception with a temperature of 85.6°C.
Water samples were collected generally in 6 separate aliquotsfor analyses of major and trace constituents, monomeric aluminumsilica, stable isotopes (18O and D) and tritium.
Temperature, pH, conductivity and alcalinity were determined inthe field. Major and trace chemical costituents were analyzed inENEL laboratory in Italy; isotopes were analyzed in IAEA and DSIRlaboratories.
2) Previous studies
As mentioned above, in the period April-May 1977, a prefeasi-bility study was carried out by the Italian company Aguater [1],24 water points and some gases were sampled.
120
Subsequently, in 1978, some springs and fluids encounteredduring the drilling of 6 thermal gradient test holes were re-sampled. The temperatures could not be measured with exactitude,but at least in two cases they were above 100°C (WE3 = 121°, WE5 =115°) .
The geochemical interpretation of collected data was based onstatistical factors and on the individuation of leakage anomalies.No particularly favorable results emerged in this connection.
In the summer of 1976 a geological study was conducted [I], themost important results of which are summarized in the nextparagraphs.
2.1 Stratigraphy
The stratigraphie sequence in the area is made up of Miocene toRecent age formations, essentially volcanics or sediments ofvolcanic origin.
The lowest unit of the sequence is the "Quemez Formation" witha radiometric age of 16.1, 9.4 m.y. This is made up of numerousignimbritic units with an estimated maximum thickness of 300 m.
The base of the "Quemez Formation", not outcropping, isprobably made up of older volcanites (lower Miocene) of theFormation of Yoza which outcrops east of Rio Empexa.
At the summit of the "Quemez Formation", Pleistocene volcanitesoccur which are linked to the renewed andesitic volcanic activityduring the Quaternary period with differentiated dacitic andrhyolitic products.
In heteropia with the products of the Quaternarian volcanism,sedimentary lacustrine and fluvial series of a prevalently muddyand conglomerate nature with local peat intercalations arepresent.
121
Volcanics cover the entire period of the Quaternary glaciationsduring which they were intensively moulded. The related morainicdeposits are associated to the glacial phases.
The only post-glacial volcanites are found between the CerroMilluri and the Cerro Pilaya Khollu.
2.2 Tectonics
A compressive phase of the lower Pliocene (Quechua phase) hasweakly folded the "Quemez Formation". This was followed by atensional phase with the development of hörst and grabenstructures. The latter one gave origin to closed basins during theisostatic uplift of the erogenic chain and, as a result of theprogressive development of evaporitic processes, have become thepresent Salars (Empexa and Uyuni).
Two principal systems of regional fractures can be indi-viduated: the first and the oldest with W-NW, N-NE direction, andthe second, still active, with a NE, NW direction.
Locally, structures of vertical collapse connected to volcanicactivity may be individuated.
2.3 Volcanism and geothermal implications
The area of recent post-glacial volcanism is located betweenCerro Milluri and Abra de Napa. A number of acid volcanoes occurhere: C. Pico Loro, C. Cayti, C. Mucellcani. The volcanic productsof this area show an evident evolutive trend from andésites -»dacites -<• rhyolites in relation to a process of subsurfacedifferentiation caused by fractional crystallization. This seemsto suggest the presence of a magmatic body in the process ofcooling (the dating of an ignimbrite sample of the volcanites ofthe Cerro Pico Loro and Huallcani gave a value of 0.8 m.y.).
122
The magmatic chamber, of great dimensions, could be centred atthe swelling structures along the belt of thermal anomaly whichstretches from Abra de Napa to Fuente Towa, and which ischaracterized by springs and thermal manifestations.
As far as the geothermal model is concerned, distinct reservoirunits and covers cannot be clearly individuated. The outcroppingvolcanites present locally a fair degree of permeability, whichbecomes high at times, both of primary origin (pyroclastics andvolcanic breccias) and of secondary origin (fissures). Thelacustrine terrains and volcanic sediments, whenever present, actas an impermeable cover. Nevertheless, the same outcroppingvolcanites reveal quite a variable degree of permeability and, inmany cases, none at all as a result of self sealing because ofhydrothermal alteration processes.
3. Classification of water samples
The classification of waters was carried out by means ofcorrelation diagrams based on main chemical costituents. In theclassic formulation of the Piper diagram all samples are found inthe quadrant of the alkaline-earth sulphate ones and in thealkaline-chlorides ones. In order to obtain a betterdiscrimination, we have preferred to use two modified diagrams, asin figs. 2 and 3, which consider Cl and S04 respectively asseparate anions. The two diagrams differ only in the distributionof the points in the two upper quadrants.
Apart from Ell, a spring emerging west of the salar, coldwaters are situated between ten and a few hundred metres abovethe level of the salar. The maximum temperature of these samplesis 14 °C and the T.D.S. is less than 1000 mg/1. As shown in fig. 3and in table 1, most of these have a high sulphate content andsometimes acid pH.
123
0 Cl- 100 S04 —— 100
4-(D
100 HC03- + S04- 100H——————t-
nQJ
HC03- + Cl-
F i g . 2 - Piper diagram w i t h Cl as separated am on. F i g . 3 - P iper diagram wi th sulphate as separated anion
These characteristics are uncommon in freshwaters, but can beexplained locally by the presence, in the past, of intensehydrothermal activity. This activity led to the formation oflarge, intensely altered zones and to numerous sulphur depositswhich culminate in the Mina Conception. During circulation, theair-saturated shallow waters encounter these formations andsulphur is transformed by oxidation into sulphuric acid and thento sulphate.
All these surface samples belong to Group A, subdivided intosubgroups Al and A2 respectively located in the NW and NEquadrant of fig. 3. E5 (t = 22°C, T.D.S. = 980 mg/1) , a surfacewater slightly contaminated by thermal fluid, falls just below theNE quadrant.
The other water points, which make up Group B, are located inthe quadrant of alkaline-chloride waters. Normally these watersare associated with a deep origin, but the chemical composition ofmany of these, in the Empexa valley, may be explained with theleakage of evaporitic deposits. These deposits derive from theprogressive receding of the Empexa salar, as shown by the numerousconcretions of calcium and magnesium sulphates and carbonateslocated few kilometres away from the present edge defining itspast extension.
In fig.3 the position of the fluids originating from theoreticisochemical leaching of three brine samples of the salar (SE 135,SE 140 and SE 141) sampled in 1976 for a mining study [2], isclose to the warmest thermal fluids.
The points distribution in fig. 3 allows only a dilutionprocess of the B group samples with Al or A2 freshwaters withoutdiscriminating in any way between thermal springs and highsalinity hypothermal or cold waters.
125
The A-B section of the fig. 2 Piper diagram vs. T.D.S., asshown in fig. 4, although confirming a dilution process does notsupply further information.
Fig.4 - Correlation diagram between T.D.S. and the A-B sectionin the Piper diagram of Fig.2.
This suggests that, in most manifestations, deep and sub-superficial circulations are widely interconnected. Fluids risingfrom the geothermal reservoir interact with the superficial salinedeposits and the original internal ratio of the major costituentschanges. So it is very difficult not only a reliable individuationof the original deep composition, which is necessary for anygeothemometric evaluations, but also an estimate of the actualextension of the circuit.
126
The Na/K ratio contributes greatly to the solution of theseproblems. The points distribution in fig. 5 shows two trends;samples E6, 7, 8, 11, 20, 21, 22, 23, 24 and WE5 fall on line A(Na/K = 50), while samples E13, 14, 15, 16, 17, 19, 26, 28, 29,WE1, WE4 and WEC fall on line B (Na/K = 16.7). This last one isvery close to the average Na/K ratio of the three brine samples ofthe salar SE135, SE140 and SE141 (= 15.5). The rest of the B groupsamples fall in the area between the two lines A and B.
30
20
Na = 2610
179
Slit-Na = 3700.0
OLu
10
x O
0-0 100 200 300
NA(MEQ/L)Fig.5 - K vs. Na diagram.
400 500
127
4
0
o
ô' /
23 /
o ° 0/
/ ^
o0 /n - " ^SV P^
o"
/o o / o
29 /
/
,, - "
) 20 4
&/Wo /
/ 0
° // i
18O
I
i ,- " " °o --
o
0 6
/''
^
' ' #Qj<° 0
2 ^
°
I I 1 I I I I ! I
0 8
.-- " °/
/'
s
^ ""
) ! 1 I 1 1 I 1 1
0 1C
20Na= 21p S
O
o
1 1 [ ! 1 1 1 1 1
)0 12!0NA(MEQ/L)
Fig.Bb - K vs. Na diagram.
As regards the water points of the first trend (line A): E6, 7and 8 (Rio Quebrada Calorno), are located very close together inproximity of the Cerro Wichu Kkollu. They have temperatures of30°, 35° and 15°C; T.D.S. are around 10000 mg/1 for the first twoand 5000 mg/1 in the last. The Ell sample is a cold spring whichemerges from the westernmost edge of the salar with T.D.S. of 5000mg/1. E20, 21, 22, and fuentes Towa (23, 24, 25) are a group ofvery interesting springs located in the Estancia Towa area, whilethe WE5 test hole is situated roughly 2 km west of the fuentesTowa. The temperatures of these 6 springs range from 22"C for E21,to 87° for E24, for the test hole, at a depth around 150 m, wasregistered a temperature of 115°C. The T.D.S., around 4000 mg/1 inE23, 24, 25 and WE5, increases considerably (15000 - 32000 mg/1)in E20, 21, and 22 situated between 3 and 5 km north of fuentesTowa.
128
The main chemical differences among these samples regard thepercentages of sulphate and of earth alkaline contents. These arehigher in the water points E6, 7, and 8, but are practicallyhalved in E20, 21, and 22; for the hottest samples (fuentes Towaand WE5) , the sulphates percentage is the same as in the threeprevious water points, but the concentration of the earthalkalines practically disappears.
Among the samples falling on line B: E13, 14, 15, 16, and 17are located in a restricted area a little north of MinaConception. Their temperatures range between 14° and 75°C, whilethe T.D.S. varies between 4000 mg/1 and 32000. Spring E16, whichemerged at 75°C at the salar edge with a 23000 mg/1 T.D.S., wascovered in 1986 by the dump of Mina Conception; its pH (1.7) wasextremely low compared to the S04 content (50 mg/1) and theconcentration of Cl (in meq/1) was considerably higher than thesum of Na and K. Very little is known of the E17 sample (17°C and35000 mg/1), collected not far from the preceding one, of whichmaintains the characteristics, except for temperature. The threesamples E13, 14, and 15, which emerge 2-3 km north-west of E16,have a temperature of 34 °C, a maximum T.D.S. of 5400 mg/1, SO4always exceeding 1000 mg/1 and Cl content stechiometrically equalto the sum of Na and K. E19 (14°C, 3000 mg/1) located nearEstancia Towa, because of its moderate acidity and Cl in excess ofthe sum of Na and K is similar to E16 and 17, although has a muchhigher percentage of SO4. The E26, 28 and 29 samples (temperaturesbetween 10° and 35°C, T.D.S. between 1100 and 2200 mg/1), aresituated on the south eastern edge of the area; their chemism isidentical and characterized, once again, by an excess of chloridescompared to the sum of Na and K, apart from the higher content ofSiO2 in E29 (the warmest of these springs, although less saline) .
129
The other samples, E9, 12, 18, and WE2, located in areas ratherdistant from one another, fall in the diagram of fig. 5 in anintermediate position between the two lines A and B.
Finally, in fig. 6 and 6b, sum of Na and K vs. Cl, the samplesare well discriminated and may be distinguished thus:
- a cluster of points constitued by Fuentes Towa and thetest holes WE2 and WE5, defined as subgroup Bl;
- samples falling on line A (subgroup B2);- samples falling on line B (subgroup B3), all
characterized by an excess of chlorides as regardsthe sum of Na and K, including the three representativebrines of the Empexa salar.
500
OLu.400
OUJ
300
200
100
0
'V
£^'°^ I 1 1 ! 1 1 l ! 1
D 1C
/;6X / ^
X*
! 1 1 1 f 1 1 I 1
)0 2C
20 ^
-P*" // ^
X
1 1 ! J i 1 1 I 1
0 3C
0
^,"X «/ O
1 I t l I 1 1 I I
10 4C
x / "X
/-'s'
'
I 1 1 1 I 1 [ 1 [
0 5C
ö
31 Jj -
Na =36950^. . -ißu ,Na = 56400^
^a'Na = 5360
1 1 1 1 i 1 1 1 1
0 6C0CL(MEQ/L)
Fig.6 - Na + K vs. Cl diagram.
130
120
OLJ
+
80
Oby 40<f
XX ,
X X
1 1 1 M 1 1 1 1
D 2
,
/oX »c
X ig XOx ox
X 28 XX ^:ex
1 1 1 M 1 1 1 1
D 4
SUBGROUP E\ ,_'
i' csi?v
', 0 ^ / 3-
v^»X ° O x '
O Xx
XX
1 1 M 1 ! 1 1 1
D 6
i 1 X
XX
/ -
"" r?> ' -X
XX
XX
1 1 1 1 1 1 1 1 1D 8
o^
0«x^
^f'<y/>/!2
^"
^""
1 1O
1 1 I II 1 ! 1 10 1C
XX
XX
XX
XX
XX
XX
I I I I I I I I I0 12
O x
9
0 xX
I I 1 I I I ! I
0 1 4-0CL(MEQ/L)
Fig.6b - Na + K vs. Cl diagram.
It is supposed, therefore, that the composition of all thewater points which are situated along the latter line is greatlyinfluenced by leakage of evaporitic salt deposits, a process whichhas certainly modified the original ratios between the differentconstituents even in the warmest fluids. An additionalconfirmation of this hypothesis is represented by the ratio Cl/Br(mg/1) ; this, in the samples E23, 24, is slightly over 700, notvery far from the typical one for formation waters, whereas forthe others it varies between 3000 - 4000, values which distinguishstatistically waters that interact with the evaporites. Theprincipal variations which occur in this case as regards theoriginal composition in the reservoir, are the considerabledecrease of the ratio Na/K and the increase of the originalconcentration of Ca and Mg.
131
Even the samples with a higher temperature which belong tosubgroup B2 present modifications, although lower than those ofsubgroup B3. These are a result of the last phase of circulation:in effect, while the ratio Na/K remains similar to that of FuentesTowa and of WE5, the concentrations of earth-alkalines and theratio Cl/Br increase considerably.
Finally, the samples of subgroup Bl, for lack of any surfacecontamination, can be considered the most representative of thedeep fluid; its possible modifications depend only on processesof riequilibration during its uplift to the surface.
4. Geothermometric Considerations
Fig. 7 and 8 show the Na, K, Mg triangular diagram according toGiggenbach [3]. The difference between the two figures consists ofthe different scales used for the Na, K, and Mg concentrations. Inthe latter case, this permits a clearer visualization of thefluids thermically more degraded and/or more mixed withfreshwaters.
In Fig. 7 the E23 and 24 (Fuentes Towa) samples are situatedslightly above the "full eguilibrium" curve, at an estimated deeptemperature of about 170 "C. The shift is probably due to theboiling and degassing phenomena which occur during the upflow withsubsequent precipitations of Ca and Mg. The E25, WE5, E20, 21, and22 samples, fall on a line which joins Fuentes Towa to thef reshwaters (right corner) . This is in accordance with anenrichment process in earth-alkaline components at constant Na/K.
Fig. 8 shows a second trend, which joins points E16, 17, WE4,WEG and E28. This first two as a proof of the leakage process ofevaporitic deposits discussed in the previous paragraph, arelocated in proximity of the points representing the brines of theEmpexa salar; we may, however, think that they could, originally,
132
•OQ No/1000
20 60 8040K/100 % Mg
Fig. 7 - Na, K , V M g tr iangulär diagram.
, Na/400V
100
MG ÎO.5
40K/10 x Mg MgfO.5
Fig.8 - Na, K, ViMg triangulär diagram.
133
be found in a higher position from which they moved away becauseof a Na/K ratio decrease (evaporite contamination). Dilutionphenomena of the E16 and 17 samples could originate points E28,WE4, and WEG.
The temperatures that can be calculated from SiO2 [4], assumingthe equilibration of chalcedony as the most probable, on the basisof values calculated with ionic geothermometers, resulted for E23and 24 175° and 186°C respectively.
Therefore, the results obtained using the most common solutegeothermometers are in accordance among them, (also the empiricgeothermometer Na/Li [5] gives values of 175° and 180°C). Thislead us to assume that the maximum temperature in the reservoirdoes not exceed 180°C.
4.1 Examination of the State of Equilibrium
By using the activities of aqueous chemical species computedfor homogeneous equilibria at temperatures ranging from 150° to20CTC, it is possible to compute the degree of under- or super-saturation of the aqueous components with different mineralogicalphases.
This can be expressed for any mineral in terms of log AP/K.AP/K refers to the activity product of the species concerned in agiven solute chemical equilibrium compared with the equilibriumconstant K for that hydrolysis reaction at a prefixed temperature.This ratio is greater than zero for supersaturated minerals andless than zero for the undersaturated ones. These log AP/K valuesare plotted vs. a proper temperature range (based for example ongeothermometers results) ; the paths of convergence of theresulting curves to the zero value establish the possible mineralassemblages of the reservoir and enable the identification of thefluids equilibration temperature by using only their chemicalanalyses and some proper constraints.
134
The E24 spring can be considered the most representative fluidof the Empexa deep reservoir, but pH, calcium and magnesiumcontent at the sampling point are the result of riequilibration atshallow levels after degassing; this process explains the thicktravertine deposition characterizing all the hot springs aroundthe Fuentes Towa area. The only simple and reasonable way tocompute the possible concentration of these components in the deepfluid is based on. the constraint of calcite equilibration at180°C.
Fig. 9 shows the log AP/K values for the E24 spring, correctedcomposition are plotted vs. temperatures ranging between 150" and200 °C. The fluid is well equilibrated with both a series ofminerals that are typical components of volcanic rocks present inthe Empexa area and the phases deriving from their hydrothermalalteration at a temperature of around 175°C. This confirms thegeothermometers values.
:Q_
cno
3.0 - i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i
-1.0 -
-2.0 -
Temperature °CFig .9 - Log AP/K vs. T diagram.
135
Another interesting aspect to be pointed out is the calciumconcentration, computed according to this method, in the reservoircondition (23.4 rog/1); this is very close to the 28.1 mg/1 foundin the gradient test hole WE5 during production.3.4 Remarks on the isotopic data
Before this survey no isotopic analyses of thermalmanifestations and cold springs had ever been performed in theEmpexa area. The results of the 1986 sampling are shown in table1, the relative 618O vs. <5D diagram in fig. 10. With the exceptionof samples E21 (stagnant pool) and FE (condensed steam of the MinaConception fumarole), no fluid exists that significantly deviatesfrom the meteoric line; in other words no appreciable exchanges of18O between fluids and host rocks are observed. As the hypothesisof chemical equilibration at depth is strengthened by the lowtritium values of the waters, the lack of isotopic shift can beexplained only in terms of the low temperature in the reservoir.
OCN
~>u
\-
130-
/^<\,®'/<3)©"'0
,-"
)
» '
6'^^ -^ -^
ACT
©
^-
®
I I 1 I I 1 I I I-16 -15 -14 -13 -12 -11 -10 -9
180 IN H20 (%o)Fig.10 - oD vs. 0180 diagram.
136
This confirms the estimated values according to the solutegeothermometers and the AP/K logs. The 159 °C temperature
n ocomputable for the exchange reaction of -LOO between dissolvedsulphate and water for the E23 sample also agrees with theprevious results.
5) Conclusions
The general situation in the Rio Empexa valley, on the basis ofthe information available and of this geochemical survey, can besummarized as follows:
a) - Three water groups with well identified chemicalcharacteristics exist.
The first is a surface one constituted of cold meteoric watersemerging at a higher level than the salar, with low salinity andmainly of an earth-alkaline sulphate type.
The second of subsurface character, includes cold and thermalsprings spread over the whole Empexa area, with salinity rangingfrom medium to high values. The chemistry, due to the leakage ofevaporitic deposits, is mainly alkaline-earth-alkaline chloride;the chloride content is higher than the sum of Na and K.
The third, representative of a deep circulation, consists ofthe Fuentes Towa and others hot springs located very close tothese. In this zone favorable structural patterns for the uprisingof thermal fluids are present. The chemistry is mainly sodiumchloride with the exception of the most peripherical springs thatcan leach evaporitic deposits; the salinity and the temperature atdepth should not exceed 4000 mg/1 and 180°C.
b) - There is a general absence of leakage anomalies and a verylimited presence of free gases. These, with the exception of theMina Conception fumarole, are completely lacking in H2S and havean extremely low content of H2. These conditions make most of the
137
gas geothermometers practically useless; with the use of theformulation based on carbon monoxide the calculated temperature isapproximately 100"C higher than that obtained by considering theliquid.
Despite the fact that the thermal information of the gasesnormally refers to the deepest part of a geothermal system andthat notable differences can sometimes be found in the calculatedtemperatures based on equilibria involving different phases, thegas results in this area are unreliable.
c) - In no sample was observed an appreciable isotope shift.On the basis of all this, the conclusions that may be drawn for
the Empexa area, at least from a geochemical point of view, arenot very promising. In theory, higher temperature fluids could bepresent in deeper levels than ones in which the waters of E23 and24 type circulate. With these premises, a geoelectric survey couldbe of use for the evaluation of subsequent developments.
References
[1] APPROVECHAMIENTO DE LOS RECURSOS GEOTERMICOS DEL
SUDOESTE BOLIVIANO - Areas de Salär d1Empexa y LagunaColorada - Aquater 1979.
[2] ERICKSEN, G.E. & VINE, T.D. (1976) - Preliminary re-port on the lithium rich brines at Salär de Uyuni andnearby salars in southwestern Bolivia.
[3] GIGGENBACH, W.F. (1980) - Geothermal solute equilib-
ria. Derivation of Na - K - Mg - Ça geoindicators.GEOCHIMICA ET COSMOCHIMICÄ ACTA, 52, 2749-2765.
[4] FOURNIER, R.O. & POTTER, R.W. (1982) - A revised andexpanded silica quartz geothermometer. GEOTHERMALRESEARCH COUNCIL BULLETIN, V. 11, 3 - 9 .
138
[5] FOULLIAC, C. & MICHARD, G. (1981) - Sodium-lithiumratio in water applied to the geothermometer of geo-thermal waters. GEOTHERMICS 10, 55 - 70.
[6] REED, M. & SPYCHER, N. (1984) - Calculation of pH andmineral equilibria in hydrothermal waters with appli-cation to geothermometry and studies of boiling anddilution. GEOCHIMICA ET COSMOCHIMICA ACTA, 48, 1479 -
1492.
139
GEOCHEMICAL REPORT ON THE SAJAMAGEOTHERMAL AREA, BOLIVIA
G. SCANDIFFIOEnte Nazionale per 1'Energia Elettrica,Pisa, Italy
J. RODRIGUEZUniversidad Mayor de San Simon,Cochabamba, Bolivia
Resumen-Abstract
INFORME GEOQUIMICO SOBRE LA ZONA GEOTERMICA DE SAJAMA, BOLIVIA.
La zona geotérmica de Sa jama, al noroeste de Bolivia, esta muy cerca dela frontera con Chile y a unos 60 km de distancia de la frontera con el Peru.
La actividad volcànica, durante el Terciario, se relacionaba con dos sis-temas principales de fallas, formados durante la fase orogénica del periodo,con tendencias N-S y E-0. Mas tarde se desarrollo un vulcanisrao cuaternariorelacionado con sistemas NO-SE y NE-SO. Este ultimo vulcanismo se caracteri-zaba por volcanes compuestos que producian lavas y rocas piroclâsticas, quecubren ahora las ignimbritas terciarias.
Las actividades térmicas mas importantes, que se manifiestan en un flui-do profundo que llega a la superficie tras su evaporaciôn, se producen juntoal rio Junthuma, a una altura de 4 400 m sobre el nivel del mar, y se relacio-nan directamente con el sistema NO-SE. Se dan otros casos a 7 km al este y200 m mas abajo, con menos salinidad, que pueden considerarse como flujo desalida lateral del mismo sistema. Este flujo latéral parece estar diluido conagua meteorica que tiene una composicion isotôpica mas negativa que las aguastérmicas junto al rio Junthuma.
Segûn geotermometros ionicos, la temperatura del depösito es de entre 230y 250°C; los datos isotôpicos apoyan estos valores.
Solo se han recogido muestras de gas en depôsitos alimentados por fluidoresidual (tras su evaporaciôn); estas muestras carecen de oligoelementos, porlo que no pueden utilizarse como geotermometros y no son representativas delestado del depösito.
141
GEOCHEMICAL REPORT ON THE SAJAMA GEOTHERMAL AREA, BOLIVIA.
The geothermal area of Sajama, Northwestern Bolivia, occursvery close to the border with Chile and about 60 km away fromthe border with Peru.
During Tertiary the volcanic activity was related to twomain fault systems, formed during the orogenig phase of theperiod, with N-S and E-W trends. Later on a quaternaryvolcanism developed in relation to NW-SE and NE-SW systems.This latter volcanism was characterized by stratovolcanoesproducing lavas and pyroclastic rocks, that are now overlyingthe tertiary ignimbrites.
The most important thermal manifestations, representing adeep fluid coming at the surface after flashing, occur near RioJunthuma at an elevation of 4400 m a.s.l. and are directlyrelated to the NW-SE system. Seven kilometers eastward and200 m downward other occurrences are found, with lowersalinity, that can be regarded as a lateral outflow of the samesystem. This lateral outflow seems diluted with a meteoricwater having an isotopic composition more negative than thethermal waters near Rio Junthuma have.
Reservoir temperature, according to ionic geothermometers,is between 230 and 250°C; isotopic data support these values.
Only gas samples from occurrences fed by residual fluid(after flashing) have been collected, they are depleted oftrace components and so cannot be used as geothermometers,being not representative of reservoir conditions.
142
1). Geologic outline
In the year 1977 a geological reconnaissance study wascarried out by Geotecneco [1], the results of which aresummarized in this chapter.
The Sajama area consists of Tertiary and Quaternaryterrains of volcanic origin. It is characterized by numerousstrato-volcanoes producing lavas and pyroclastites which havegrown over an older ignimbritic cover with vast lateralextension. The most recent volcanic activity occurs west of RioSajama, along the border between Bolivia and Chile where thevolcanoes Huallatani (Chile), Parinacota, Pomerape, Kakepe andCondoriri volcanoes are present.
1.1 Stratigraphy
The most ancient stratigraphie unit is an ignimbriticsequence of unknown thickness. The upper part, with a maximumthickness of some tens of meters, is composed of intenselyfractured ignimbrites. The lower levels consists, instead, oftuffs intercalations with variable, but often very reducedpermeability. The base of the ignimbritic body is notoutcropping. It probably rests in stratigraphie unconformity onthe sedimentary sequence of Corocoro. The latter occurs inother places in the region with a thick succession ofprevalently muddy, Tertiary continental sediments containingarenaceous horizons, conglomerate and tuff levels.
Above the oldest ignimbritic unit, volcanic units ofvarious age and composition, which refer to distinct centres ofemission, are found. These do not form uniform covers. In
143
chronological order they are:- Andesitic porphyritic lavas of Nevado Sajama.They are probably of Pleistocene age as proven by the
intense development of forms of glacial erosion associated tomorainic deposits and traces of fossil fumarole activity.
- Intermediate andesitic lavas, partly vesicular, of theCerro Quisi Quisini.
Acid porphyrites of Cerro Condoriri, Nevado KakepeJunthuta, and Nevado Payachata.
In the most pronounced depressions of the Sa jama basinincoherent deposits with fine and intermediate granulometryhave accumulated, composed of tuffs and ash materialsintercalated with muddy levels. The origin of these deposits isattributed to episodes such as mud flows (Lahars), ash flow andexplosive activities (ash fall - pomices) connected to the mostrecent volcanic activity. These terrains have been abundantlyeroded.
1. 2 Tectonics
The area is characterized by tensional structures. Theprincipal structural elements have conditioned both thevolcanism and the hydrologie surface structure.
Two principal fractures systems may be distinguished. Thefirst, and seemingly older system, with N-S/E-W direction onwhich the Rio Kasilla and tracts of the Rio Sajama havepositioned themselves. Traces of this system can beindividuated on the Nevado Sajama.
The second, more recent system with NW-SE/NE-SW direction,has influenced to a larger extent the present morphologyconditioning the alignment and the development of the most
144
recent eruptive centres. The Rio Junthuma has its locationalong an element of this system.
1.3 Volcanism and geothermal implications
On the basis of stratigraphie data, a possible sequence ofvolcanic events is the following:
- first phase with intense fissure eruptive activity,probably along the oldest structural N-S/E-W systems, withemission of viscous acid products which form the basalignimbrites.
- There follows a phase in which the eruptive activitybecomes less frequent, it is concentrated along limited sectorsof the fractures system, forming central volcanoes. Theproducts emitted are andésites with a porphyritic texture. Tothis phase belongs the formation of the Sajama Volcano but thesame materials make up the lower parts of other volcanicstructures in this zone.
- Volcanic activity moves west as time passes with centraleffusive, and more rapid and tranquil eruptions of intermediateandesitic lavas (Cerro Quisi Quisini).
- There follows the most recent phase of a partiallyexplosive nature, with the emission of great quantities ofporphirytic acid material. This is composed of rhyolites andrhyodacites with phenocrystals of plagioclase, pyroxene, andbiotite (normative quartz > 20%) .
The centres of this activity are: Cerro Condoriri, NevadoKakepe Junthuta, Nevado Payachata and Huallatani. Among these,the most recent seem to be the Huallatani volcano, still infumarole activity, and the Payachata, which has two eruptive
145
centres; the Cerro Parinacota, and the Cerro Pomerape whichshows evident collapse structures and has a solfataricactivity. The Nevado Condoriri and Nevado Kakepe are older andwithout present manifestations, even though the latter volcanostill shows thermal anomalies (hydrothermal manifestations).
- After the deposition of acid porphyrites comes a periodof mud flows with formation of lahar-type deposits in the mostdepressed zones of the Sajama basin. These deposits are, atpresent, highly eroded, and are mostly along the Rio Junthuma.
- On top of the lahars there are deposits of ash flow,partly consolidated and containing also organic material. Thesummit of these deposits is transformed into paleosoil. Thiswould indicate a period of lacking eruptive activity duringwhich vegetation developed.
- The paleosoil is covered by a level of 30 - 40 cm ofuniform thickness, of piroclastic products, mostly ash andpumice of acid composition with sedimentary lithic intercalatedfragments. This level records a particularly violent explosiveevent, perhaps phreato-magmatic, during which materialsbelonging to deep stratigraphie levels have been torn from thewalls of the pipe (Corocoro formation?). This episode could beattributed to the Cerro Parinacota or Pomerape or even to themost recent Huallatani, despite this being the furthest awayfrom the deposits.
- Above the pumices occur again levels of fine and mediumash flow material which form the present soil.
From a geothermal point of view there are numerousindications of a volcanologie nature in favour of thehypothesis that today there exists a not very deep magmatic
146
body in the process of cooling. Among these there are: activefumarole and solfataric activity of some volcanoes; the recentage of the last volcanites emitted; their evolutive trend fromandésites to rhyolites with femic phenocrystals as witness of aacid differentiation process caused by fractionalcrystallization in a quiescent stage; the overall volume of theyoungest differentiated volcanites suggesting the presence ofmagmatic chambers with considerable possible storage and longcooling periods.
As regards the hydrogeologic model, the relict laharsdeposits, together with the ashes, form an impermeable coversufficiently continuous and extensive in the lowest parts ofthe basin. The outcropping volcanites at the highest altitudesalong the slopes of the volcanoes have quite variable degree ofpermeability. In theory, these are permeable because offracturing and could provide an important recharge area, butthermal fossil manifestations in all the zone must be noted asevidences of self-sealing processes connected with a previousrather intense activity. This makes the permeability of thevolcanites very discontinuous with rapid horizontal variationsand the development of independent (closed?) circuits, as shownby the existence of cold and warm springs a few meters awayfrom each other. The identification of the reservoir is also aproblem. This should develop under the strato-volcanoes in thebasal ignimbrites, the permeability of which is ascertainedonly for the first tens of meters. The volume of the entireignimbritic body does not seem sufficient to form a reservoir;therefore, if this reservoir exists, it should continues atdepth in the pre-volcanic substratum. We have no directknowledge of this substratum, apart from the sedimentary
147
intercalation in the pumices, deriving from the fragments tornby the explosions. One hypothesis is that this substratumconsists of the continental terrigenous succession of Corocorobut the presence of this group under the Sajama area is notcertain. Furthermore the Corocoro group presents a considerablevariation in composition and, therefore, in permeability (claymembers, arenaceous conglomerates, etc.). As a result ofintense tectonic deformation, the members of the sequence arenot always mantained. Therefore, even if the existence of theCorocoro Group in the Sajama area were ascertained, this wouldnot necessarily constitute a permeable reservoir.
2). Sample collection and field determinations
The Sajama geothermal area is located in the north-westpart of Bolivia, very closed to the boundary line with Chileand about 60 Km away from the border with Peru. The area, withan average elevation of 4400 m. a.s.l., shows an unevenmorphology; the most important manifestations are localized ina small valley along Rio Junthuma.
Two more springs of lower flowrate occur at a level ofapproximately 4200 m a.s.l., located some 7 Km east of the RioJunthuma ones. The sample location map is shown in fig. 1
The survey in this area was carried out over a period ofseven days from the 5ht to the llth of June 1988. A total of 15samples of hot and cold springs were taken, 3 of both freetotal and condensate gases. Unfortunately more than 50% of gassamplers were damaged during the freight to Italy.
Water samples were collected generally in 6 separatealiquots for analyses of major and trace constituents,
148
}f<\f 'Äf ^a^V V
T»M60 QUEMAOO
HUALL/iTANI
= Cold waters Q = Hot watersFig. I - Location map of Sajama area samples.
149
monomeric aluminum silica, stable isotopes (180 and D) andtritium.
Temperature, pH, conductivity and alcalinity weredetermined in the field. Major and trace chemical costituentswere analyzed in ENEL laboratory in Italy; isotopes wereanalyzed in IAEA and DSIR laboratories.
3). General outline of the Rio Junthuma manifestations
In this area (fig. 2) occur several active and fossilmanifestations: the active ones (more than fifty), showingtemperatures ranging between 50 °C and over 85° C (that is theboiling point for this altitude " 4400 m a.s.l.), aredistributed for a length of two kilometers. Their locationrange between some centimeters to a few tenths of meters fromRio Junthuma. In some springs occurs appreciable gas and thetotal hot water flow rate can be estimated around 45 1/s; fig.2 shows how manifestations are located in two structurallyseparated zones. The first one, northward, provides moreoccurrences at a temperature close to boiling than the southernone does. This latter one, however, besides showing a slightlyhigher conductivity for the most important water points,presents mud pools and at least two fumaroles. Sampling ofthese manifestations was impossible because of the dangerousaccess to them.
150
1.10000
Fig. 2 - Location map of the thermal manifestations
of the Rio Junthuma .
151
4). Water chemistry
4.1. Classification of the water samples
The analytical results of the fluids sampled during thesurvey are reported in tab. 1 (waters) and 2 (gases).
The classification of the samples was carried out by meansof cartesian diagrams and correlations existing between somechemical components.
The classic Piper diagram is shown in fig. 3. The latter isdivided into four quadrants and samples are generallydistributed as follows. In the first NW are found the earth-alkaline-bicarbonate waters, generally linked to shallowcirculation; in the second SW the alkaline-bicarbonate ones; inthe third SE alkaline-chloride waters occur, normallyconsidered of deep origin; in the fourth, NE, the earth-alkaline-sulphate ones.
Among the samples of the Sajama area only SA16 and 16Boccurring before the Rio Junthuma hot springs start, arelocated in the NW quadrant; both are low salinity cold waterswith an anomalous SiO2 (75.5 mg/1), the first one is a stream,the latter is the Rio Junthuma itself. Samples SA41 and SA42fall in the NE quadrant; SA41 is a streamwater, SA42 (31°C,T.D.S. around 500 mg/1), a spring located 30 Km to the north ofthe main manifestation area, on the base of its chemicalcomposition can be considered as a freshwater conductivelyheated. The SiO2 and SO4 content, respectively 120 mg/1 and 226mg/1, is higher than for the others freshwaters. Regardingwater points located in the SE quadrant: SA40, collected nearSA42, is a low salinity hypothermal spring with a noticeablesulphate content; SA37 (29°C, T.D.S. 1427 mg/1), is another Rio
152
TABLE 1 - Lab. analytical results for water samples.
OODE
SA1SA11SAU
SA13
SA16
SA16BSA18SA21
SA35SA36S A3 7SA38SA39SA40SA41S A4 2
Tf
84.568.971.787.812.09.8
51.553.386.579.029.139.345.219.312.331.0
OODE
SA1
SA11
SA13SAHSA16SA 168SA18SA21SA35SA36SA37
SA38SA39SA40SA41SA42
pHf Condf Alkf Ça Hg Na K Cl S04 Si02mS/cm meq/l mg/l mg/l mg/l mg/l mg/l mg/l ipg/l
7.30 6170 4.06 18.5 2.00 1250.0 127.0 1790.0 339.0 192.06.19 5830 3.18 12.8 0.20 1420.0 144.0 1950.0 372.0 240.06.30 6350 3.11 7.4 0.24 1380.0 138.0 1950.0 371.0 236.07.43 6590 3.30 25.0 3.20 1180.0 118.0 1630.0 313.0 213.06.45 516 1.40 14.4 8.00 9.9 3.1 1.6 15.0 78.86.31 168 1.56 14.6 8.20 9.8 3.4 1.7 14.9 78.4
6080 8.9 0.48 1340.0 123.0 1870.0 358.0 207.06160 7.9 0.07 1430.0 129.0 1930.0 374.0 243.05240 43.4 7.60 1140.0 115.0 1560.0 294.0 201.05670 28.7 2.80 1290.0 127.0 1740.0 340.0 214.02240 18.9 6.50 442.0 46.5 591.0 124.0 119.02270 43.5 28.30 439.0 56.0 476.0 188.0 155.01670 27.1 17.10 300.0 49.0 319.0 163.0 148.0
193 14.8 6.90 31.8 3.5 2.8 62.0 74.1270 16.7 9.00 20.3 4.6 6.3 65.3 64.482 68.8 18.00 26.4 13.2 0.4 226.0 120.0
Date pHl Condl A lk l Ralkl As Alm Cs Zn Pb SbmS/cm meq/l meq/l mg/l mg/l mg/l mg/l mg/l mg/l
06/06/88 7.33 5.85 4.00 3.80 4.40 0.00 1.90 0.00 0.00 0.0006/07/88 7.21 6.35 3.16 2.96 4.70 0.00 2.30 0.01 0.00 0.0006/06/88 7.21 5.40 3.24 3.04 3.90 0.00 2.10 0.00 0.00 0.0006/07/88 6.90 6.47 3.16 3.00 4.80 0.00 2.30 0.00 0.00 0.0006/07/88 6.32 0.18 1.34 1.34 0.00 0.00 0.00 0.00 0.00 0.0006/11/88 6.31 0.17 1.38 1.38 0.00 0.00 0.00 0.00 0.00 0.0006/08/88 6.87 6.29 2.92 2.76 4.70 0.00 1.20 0.00 0.00 0.0006/08/88 7.02 6.25 2.84 2.68 4.80 0.00 1.20 0.00 0.00 0.0006/08/88 7.36 5.21 5.68 5.48 3.90 0.00 1.90 0.00 0.00 0.0006/08/88 7.18 5.67 4.18 4.02 4.10 0.00 2.10 0.00 0.14 0.0006/08/88 6.76 2.31 2.40 2.40 1.30 0.00 0.68 0.00 0.00 0.0006/09/88 7.05 2.35 6.84 6.74 1.80 0.00 0.45 0.00 0.00 0.0006/09/88 7.24 0.28 4.82 4.72 0.95 0.00 0.34 0.00 0.00 0.0006/09/88 6.60 0.30 1.12 1.12 0.14 0.00 0.00 0.00 0.00 0.0006/10/88 6.56 0.14 0.92 0.92 0.86 0.00 0.00 0.00 0.00 0.0006/13/88 7.13 0.63 1.38 1.38 0.00 0.00 0.00 0.00 0.00 0.00
H3S03mg/l
198.0214.0217.0184.0
0.50.4
208.0216.0175.0197.064.747.930.50.81.40.3
1mg/l
0.700.800.701.00
0.6U0.700.500.500.300.30
NH4mg/l
0.6600.7801.1000.5600.0000.0000.4100.4000.3900.4800.1400.2900.3100.0000.0000.000
TDSmg/l
3948437236924331
134139
<,144434635653964142714501021202196476
H2Smg/l
1.90.00.00.80.01.30.00.00.8Û.Û0.00.60.80.00.00.0
A. S. Lm
4380441044104410443044304400441044154425437042904290440044004410
Limg/l
6.707.207.405.900.010.017.007.203.006.702.202.201.500.090.020.01
018_H200/00
-13.99-12.73-14.14-12.68-16.52-16.54-12.83-12.56-14.00-13.74-15.63-16.60-16.92-17.65•16.92-17.26
Rbmg/l
1.101.301.301.100.000.001.201.201.101.200.390.300.230.000.000.06
D_H20o/oo
•107.9-108.6-110.8-107.0•120.9'-118.0-105.6-105.6-110.2-110.2-115.6-123.1-123.3-126.7-123.3-125.4
Brmg/l
5.705.905.804.600.000.005.205.904.505.101.701.400.860.000.000.00
H3_H20T.U.
0.9
0.8
2.72.30.11.10.60.71.81.00.60.61.50.6
F
mg/l
1.902.102.201.800.250.252.002.101.901.800.810.430.310.300.440.17
018.
AIT
mg/l
0.0330.0220.2500.0000.0000.0440.0000.0470.0000.0000.0000.0000.0000.0000.0230.036
.504 S34_
Femg/l
0.060.000.210.000.000.160.150.020.000.010.450.030.000.000.010.04
S04
Srmg/l
2.502.202.201.800.140.152.102.202.502.400.910.900.610.200.170.50
Bamg/l
0.1300.1200.1000.1000.0050.0060.1000.1100.1300.1200.0500.0200.0050.0040.0200.006
0/00 0/00
6-2
3
.00 11.
.00 12.
.90 11.
.50
.20
30
TABLE 2
Sajama gas sample analysis (vol, 7«)
CODE C02 H2S H2 CH4 N2 CO (ppmV)
SA14A 99.78 0.0 0.0 0.007 0.0 11.20
SA-36 99.52 0.0 0.0 0.011 0.47 9.65
-f
-f(DZ .
0 Cl- -t-———— 1 _____ 1 ———— 1 ———— 1 ————
+BA16B
—— _.....,.. ! . . - . -J —————————— J ——— ..._....!.-. . . . . . . ——
SQ4 — 100———— i ———— i ———— i ———— i ———— '
+SA42
+ SA41
+ SA40
+ SA37
+$44(3*
n- 0)
2(D
3
100 HC03- + COS— 0Fig.3 - Piper diagram for Sajama area water samples.
Junthuma sample taken a little downstream from the lastoccurring manifestation; SA38 and SA39 are the two thermalsprings located ~7 km eastward of the main springs area. Theseshow temperatures of 39.5°C and 45.2°C with T.D.S. of 1450 and"1000 mg/1 respectively. All the other water points, collectedalong the Rio Junthuma, have temperatures ranging between51.5°C and 87.8°C and T.D.S between 3700 mg/1 and 4400 mg/1.
154
In fig.4 the Piper diagram was plotted with chloride as aseparated anion: all samples appear well correlated on astraight line.
0 Cl- 100
+ro
4.SA41
+SA40
n.. Ql
3:IQ
100 HC03- + 504— ß OFig.4 - Piper diagram with Cl as separated anion.
In fig,5 the correlation between total salinity, expressedin meq/1, and an A-B section of the previous diagram confirmsa dilution process of the deep hot water with a local groundwater.
Only SA38 and SA39 slightly shift from the dilution trend.This is mainly due to a percentual increase in alkalinity andsulphates. As later on explained, considering the isotopic datathese springs are characterized by dilution with a ground waterthat is not the same interacting with the thermal occurrencesalong Rio Junthuma.
155
O
-14
Fig. 5 - Gorrelation diagram between TDS and A-B sectionin the Piper diagram with Cl as separated anion
Besides this difference SA38 and 39 can be related to thesame deep thermal inflow that feeds the main system. In factconsidering the representative points of all the thermalsprings in the study area in the triangular diagram correlatingLi, Rb and Cs [fig.6], they result quite close.
Concluding their location 7 km to the East and theiraltitude, "200 m below the Rio Junthuma springs, make of SA38and 39 a likely lateral outflow of the same system.
156
' l1} T I I l i l l | l i l l l l l l l100 80 60 20 0
Rb * 4 (mg/!)Fig.6 - Li, Rb, Cs triangular diagram,
4.2. Geothermometry
The temperature of the deep fluid on the basis of thetriangular Na, K, Mg diagram [2] (fig.7) should run not below230°C. The mixing process of this deep water with differentpercentages of freshwater is highlighted by a perfectlystraight lining up of the water points until the bottom-rightcorner of the diagram (shallow waters representative zone).
157
Na/1000V
100MgtO.5
Fig.7 - Na, K, \/Mg triangulär diagram.
Again the plotting of 10*CMg/(10*CMg+CCa) vs.10*CK/(10*CK+CNa) [2] (fig. 8) supports this dilution with theonly difference that the extrapolation of the line, connectingthe samples representative points, up to the theoreticequilibrium curve at the different temperatures gives areservoir fluid at 250°C of temperature.
In fig. 9 [3] are shown the log AP/K values for SA21 springcomposition vs. temperatures ranging between 150 and 260°C(with the only constraint of the calcite equilibration at thedifferent temperatures). The curves for a series of volcanic
158
10 Mg
l l l l f l l l J l i l ! l l l l l l l l l l l l L l l l l l l l l l l l I l i i l I l
10 K
o.o
Fig.8 - 10K / (10K + Na) vs. lOMg / (lOMg + Ca) diagram.
Sample SA21
Added Carbon (o solution133.8 mmol/Kg (at 240»C)
l l l l l l l l l | l l M l l l l l | l l l l l l l l l | l l l l l l l l l | l l l l l l l l l | l l l l l l l l l | l l l l M l l l | l l l l l l l l l |'100 120 140 160 180 200 220 240 260Temperature C
Fig.9 - Log AP/K values vs. tempera ture-
159
rock components, and the phases derived from their hydrothermalalteration, converge at a temperature around 240°C. The curvesmatch at a value of log AP/K below zero, this could beexplained by a poor computation of theoretic pH values due to alack of the pH value in the field. In the same way can beexplained the apparent equilibration of fresh minerals at alesser temperature of the altered ones.
For this reason it is been assumed as the equilibrationtemperature value the one correspondig to the lines crossingpoint instead of the conventional log AP/K = 0.
As far as concerning the SiO2 geothermometer [4] may beinteresting to examine fig. 10, here SiO2 is plotted against1000/T°K.
Oto-(N
o:
cnEo
"—^LO-T—' ,
(N :o :c/)gj
o.in-
O--T-T1.50
T Amorphous Si l ica
i i i i r t i | i i r i i I T i i j i i i i i i i i i [ i i i i f i2.00 2.50 3.00
1000/T3.50 4.00
Fig.10 - Si02 concentration vs. 1000/T0 K.
160
Many representative points fall along the solubility curveof amorphous silica, this agrees with the presence ofignimbritic layers at depth and with an equilibration processat the temperature of outflow. Because of it, the Si02 contentcannot be used to estimate the temperature at depth.
4.3. Remarks on the isotopic data
No isotopic analyses of thermal manifestations or coldwaters present in this area had ever been performed before,besides in the course of this survey only very few cold springswere found; so the relative results presented in tab. 1 areonly enough to draw a tentative meteoric model of thecirculation patterns.
The highest chloride content and the most isotopicallypositive springs SAH, SA14, SAIS and SA21, despite theirtemperatures are not the highest in the Rio Junthuma area, havebeen considered as residual waters originated from a deep fluidsingle step boiled from a 260°C reservoir temperature to 85°C.As the representative points of these springs in fig. 11(diagram 5180 vs. SD) are slightly scattered, to reducepossible errors to a minimum, the boiled water composition (BW)has been computed extrapolating to zero the tritium content inthe three diagrams of fig. 12, 13 and 14, respectively 3H vs.S18O, 3H vs. 6D and 3H vs. Cl.
The values obtained in such a way: 518O « -12.65, <SD « -105.1 and Cl « 1950 mg/1 are comparable to those resultingaveraging the analytical results for SA14, 18 and 21. On thebase of the single step flash the 260°C deep water composition(DW) should be: 518O = -14.64, SD = -116.6 and Cl = 1290 mg/1.
161
-95
•105
OC\JX
-1 15
-125
•1354-7-18 -16 -15 -14
180 IN H20(%o)Fig.11 - £> D vs. £ 180 d iag ram.
-12
o. u -
2 5 -£,•.•*-) „
2.0 -_
1.5 -E
1.0 -E
0 5 ~*— ' * «— ' _
0.0 --1?
/
40 ,42 ;o /o
16o
/I5\s
\Q —————————\ _ /
38
ao
x3o\
\o
35
1
\
21
^l l 1 l 1 1 l l l 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 I ! 1 1 1 1 1 1
3.0 -17.0 -16.0 -15.0 -14.0 -13.0 -12180 IN H20(%o)
oC\ln:
n:
Fig.12 - Tritium content vs. 180 diagram.
162
O(NX
3.0
2.5
2.0
1.5 T
1.0
0.5
0.0
-0e
,16B
O
i;oQQ
O:O1
-130 -125 -120 -115 -110D IN H20(%o)
-105 -100
Fig.13 - Tritium content vs. o D diagram.
3.0
2.5 -E
HOCNn:
2.0
1.5
1.0
0.5
0.0
O
-e-
400 800 1200CI (mg/l)
Oo,3 O
31o
1600 2000
Fig.14 - Tritium content vs. Cl diagram.
163
The resulting oxigen shift from the GMWL is around 2 deltaunits of 180 and the recharge water should have an isotopiccomposition similar to that of Rio Junthuma.
Considering now samples SA1, 13, 35 and 36 they both forsalinity and tritium content, look like a dilution withpercentages between 15% and 35% of local groundwater.
This process is supported by their location on the diagramsin fig. 12, 13 and 14.
In the c518O vs. SD diagram of fig. (11) they occur inintermediate position with respect to lines A and B. The firstone, A, represents the dilution line along which fall allpossible mixture between the local groundwater (SA16B) and theundiluted single step boiled deep water BW. Curve B shows inturn the theorical isotopic composition of the residual watersoriginated, according a single step flash, from the possiblemixtures between local groundwater and deep water rangingtemperatures between 85°C and 260°C.
The first hypothesis (superficial mixing) is automaticallyexcluded because SA1, 13, 35 and 36 have temperatures alwaysclose to or even higher than boiling temperatures, despite theevident dilution (chloride and tritium content). Also thesecond hypothesys (mixing at the depth) can be excluded, thelocation points of these springs in fig. 11 could be explainedonly by low contributes of local groundwater that again do notfit the analyzed chlorides and tritium.
The SD vs. Cl diagram (fig. 15), shows that such mixturesarise at a subsuperficial level. The representative points ofSA1, SA13, SA35 and SA36 are well below the curve B (theoricallocation of residual waters originating from diluited deepfluid) and can be explained on the basis of a boiling process
164
for the undiluted deep water up to about 120°C, and afterwardsa mixing with the local groundwater (SA16B). This fit properlyboth the analytical chlorides and tritium contents.
Finally considering the location of samples SA38 and 39 inthe diagrams 518O vs. 5D (fig. 11) and SD vs. Cl (fig. 15) isevident:
1) The groundwater that acts as superficial end member forthe mixing has an isotopic composition more negative than theone interacting with the Rio Junthuma springs; the tritiumcontent very close to 0 entails long circulation time peculiarto regional patterns. The more positive values for samples SA16and SA16B can be better explained with daily cicling process offreezing and melting than with a lower infiltration level.
2) The deep component again seems to be an actual reservoirfluid more than a residual water. This confirms the hypothesisof a lateral outflow.
-95
-135 1000CI (mg/l)
Fig.15 - oD vs. Cl diagram.
1500 2000
165
4.4. Extimation of the Rio Junthuma springs total flowrate
Given the particular morphology in which manifestationsalong Rio Junthuma lies, it is possible to compute the totaldeep water flowrate. As measured water flow rate at points Qland Q2 (fig.2) are respectively 106 1/s and 150 1/s, the deepwater contribution should be 44 1/s (~30%).
The chlorides and tritium contents of BW, SA16B and SA37agree closely with this value.
5). Gases
The only two available gas analyses (tab. 2) belong tosprings which are fed by a deep fluid that underwent a boilingprocess during its raise.
Such gases, besides being not representative any more ofthe deep condition, are therefore depleted of the tracecomponents.
In order to estimate a geothermometry or, in any case, tohave an idea of the original reservoir composition it should beadvisable to sample gases from mud pools or fumaroles fed bythe steam produced with the boiling process.
References
[1] APPROVECHAMIENTO DE LOS RECURSOS GEOTERMICOS DEL
SUDOESTE BOLIVIANO - Areas de Salär d'Empexa yLaguna Colorada - Aquater 1979.
[2] GIGGENBACH, W.F. (1980) - Geothermal solute equilib-ria. Derivation of Na - K - Mg - Ça geoindicators.GEOCHIMICA ET COSMOCHIMICA ACTA, 52, 2749-2765.
166
[3] REED, M. & SPYCHER, N. (1984) - Calculation of pH andmineral equilibria in hydrothermal waters withapplication to geothermometry and studies of boilingand dilution. GEOCHIMICA ET COSMOCHIMICA ÄCTA. 48,1479 - 1492.
[4] FOURNIER, R.O. & POTTER, R.W. (1982) - A revised and
expanded silica quartz geothermometer. GEOTHERMALRESEARCH COUNCIL BULLETIN. V. 11, 3 - 9 .
167
GEOCHEMICAL AND ISOTOPIC EXPLORATION OF THEGEOTHERMAL AREA OF PAIPA, CORDILLERA ORIENTAL,COLOMBIA
R BERTRAMI*1, A. CAMACHO**, L. DE STEFANIS***,T. MEDINA**, G.M. ZUPPI***
* Ente Nazionale per 1'Energia Elettrica,Pisa, Italy
**IAN,Bogota, Colombia
***Dipartimento di Scienze della Terra,Università di Torino,Turin, Italy
Resumen-Abstract
EXPLORACION GEOQUIMICA EISOTOPICA DE LA ZONA GEOTERMICA DE PAIPA, CORDILLERAORIENTAL, COLOMBIA.
La explotacion de la zona geotérmica de Paipa (150 km al N de Bogota) serealize durante el période de 1987-1989, obteniéndose los siguientes resulta-dos y conclusiones principales:
I - Las aguas geotérmicas tienen una faciès de cloruro-sulfato sôdico.
II - El componente gaseoso principal es CO , aunque también estân pré-sentes CH , H S y N .
Ill - La composiciön isotôpica estable del agua muestra efectos importan-tes de evaporaciön y de separaciön del vapor. El fraccionamientoisotôpico indica una temperatura entre 200° y 230ÖC.
IV - La presencia de componentes que indican pérdida de vapor geotérmico,taies como NH. , H„ y H„BO„, sugiere 14 2 3 3depôsito profundo de agua a alta temperatura.taies como NH, , H„ y H B0„, sugiere también que existe un4 2 3 3
V - Todos los geotermômetros quimicos senalan una temperatura del depô-sito de 220°C o mas.
El modelo de la zona geotérmica que se saca de los datos geoquimicos eisotôpicos indica que existe, al menos en la parte sur, un protnetedor poten-cial geotérmico.
1 Present address: ENEL Pietrafitta, Perugia, Italy.
169
GEOCHEMICAL AND ISOTOPIC EXPLORATION OF THE GEOTHERMAL AREA OF PAIPA, COR-DILLERA ORIENTAL, COLOMBIA.
In the period 1987-89 the geochemical and isotopicexploitation of the Paipa geothermal area (150 km N of Bogota)was carried out with the following main results andconclusions :I - The geothermal waters have a sodium chloride-sulphate
facièsII - The main gas component is C02, but also CH4, H2S and
N2 are presentIII - The stable isotope composition of water shows important
evaporation and steam separation effects. The isotopicfractionation indicates a temperature between 200° and230° C
IV - The occurrence of components indicating loss ofgeothermal steam, as NH4, H2 and H3BO3 also suggest theexsistence of a high temperature water reservoir at depth
V - All chemical geothermometers indicate a reservoirtemperature equal to or higher than 220° C
A model for the geothermal area, derived from geochemicaland isotopic data, indicates a promising geothermal potential,at least in the southern part.
1 Introduction
The Paipa geothermal district, located 150 km north ofBogota and 250 km east of Nevado del Ruiz volcano (Fig.l),belongs to the Altopiano Cundi-bayacense in the CordilleraOriental; its geological setting, typical of this region,consists of a Mesozoic-Cenozoic substrate, generally actingas a geothermal reservoir, overlying a Precambrian andPaleozoic metamorphic basement (Fig.2). The Cretaceous-Miocenesediments are formed by shales alternated by quartziticsandstones, mudstones and dolostones. Levels of evaporites(Upper Cretaceous) and coal (Eocene) are present (1,2).
170
Figure 1 Geographical location of Paipa geothermal district.The area belongs to the Altopiano Cundi-bayacense inthe Cordillera Oriental.
The volcanic rocks, intruded in previous sediments, arecomposed of an extensive sequence of riolitic and porphiriticlava flows and pyroclastic deposits. The Upper Pliocene toPleistocene sediments are an assemblage of coarse grainedvolcanogenic rocks derived from Cuerpo Volcanico de Oolitasand Pan de Azucar (1,2).
The Paipa geothermal field is strictly connected to theColombian Andes tectonics, which strike the north-southparallel to the Ecuador-Colombia trench. The geothermal field,in fact, is located in the central axis of the Tunjasinclinorium and is limited to the east side by the NNE-SSWEl Salitre fault (1,2,3).
171
BANGS
V W V Vv v v v v v v vv v v v v v v v v v v vV v v v v v v v v v v v v v v vv v v v v v v v v v v v v v v v vv v v v v v v v v v v v v v v v v v vv v v v v v v v v v v v v v v v v vv v v v v v v v v v v v v v v v v v v ,v v v v v v v v v v v v v v v v v v v v/ v v v v v v v v v v v v v v v v v v v v v v/ v v v v v v v v v v v v v v v v v v v v v v v/ v v v v v v v v v v v v v v v v v v v v v v v v v "/ v v v v v v v v v v v v v v v v v v v v v v v v v v/ v v v v v v v v v v v v v v v v v v v v v v v v v/ v v v v v v v v v v v v v v v v v v v v v v v v/ v v v v v v v v v v v v v v v v v v v v v v/ V V V V V V V V V V V V V V V V ..p/ V V V V V V V V V V V V V V V V "/ v v v v v v v v v v v v v v v v v/ v v v v v v v v v v v v v v v v v ,/ v v v v v v v v v v v v v v v ,/ v v v v v v v v v v v v v
/v v >f V VV V l
l V V V\OV V V V N/V V W V//V V V V V N' v v Y_»ri/ v v v v v j
/wv XL/VV wv vv|i v v v v v v v v v v v -j\y / w v w v v " * ' ' ' vl
/ v v v v v v/VW l**it V V V V V V V \/ v v v ff-tlilSfLy v v v v v 1/ v v yM^SS^ v vv v \j V V \ v^l^l^v V V V V V V V V ^/ V V V fââî'v V V V V V V V V V V V V V V V N
PLAYA. W W W . v v v v v V V lv v v v v v v v v v v v v O/
V V V V V V V V V V V V V V | 'v v v v v v v v v v v v v v v *
v v v v v v v v v v w v v v v v >/wvvwvwwvvvwvwvwvwvvwvwvwwwvw/ V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V '/ v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v w v ;/ V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V l/ V W V W V W V V V V V V V V W V W V V V V V W V W V W V V V W '/ V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V '/ V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V '/ V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V '/ V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V '/ v v v v v v v v v v v v v v v v v v v v v v v v v v v v v ;/ V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V '/ V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V j/ V V V V V V V V V V V V V V V V V f V V V V V V l/ V V V V V V V V V V V V V Vv v v v v v v v v v v v v v v v v v v v v v v v vlv v v v v v v v v v v v v v y yV V V V V V V V V V V V V V V V V V i* **M*.'1Xi«y-.V-V V V V V V V V V V V 'v v v v v
' W W W W 1' V V V V V V V V N
£ V V V V V '*V W V N
• 6
/ v v v v v v v v v v v v v v v v v/ v v v v v v v v v v v v v v v v v/ V V V V V V V V V V V V V V V V VJ/ V V V V V V V V V V V V V V V V/ V V V V V V V V V V V V V V V Vv v v v v v v v v v v v v v v v v/ V V V V V V V V V V V V V V V V/ V V V V V V V V V V V V V V V V/ v v v v v v v v v v v v v v vV V V V V V V V V V V V V V V VV V V V Vjv vv v yo".0« °Xv v w vW V
/ V V V V V V V V V N^13 v v v v v v >V V W V fV V VVV V V V >v v v v v v v v v v v v sv v v v v v v v v v v v v s
V V V V V V V V V V V V Nv v v v v v v v v v v v ^V V V V V V V V V V V )V V V V V V V V V V 1 -
V V V V V V V V V V V f tv v v v v v v v v s
' Q H
V V V V V V V V Nv v v v v v v v ^
fcCUERPO^*sVOLCANlCO'^
OLITASl/ v v v v v v v v v v v
/ v v v v v v v v v v s( • V V V V V V V N
/ V V V V V V N' v v v v v y.>
Figure 2 Geological setting of the Paipa area.tip: Cretaceous-Eocene sediments (shales alterned withquartzitic sandstones, mudstones and dolostones).dotted: Upper Pliocene-Pleistocene sediments(coarse-grained volcanogenic rocks).dark: volcanic rocks (riolitic and porphiritic lavas)gray: alluvial sedimentsdash: lake Sochatogalines: main faults
172
The main aim of the present study is the evaluation ofthe chemical and isotopic characteristics of water and gassamples collected by Colombian members of IAN and IAEAexperts, in order to define the relations between thedifferent types of water in this area as well as the probabledistribution of temperature at depth.
2 Field and analytical methods
Ground-based field work was carried out in July 1987 andin August 1989 at all the hot discharges and steaming groundspresent in the area. Samples of cold waters were collectedat the same time from streams and springs adjacent to thethermal discharges. The hot water points (> 50°C) of thisarea received attention during reconnaissance studies of thegeothermal energy potential of Colombia since the early 1980s(4,5); therefore, detailed sketches or models for the Paipageothermal system are available. Altogether 19 water sampleswere collected for chemical and isotopic analyses.Temperature, Eh, pH and conductivity were measured in situusing portable meters. All samples were filtered throughMillipore membrane filters.
The major dissolved constituents were determined atInstitute de Asuntos Nucleares (IAN), at ENEL laboratory(Lardarello) and at Dipartimento di Scienze della Terra ( DST- Torino) by atomic absorption spectrometry (cations), by ionchromatography (anions) and by titration (alkalinity) usingstandard methods. Moreover, some analyses of anions (HC03-,SO4=, S=, PO4=, N03-) have been performed in the field usinga portable spectrometer.
The gas samples from steam vents as well as fromdischarges were collected in evacuated vessels previouslyfilled with 500 ml of 4N NaOH; the gases were analysed inENEL laboratory by a combination of wet chemical and gasChromatographie methods. The non absorbed gases were analysedby chromatography.
Isotopes of water molecule were analysed in the IAEAlaboratory, while Sulphur-34 and Oxygen-18 of dissolved
173
sulphates precipitated with BaCl2 carried out at the Paris-SudUniversity.
The results are reported in tables 1-2 (chemistry) andtable 3 (isotope).
3 Chemical composition of water discharges
3.1 Major elements
On the basis of a preliminary survey of the analyticaldata given in table 1 and reported in figure 3, the majorpart of water can be classified as acid-sulphate-chloride,as defined by Giggenbach (6). The chemical compositionappears to be rather homogeneous and only cations can markthe differences between the cold and hot waters. Hightemperature waters are mainly characterized by thesignificant presence of Na-K, whereas waters from a partialor total superficial origin show a variable Ca-Mg component.
Moreover, the temperature of hot discharges rangingbetween 30 and 75°C does not present a regular arealdistribution, as could be suggested by the groundwater flowinside the Paipa district. On the other hand the Ca andMg content increases from Rio Salitre-La Playa (P08 ) toBanos Termales (P01-P03, P05, P20) following the watermovement, irrespective of temperature.
As shown in figure 4, it is correct to consider P08the closest sample to the chemical end-member of the Paipawaters, even if it indicates an intermediate temperature(50°C). The second pole is formed by environmentaltemperature waters that flow in the shallow aquifers andare characterized by a neutral chloride chemistry. In thiscase, the chemical content is mainly controlled by thehydrolysis of volcanic porphyrites as indicated by samplescollected in the hydrographie network (Q.Honda) or by thesolubility of the Cretaceous-Eocene deposits (Mina deCarbon). However all the samples do not containbicarbonates. A gradual intermixing between two groupsappears.
174
tab. 1 Chemical data of water samples - L indicates laboratory analysis; F, field analysis. Elements areexpressed in p.p.m.
SampleP-01
P-02P-03P-04
P-05
P-06P-07P-08
P-09P-11
P-13P-18P-20P-22OHMC
Year8789878987898789878989878789878789898989898989
TF°C52550652551 872574021 520 173272019 745956 556 338024 8242
51 2720166128185
CondFUS/cm3350035500595004100036304130
2420072
483001018140152
3830040900630020
5670
pH F
6 768ES686869586 47276456 46 84 7705962
736878753 1
Eh F
1052598
-3122010544-42-30150-91-2321473722559829-320
236
ALK F
43
40
41
335265OB
43
1 842406505
CondL
40000390003300040000
51900
42002100024700
95200004400049500
158400041500
pH L
6 6
65
72
657747
52
64
7673
Ca++1349014480129832116363033322154913127670111
450
Mg++2316151615163451312
14046325
5806
1
17154
05300
Na+
146001120013300105001250011000963720
91155300
1478131770813000
1305223
11000102001500
252
K+
23001200365012002900120015977
23755923
100015931200187073
1100950155046
HCQ3L564548425043233529
26 9092650456 61 56444045
Cl-580054005700540058005300438164
3698270043
32707430480053202756
5300520072553
S04 =
19700182001840019200182001840014351520
10100690023
1031823814162502361915
8121900018500750
14500
H2S
1 41 4
! 6
081 6
1 51 5
Si024847564260435
85824828615834539948
4340
13511 6118
NH4 +
72
35
83
32
468252
U+
70
90
804
442
100
2
8075102
1 5
Fe08804094050750402049030503503
0303030509
1550
Sr++
2611
04
07
04
03
041 303
08
Ba-H-
1010
1502
12
03
0515
AI tot
06
0030 11
02004
008
0509
190
B53
33
28
25
421 61 646
03021
33
F-
17421429163303007172429
10218537083329
32616
Br-
25
221905
100 13
21
1825
N03-
08
18
24
1 4
0746
65
1 609771 7
P04 =
55
7
33031 602
11 5
02330723
1 2
tab. 2 Chemical data of gas samples. Analyses are expressed in per cent,
Sample
P-01
P-02
P-03
P-06
P-07P-13
P-13aP-20
Year
87898789878987898787898989
H2
88
0.06
0.009
0.05-
0.0010.0090.006
02+ Ar
0.233.290.057.350.061.430.1
0.770.070.110.120.03
N2
1.411.80.33
21.810.3
0.321 01
16 860.371.661 032 16
CH4
0.050.770.020.780.010.060.294.370.060.610.060.79
C02
98.384.14
99.670
99.698.11
98.677.599.597.6
98.7997
13.94
H2S
0.021
0.027
0.0550.012
—
-
:0.003
Happm
10.3
0.82
38
5.8
C2H6ppm
2.3
0.63
15
3 3
COppm
4.7
2.16
2.69
n.d.
0.717
tab. 3 Isotopic data of water samples, referred to internationalstandards.
Sample
P-01
P-02
P-03
P-04
P-05
P-06P-07P-08
P-09P-11
P-18P-20P-22Q.H.M.C.
Year
87898789878987898789898787898787898989898989
5 D%. SMOW
-68.0
-67.2
-67.0
-72.0
-68.4-63.2
-67.4
-72.6-67.4-68.0
5 180%. SMOW-6.55-6.61-6.68-6.89-7.05-7.32-9.47-9.67
-8-7.91
-15.01-8.44-6.54-6.08-8.69-9.97-9.99-6.72-6.9-8.73-9.25-9.57
TT.U.
0.3±0.2
0.0+0.2
0.3+0.2
0.4+0.2
2.0+0.23.7+0.3
0.4±0.2
0.3+0.20.010.20.0+0.2
5 34S(S04)
8.90
10.20
8.00
9.5011.00
10.00
9.109.70
10.307.60
5 180(S04)
4.0
6.4
11.5
7.217.0
6.0
4.95.1
13.5
15.0
100
100CI
100
Figure 3 Relative HCO3, Cl, SO4 contents (mg/kg) of waterscollected in 1989 and listed in table 1. The majorpart of waters can be classified asacid-sulphate-chloride.
176
8\ 100O/
Figure 4 Particular of relative Na+K, Mg, Ca contents. Onlycations can mark the differences between surface anddeep waters. Sample P08 could be considered ratherclose to the chemical end member of Paipa deep waters.
The S:C1 ratios are smaller than those of the magmaticdischarges in the Nevado del Ruiz volcano (7-10) whichsupports the assumption that the chemistry of these watersis due to the partial absorption of deep gases (9, 10).
Although the water temperature is higher than 30 °C,thermal spring discharges show a low content of dissolvedsilica. This could be explained either by an aquifer matrixrelatively depleted in quartz and silicates, or by watersheated without reaching complete chemical equilibrium. Thissecond process needs slow reactional kinetics (11, 12)which could be favoured by the presence, in the volcanicrocks, of quartz phenocristals (1-3). The undersaturationwith respect to quartz no longer appears in the equilibrium
177
diagrams (Fig.5) of aqueous phases at a temperature of60°C. Therefore, as at this temperature, which correspondsto the mean temperature of all the Paipa thermal springs,waters are always saturated with respect to quartz, it iscorrect to consider that in rising up, the fluids couldbe thermically equilibrated with external conditions. Thenew equilibrium allows precipitation of exceeding SiO2, asindicated by the presence of hydrothermal calcedony in thesedimentary formations.
16.0
1 2.0
m01o
80
4.0
0.0
Q U A R T ZS A T U R A T I O N
OH
A M O R P H O U SS I L I C A
S A T U R A T I O NMC
-6.0 -4.0 -ao
Figure 5 Equilibrium diagram of acqueous phase at 60°C. Thefluids are equilibrated with external conditions: thisallows precipitation of excess SiO2 as indicated bythe presence of hydrothermal calcedony in thesedimentary formations.
178
At the same time, figure 6 showss the different stagesof maturity reached by waters during dynamic hydrothermalalteration. The most important alteration process consistsof attack by acids formed through absorption of deep gasesinto groundwaters and leads to rock destruction (6, 11,12). In the Paipa geothermal district, separation of theC02 rich vapour phase and its absorption into water generatean environment of increasing acid alteration. However, whenthe volume of waters reacting with gases is small (P08),or when the exchange reaction time between the water andthe gases is long (Pll), the continuous gas flow permitsthe waters to reach equilibrium with C02, leading to arapid increase in their reactivity. Besides, when the gasvolume is smaller than that of water (P18) or when theexchange reaction time is short (QH), the amount ofdissolved CO2 is reduced, decreasing the reactivity of thewater towards the aquifer matrix. The principal processes
Na1JÛOO
so
/ / / / s h a l l o w X\/ f i f w a t e r s \
/ 220°/ /Z00° / 11 \/ / / / 60*
100
Figure 6
50 /
QH
Relative Na, K, Mg, content of thernIsotherms calculated according to Giggenbach equations(6) are given together with the full equilibriumcurves.
179
affecting geothermal fluids during their further rise tothe surface are summarized in figure 7, where dissolvedC02 is reported with respect to discharge temperature.Dissolved CC>2 shows the same order of magnitude in hot asin cold waters. Therefore, waters do not precipitate thetravertine deposits in all the geothermal district, ascould be suggested by the high partial pressure. In thecase of Mina de Carbon and of water point P06 (steamheated pool), superficial oxidation of H2S provides a sourceof acidity and an excellent environment for intense rockleaching. The waters thus show a high content of dissolvedsulphate.
o-ou
MC08
-1-
-2-
-3 —
11
O
o
4
0
<00!180
500
3
020
J2L10 30 50 70 t C
Figure 7 Dissolved CÛ2 as fonction of discharge temperatures.
3.2 Trace elements
The most significant trace elements in Paipa waters,halogens (F and Br), B and Li, could complete interpretationof the water origin and circulation. By including analyticaldata obtained during a comprehensive study of Nevado delRuiz gases (7, 9, 10, 13), the relative Cl, Br and F
180
contents are compared in (Fig. 8). The most striking featureof this plot is the pronounced mixing line between theNevado del Ruiz gases and magma and marine evaporites (14).The volcanic pole is formed by samples P06 and Pll, whereasP04 represents the evaporitic pole. The thermal watersamples appear to be partially enriched in F and Cl,confirming the attack of strong acids (HC1 and HF), formedthrough the absorption of magmatic gases, on the aquifermatrix. The destruction of evaporitic layers follows aselective evolution, thus enriching thermal waters in thedissolved chlorides. However, agreement of Br:Cl and F:C1ratios is not merely a coincidence. Other "markers"dissolved in the waters, such as B and Li, plotted in
Nevado del RuizV V
100 Br
Figure 8 Relative Cl, Br, F contents plot. The most strikingfeature is the pronounced mixing line between magmaticand evaporitic points.
181
figure 9 confirm the double origin of fluids. The moststriking feature of this plot is the pronounced mixingline described by cold groundwater collected inside thegeothermal area and trending towards the Li pole. SimilarlyB:C1 and SO4:C1 ratios suggest that acid waters are mixturesin the depth of water from an acid-sulphate-chloridereservoir, with various proportions of shallow groundwaters.The principal source of sulphate and halogens may be doneby high-temperature deep gases interacting with thesedimentary deposits that cover the reservoir.
CI/100
100
gases f rom N e v a d odel Rui z
m a r i n ee v a p o r i t e s
100 Li
Figure 9 Relative Cl, Li, B contents plot. The double originof fluids is confirmed. The diagram emphasizes themixing at various proportions of waters from anacid-sulphate-chloride reservoir and a shallowaquifer.
182
marineevapor i tes
Nevado del Ruim a g m a
Paipa therma lw a t e r s
Nevado del Ruizgases
100 50B
Figure 10 Relative Cl, B, SO* contents plot. The principalsource of suphate and halogens may be the hightemperature deep gases interacting with thesedimentary deposits.
However the lack of any intermediate water suggeststhe existence of two separate circulation systems: a deepone, concerning water rock interaction and involvingsedimentary rocks of Cretaceous-Eocene age, and a shallowone, where cold waters interact with volcanic rocks oradsorb deep gases of steam vents.
4 Gases chemistry
The areal distribution of gas discharges shows the absenceof steam vents in the southern sector of the Paipa areainside the Cuerpo Volcanico de Olitas. Moreover, a preliminaryanalyses of gas chemistry (Tab.2) indicates that the
183
discharged gases are mainly formed by C02, followed by N2,and show a rather constant chemical content. Because of thecomparatively high discharge temperatures, the relativeproportions of more soluble (H2S) to less soluble (H2, N2 andCH4 ) gases in the free gas phase sampled are likely to besimilar to those found in the deep fluid, thus permittingthe evaluation of the equilibrium temperatures.
The O2+Ar content is normally lower than the erroraffecting samples during collection. The contribution ofatmospheric gases seems fairly small and is strictlycontrolled by hydrogeological conditions; in fact, in 1989,collection took place after an heavy wet period and surfacecold waters significantly diluted deep fluids, as pointed outby the high N2 content. However two major groups of gasdischarges may be distinguished (Fig.11). The samplescollected in the Rio Salitre sector indicate a high H2 andsaturated hydrocarbons content. The presence of H2 and CH4observed in all discharges of this sector suggest a veryhigh temperature of interaction with deep matrix and arelatively short residence time of the rising fluids at acomparatively lower temperature. In addition, samplescollected in the northern sector show the presence of volatilesulphur compounds. The H2S system is controlled by the redoxconditions during the upward movement of fluids; in thiscase, a short residence time of gases in a likely oxydizingenvironment near the surface and a short interaction of thefluids with areated groundwaters is suggested. Moreover, Hecontents are relatively higher than in the Rio Salitre sector,indicating a longer residence time of fluids in the depth.In other words, the reservoir cap must be more important andshow a lower permeability in Paipa than in the southernsector, where the holocene aquifer, although probably havinga higher storage capacity, does not show a very significantthickness.
Thus, the chemistry of the gases collected in thegeothermal district provides additional information about themineralogical matrix of the deep reservoir, about thefracturation intensity of the upper system and the hydraulicconductivity of the cap.
184
CO./100100
Figure 11 CO2/ N2, CH4 relative contents plot. Full andempty circles indicate respectively gas samplescollected during 1987 and 1989. Two major groups ofgas discharges may be distinguished: samples collectedin Rio Salitre district (P06, P07 and P13 ) and inBanos Termales (P01, P02, P03 ).
The spatial distribution of gases probably reflects therelative proportions of magmatic gases interacting with theshallow groundwater layer overlying the carapace. The smallamount of liquid stored in the Rio Salitre area only providesa low buffer capacity with respect to chemical variation.
185
5 Isotopic composition of waters
5.1 Water molecule isotopes
The isotopic composition of water samples is given intable 3 and the results for Deuterium and Oxygen-18 arereported in figure 12. The best fit for sample data pointsis a line described by the equation
62// = (1.44±0.20)6180-(S7.8±0.8)
intercepting Local Meteoric Line at 62 H = 74.0 and&18O = -10.5. Using the correlation linking 18O of localprecipitation to altitude, as reported by Torres et al.(15), an "isotopic altitude of recharge" of 2700 m isobtained. This altitude is in agreement with the meanelevation of Cordillera Oriental in the Paipa district.
-40.
-60-— E X C H A N G E WITH C02 (=50%)e
-80- 5 D = ( 1.44 ± 0.20) 6180 - (57.78 1 0.7?)
^ R E C H A R G E
-100.
-1 5 -10 , 18_0 0
— 5
Figure 12 Deuterium versus Oxygen-18. Sample P06 that doesnot fall on the evaporation line as all the othersamples is interpreted as a continuous isotopicexchange ( of about 50% ) between «O of localprecipitations and deep CO2.
186
The slight slope of the 62H-618o correlation line couldbe explained by the isotopic enrichment for steam heatingand non-equilibrium evaporation of deep waters at atemperature higher than 220°C. The waters have the samehydrogeological origin, as indicated by the good correlationcoefficient (r? = 0.95). A similar conclusion could bedrawn from figure 13 where 5L8O is reported in relation tochloride content :
ÔI8O = 0.75[CZ"]- 1CT3- 10.1
Moreover, the upward movement of fluid is accompaniedby boiling and phase separation, and secondarly by mixingwith cold waters. Nevertheless the lack of observed steampoints in the district allows for neither quantificationof the steam production nor the measurement the isotopiccomposition of the separated water phase. Considering thewater collected in P08 as very close to the end-member ofdeep fluids, and assuming a cooling process as boiling and
-4
-6
-8
-10
-12
-14
-16
18 l -\ ~3
ô O= (0.75CI ) 10-10.1
- -1CI mg I
2000 4000
Figure 13 Oxygen-18 versus chloride content.
187
multistage steam loss, the vapor isotopic composition shouldbe -15.2 for 618O and -82.5 for o2//. The liquid-vaporseparation strongly partitions dissolved salts, increasingin our case to about 1.2 times the chloride concentrationin the Paipa thermal waters.
The cooling processes are recognizible in the centralsector of Paipa district, where mainly gaseous vents arepresent, not only because of the major role on steam lossplayed by Rio Salitre fault but also because of theproximity to the hydrogeological recharge areas and thusbecause of superficial groundwater movement absence.Moreover, in the northern area the mixing with coldgroundwaters takes place.
However, sample P06 does not follow the scheme describedabove. This sample was collected in a small pool in theRio Salitre sector. The pool is filled with surface watersin relation to the meteorological and hydrogeologicalcycles. Sample P06 could be interpreted either as acontinuos isotopic exchange between ISQ of summerprecipitations (6180~-9%.) and CO2 or as Deuterium enrichmentof an hypothetical residual vapour phase in the wholegeothermal system.
The first hypothesis is in agreement with the C02discharged in the pool; the gas volume is larger than thatof the water. Taking in account a mean isotopic compositionof 18O in C02 equal to -14 %. (16) and a f ractionationfactor of 42.5 %. valid for a temperature of 15 °C (meanyearly temperature), it is correct to considerprecipitations isotopically depleted of 5.8 %. and exchangedfor 50%.
Due to the lack of vapour steam in the area, aspreviously mentioned, it is not possible to take in accountthe second hypothesis. Moreover, the Hydrogen exchangekinetic seems faster than the required and, at the sametime, the reactional temperature needed (more than 300°)is not in agreement with that evaluated in Paipa geothermaldistrict.
188
Tritium activity in thermal and cold waters is alwayslower than in Bogota precipitations (15, 17) during thesame period. The Tritium low level indicates a nonsignificant component of present precipitation, defining inthis way a mean residence time higher than 50 years. OnlyP06 shows a Tritium content close to that of actualprecipitations, confirming the hypothesis of an isotopicexchange betweeen meteoric waters and C02 steam vents.
5.2 Isotopes in dissolved sulphates
In table 3 the isotopic composition of stable isotopesin dissolved sulphates is reported. The 534S values ofthermal waters are consistent with a mixing between anevaporitic mineral source and a deep sulphide; but M.C.sample appears to be quite enriched in the heavy isotopeif the oxidation from a secondary sulphide is considered(Fig. 14).
18
C r e t a c e o u s e v a p o r i t e s
14
10 — O,22
O O20
" isM C
O
Figure 14 Sulphur-34 versus sulphate-chloride weight ratio.
189
In fact the isotopic composition of Sulphur with amagmatic origin ranges between 0%. and + 2%. vs. C.D. (11,12) and the 634s of Cretaceous evaporites, like those presentin the subvolcanic stratigraphy, ranges between +15%. and+ 18%. vs. C.D. (17).
Dissolved sulphate formed by the oxidation of subsurfacesulphides preserves the original isotopic signal; in thisway the solid sulphide pole in the Paipa aquifer matrixshould have a 634s of about +8%. (M.C. sample). Thiscomposition should be considered as a result of theabsorption of volatile Sulphur compounds inside carbonlevels of the Guadua formation, which forms the cap ofthe geothermal reservoir.
In a 634s vs. 618O diagram (Fig. 15) all representativepoints of thermal waters fall on a straight line. Becauseof its slope (about 1.5), this line could be consideredas a reduction line. It cannot be considered as a
+ 20
oto + 15
+ 10
+ 5
22 • 6
MC
+• 5 +10 + 15„180 O (so4)
Q/xoo
Figure 15 Sulphur-34 versus Oxygen-18 of dissolved sulphates.Star indicates isotopic composition of Cretaceousevaporites. All thermal waters fall on a straightline with a slope of about 1.5, that could beconsidered as a reduction line. Samples at lowtemperature (P04, P06, P22, M.C.) do not achieveisotopic equilibrium during subsurface oxydation.
190
precipitation line of gypsum and anhydrite since the thermalwater always remains undersaturated with respect to theseminerals. Thus, this line is interpreted as a mixing linewhere the end-members have to be identified in theCretaceous sulphates and the geothermal gases. Duringsubsurface oxydation, the isotopic equilibrium betweenmolecular Oxygen and water is not achieved for lowtemperature samples. On the basis of classical equilibriumequations (18, 19), the percentage of molecular Oxygenranges between 90 and 100%. Nevertheless it is possiblethat the C02 Oxygen participates to the sulphide oxydation.Sample M.C. indicates that the oxydation takes placeinstantaneously at an environmental temperature and a lowpH. Under such conditions the equilibration time forisotopic exchange reaction is a few hours.
The other water samples indicate reactional conditionsclose to the equilibrium and a participation of waterOxygen of more than 50%. In this way the reaction half-liferanges between 50 ( P08 ) and 300 years ( P01 ). The timeherewith obtained is in agreement with water age calculatedby Tritium content, confirming once more that the aquiferwhere all isotopic exchanges take place is confined andhas a low permeability.
Similar conclusions can be drawn from figure 16, wherethe 180 content of dissolved sulphates is plotted versusthat of corresponding waters. The isotherms representingthe mean fractionation factors for isO exchange betweensulphate and water are determined according to Lloyd (18)and Mizutani & Rafter (20) equations. The plot of thermalwaters indicates a close approach to equilibriumtemperatures, similar to those evaluated by the means ofwater isotopic enrichment in Deuterium and Oxygen-18.Moreover, cold waters oxidizing close to surface conditionswith molecular Oxygen indicate temperatures generally lowerthan those measured at the discharges.
191
-5
Figure 16 Oxygen-18 in dissolved sulphates versus Oxygen-18in corresponding waters. The isotherm are determinatedaccording to Lloyd (18) and Mizutani & Rafter (20)equations.
6 A model for the Paipa hydrothermal system
The use of classical geothermometers Na-K (21), Na-K-Ca-Mg(22) and CO (23) defines a reservoir temperature close to200 °C (tab.4). As expected, the same geothermometers appliedto cold water indicate an equilibrium temperature similar tothe one measured at the discharge. Only TNK2 gives for coldgroundwaters of Cuerpo Volcanico (Pli), a temperature indisagreement with the one measured, because of Na excess involcanic rocks.
tab. Reservoir temperatures defined by classical geothermometersSampleP-01P-02P-03P-04P-05P-06P-08P-11P-13P-18P-20P-22Q.H.M.C.
TQA100959540100809010095955550-
TNK20021020520021075190230200195200300210
TNKCM2402452401402405526550220220604040
T C02--105-
—-—80
-—
T CO250230240-
—--
175-200-—
T 18 0170-13065
11510
15050
-16015055
-45
192
Silica geothermometer, as previously shown, indicates alower temperature than the one obtained with the othergeothermometers. Thus, the Si02 content of thermal watersdoes not reflect the fluid-rock equilibrium reached in thereservoir and underevaluates dissolution temperature. By theuse of Triangular diagram (6), evaluation of analytical Na,K, Mg contents allow the definition of deep equilibrationtemperatures and the effects of water mixings (Fig.6).
The position of all data shows either a trend towardsthe partial equilibrium or a complete immaturity. Only watersample P08 reaches full equilibrium at a temperature of about250 °C. Because of the faster rate of equilibration in thesystem K-Mg with respect to K-Na, a difference of about 40°Ccan be evaluated between conditions in the shallower levels(K-Mg) and those in the depth (K-Na) for the Paipa thermalwaters. Gas geothermometers are affected by surface orsubsurface oxidation of H2S, CH4 and other non-absorbed gases.Moreover, mixing with cold waters leads to an incorrectevaluation of the last equilibrium temperature. Nevethelessfor sample P13 collected in a steam vent, temperaturesobtained with gas geothermometers must be compared with thoseevaluated by dissolved geothermometers in the close area(P08); in this sector of Paipa geothermal district, the deeptemperatures are always higher than 200°C as indicated by COgeothermometer ( 23 ).
The water maturity index (6) is always above 2.0;therefore it is correct to evaluate the CO2 fugacity in thegeothermal system using K, Mg, Ca relative contents. Allthermal waters are aligned and trend towards sample P08,confirming once more, that it could be considered like, orvery close to, deep end-member (Fig.17).
For the Paipa geothermal system a fugacity of 100 barscould be taken into account. Considering a water column inequilibrium with such a pression in an open system, a depthof about 1000 meters must be evaluated.
All thermal waters have a constant value of mole ratiosn CO2/n H20 for vapour phase (about 5 ), indicating homogeneousconditions inside the system. Starting with equation 30 of
193
40 60 100 200
KC
5 _
-1
/° \t(c)
0 1
_ 100
7 L (Km)
Figure 17 Evaluation of CO2 fugacities in Paipa area by usingK, Mg, Ca contents in discharge waters according toGiggenbach equations ( 6 ).
Giggenbach ( 6), water fugacity of about 20 bars could beobtained, indicating a reservoir temperature higher than210°C.
Thermal waters are in equilibrium with minerals formingthe sedimentary cap (tab.5). Using the heat and mass balanceequation, the composition of waters before separation wererecalculated. In this case at a temperature of 220°C, watersreach, or are close to, equilibrium with respect tohydrothermal or metamorphic minerals.
The influence of deep uprising fluids is higher in BanosTermales (northern sector) than in La Playa (southern sector),as indicated by the presence of discharges at hightemperatures.
194
tab. 5 Saturation index with respect to some sedimentary and metamorphic minerals.
SampleP-Q1P-02P-03P-04P-Û5P-06P-08P-11P-18P-20
Anhyd.-0.59
-0.618-0.51
-1.769-1.073
-3.6-1.391-3.199-0.662-0.589
Laum.-3.245
-6.15-—
-1.193-
-20.843-
0.589-
Low Alb.-1.161
-2.69—-
-0.58—
-9.741—
0.723-
Microcl.-0.16-1.68
——0.26
—-8.86
—1.7
-
K Mica-3.167
-7.08—-
-2.009—
-24.318—
1.883-
Analc.-3.009-4.468
--
-2.044—
-11.347-
-1.07-
Calced.0.240.18
-0.02-0.26
00.250.040.430.19
-0.04
Prehn.-8.421
-11.5——
-3.449—
-30.027—
-3.677-
Calc.-1.91
-2.086-1.439-3.744-1.272
-6.5-4.449
-2.34-1.579-1.651
Quartz0.6320.5660.2980.2980.313
0.750.4070.9160.5790.287
(Ji
Moreover, the southern sector, being closer to the aquiferrecharge area, is characterized by the presence of diffusesteam vents for the local hydrostatic conditions and sedimentshydraulic conductivity. The low upward movement of gases, asindicated by the low fracturation intensity of peliticformations and the depth of resevoirs as previously definedcould force the uprising gases, mainly the more reactiveones, to equilibrate with the new thermal and geochemicalconditions.
This model is valid for the sectors where volcanic rocksare covered by a significant thickness of sediments. Whenvolcanic rocks outcrop (Cuerpo Volcanico de Oolitas and CuerpoVolcanico Pan de Azucar) steaming grounds and thermaldischarges are completely absent.
The temperature difference between fully equilibratedwaters (P08 at 250°C) and partially equilibrated waters(P01-P03 at 220°C) allows the evaluation of chemical andisotopic composition of the deep end-member:
a2!! = -65.5; 6180 = -5.80; Cl- = 5720The potential source for high chloride content may include
the dissolution of evaporites within the subvolcanicstratigraphy, incorporated into fluids.
7 Conclusions
The above geothermal model was constructed for the Paipadistrict assuming uniform permeability conditions of thesedimentary cap; therefore, it is likely to indicate onlythe potential distribution of fluids inside the system.
The actual model is governed by the scarce knowledgeabout the local stratigraphy and the presence and rate offaults and fissures. Because of these geological problems,exploratory drilling in Paipa and La Playa sould be seriouslyconsidered.
196
References
(1) RENZONI, G., (1967) Geologia del Cuadrangolo 3-12 Tunja.Bol. Geol., 24, 2, Ingeominas, Bogota, 31-48.
(2) JULIVERT, M., (1970) Cover and basement tectonics in theCordillera Oriental of Colombia, South America, and acomparison with some other folded chains. Geol. Soc. Amer.Bull., 81, 3623-3646.
(3) JULIVERT, M., (1973) Les traits structuraux et l'évolutiondes Andes Colombiennes. Rev. Géogr. Phys. et Géol. Dyn.,25, 143-156.
(4) GEOTERMICA ITALIANA (1981) Estudio de reconocimiento delos recursos geotermicos de Colombia. Informe de la fasede campo, OLADE, Bogota.
(5) JAPAN CONSULTING INSTITUTE (1983) Feasibility study reportof Geothermal Power Plant in Colombia. I.C.E.L., Bogota(unpublished).
(6) GIGGENBACH, W.F., (1988) Geothermal solute equilibria.Derivation of Na-K-Mg-Ca geoindicators. Geoch. Cosmoch.Acta, 52, 2749-2765.
(7) BARBERI, F., MARTINI, M. , ROSI, M. (1990) Nevado delRuiz volcano (Colombia): pre-eruption observations and theNovember 13, 1985 catastrophic event. Journ. of Volcan,and Geotherm. Res., 42, 1/2, 1-12.
(8) STURCHIO, N.C., WILLAMS, S.N., GARCIA, P.N., LODONO C.,A. (1988) The hydrothermal system of Nevado del Ruizvolcano, Colombia. Bull. Volcanol., 50, 399-412.
(9) GIGGENBACH, W.F., GARCIA P., N. , LODONO C. , A., RODRIGUEZV., L.A., ROJAS G., N., CALVACHE V., M.L. (1990) Thechemistry of fumarolic vapour and thermal-spring dischargesfrom the Nevado del Ruiz volcanic-magmatic-hydrothermalsystem, Colombia. Journ. of Volcan, and Geotherm. Res.,42, 1/2, 13-39.
197
(10) STURCHIO, N.C., STANLEY, N.W. (1990) Variations inchemistry of acid-sulphate-chloride springs at Nevado delRiuz volcano, Colombia: November 1985 through December1988. Journ. of Volcan. and Geotherm. Res., 42, 1/2,203-210.
(11) HENLEY, R.W., TRUESDELL, A.M., BARTON, P.B.Jr., WHITNEY,J.A. (1984) Fluid-mineral equilibria in hydrothermalsystems. Reviews in Economic Geology, vol.1, Society ofEconomic Geologists, 267 pp.
(12) GIGGENBACH, W.F., GONFIANTINI, R. , JANGI, B.L.,TRUESDELL, A.H. (1983) Isotopic and chemical compositionof Parbati valley geothermal discharges, NW-Himalaya,India. Geothermics, 12, 199-222.
(13) WILLAMS, S.N., STURCHIO, N.C., CALVACHE V., M.L., MENDEZF., R., LODONO C., A., GARCIA P.,N. (1990) Sulphur dioxydefrom Nevado del Ruiz volcano, Colombia: total flux andisotopic constraints on its origin. Journ. of Volcan, andGeotherm. Res., 42, 1/2, 53-68.
(14) SONNENFELD, P. (1984) Brines and evaporites. AcademicPress Inc., 613 pp.
(15) TORRES, E., JIMENEZ, G., OBANDO, E., ALAYON, E., SANCHEZ,L. (198 ) Evaluacion del acuifero de la sabana de Bogotautilizando tecnicas isotopicas. In "Estudios de Hidrologiaisotopica en America Latina". I.A.E.A., Vienna, TEC-DOC502, 203-213.
(16) BERTRAMI, R., CAMACHO,Investigacion geotermicacolombo-ecuatoriana. Casos de
A., ZUPPI,de la
Chiles-Tufino
G.M. (1990)cordillera
y de Paipa.Informe final. I.A.N., Bogota, 35 pp.
(17) IAEA (1990) Environmental isotope data n°9: World Surveyof Isotope Concentration in Precipitation (1984-1987).IAEA, Vienna, Tech. Rep. Ser. 311, 188 pp.
(18) LLOYD, R.M. (1968) Oxygen isotope behaviour in thesulfate-water system. J. Geophys. Res., 73, 6099-7110.
(19) CHIBA, H., SAKAI, H. (1985) Oxygen isotope exchange ratebetween dessolved sulfate and water at hydrothermaltemperatures. Geoch. Cosmoch. Acta, 49, 993-1000.
198
(20) MIZUTANI, Y., RAFTER, T.A. (1969) Oxygen isotopiccomposition of sulphates. Part 3: Oxygen isotopicfractionation in the bisulphate ion-water system.N.Z.J.Sci., 12, 54-59.
(21) ARNORSSON, S., SVAVARSSON, H. (1985) Application ofchemical geothermometry to geothermal exploration anddevelopment. Geoth. Res. Council Trans., 9, 293-298.
(22) FOURNIER, R.O., POTTER, R.W. (1979) Magnesium correctionto the Na-K-Ca chemical geothermometer. Geoch. Cosmoch.Acta, 43, 1543-1550.
(23) D'AMORE, F., PANICHI, C. (1980) evaluation of deeptemperatures of hydrothermal systems by a new gasgeothermometer. Geoch. Cosmoch. Acta, 44, 549-556.
(24) BERTRAMI, R., CIONI, R., CORAZZA, E., D'AMORE, F.,MARINI, L. (1985) Carbon monoxyde in geothermal gases.Reservoir temperature calculations at Lardarello (Italy).Geoth. Res. Council Trans., 9.
199
ISOTOPIC COMPOSITION AND ORIGIN OF THERMAL ANDNON-THERMAL WATERS FROM THE MIRAVALLESGEOTHERMAL FIELD, COSTA RICA
W.F. GIGGENBACHDivision of Chemistry,Department of Scientific and Industrial Research,Petone, New Zealand
R. CORRALES, L. VACAInstitute Costarricense de Electricidad,San José, Costa Rica
Resumen-Abstract
COMPOSICION ISOTOPICA Y ORIGEN DE LAS AGUAS TERMALES Y NO TERMALES DEL YACI-MIENTO GEOTERMICO DE MIRAVALLES, COSTA RICA.
Si se va del norte al sur a lo largo del yacimiento geotérmico de Hirava-lles, en Costa Rica, se comprueba que los contenidos de deuterio y de oxige-no 18 de las aguas en movimiento que représentai! sistemas freâticos locales,
o odisminuyen un 25 /oo y un 3 /oo respectivamente a lo largo de uiiadistancia de solo 40 km. Las aguas fluviales con flujos de il 000 kg/s, quetransportan la mayor parte de las aguas de escorrentia de la zona, tienen com-posiciones isotâpicas parecidas a las del agua freâtica del norte, lo que su-giere que los vientos del norte hacen que la mayor parte de la precipitaciönse produzca en la ladera norte de la cordillera. El contenido de deuterio delos caudales de agua de pozo geotérmico y cloruro natural se corresponde conel de las corrientes del norte, lo que indica que se produce un proceso derealimentaciön a partir de las aguas freâticas del norte cuya cantidad es pré-dominante. Las aguas termales con mayor contenido de cloruro se enriquecencon oxigeno 18, lo que posiblemente se debe a que se produce un intercambioisotôpico con la roca. Sin embargo, a la vista de los recientes hallazgos ensistemas hidrotérmicos situados a lo largo de plaças con limites convergentes,se considéra como mas probable que el enriquecimiento con oxigeno 18 se deba ala mezcla del 15 al 20% con un agua "andesitica" cuyo contenido en deuterio es
o o-20 /oo y +10 /oo en oxigeno 18. La absorciôn de esta agua andesi-tica, que se desprende en forma de vapor de las masas profundas de magma enproceso de enfriamiento y cristalizaciôn, conduce inicialmente a la formacionde aguas âcidas cloruro-sulfatadas. La llegada de estas aguas a un pozo geo-térmico tras el proceso de profundizaeion sugiere que la conversion de aguascloruro-sulfatadas inmaduras en aguas de cloruradas neutras maduras se produceen nivelés comparât!vamente poco profundos.
201
ISOTOPIC COMPOSITION AND ORIGIN OF THERMAL AND NON-THERMAL WATERS FROM THEMIRAVALLES GEOTHERMAL FIELD, COSTA RICA.
Progressing from the northern to the southernextent of the Miravalles Geothermal Field, Costa Rica, thedeuterium and oxygen-18 contents of stream waters,representing local groundvaters, decrease by 25°/oo and 3°/oorespectively over a distance of only 40 km. River waters withflows >1000 kg/s, carrying most of the drainage from the area,have isotopic compositions close to those of northern ground-water suggesting that most of the precipitation, carried bynortherly winds, falls to the north of the Cordillera. Thedeuterium content of geothermal well and natural chloridewater discharges agrees with that of northern streams,pointing to recharge from the quantitatively predominating,northern groundwater. The higher chloride thermal waters areenriched in oxygen-18 possibly due to isotopic exchange withrock. In view of recent findings on hydrothermal systemslocated along convergent plate boundaries, however, theenrichment in oxygen-18 is considered more likely to be due tothe admixture of 15 to 20% of an "andesitic" water with adeuterium content of -20°/oo and oxygen-18 of +10°/o». Absorptionof this andesitic water, released as vapor from cooling andcrystallising magma bodies at depth, leads i n i t i a l l y to theformation of acid sulfate-chloride waters. Incursion of suchwaters into a geothermal well after deepening suggests thatthe conversion of immature sulfate-chloride to mature neutralchloride waters takes place at comparatively shallow levels.
INTRODUCTION
The Miravallec Geothermal Field, Costa Rica, lies some 125 kmto the NE of the capital San José. Following an initialassessment in 19G3 by United Nations experts, more detailedgeochemical investigations into its potential for electricpower production began in 1975 (Gardner and Corrales, 1977).These studies suggested that a suitable geothermal reservoirwith temperatures of around 240°C underlies the study area.Three wells were drilled from 1979 to 1980, all provingproductive. Until 1990 a further five wells were completed
202
comprising a total electric power production potential ofabout 40 MWe.
The present investigation was carried out within the frameworkof the IAEA Coordinated Research Program on the Application ofIsotopic and Chemical Techniques to Geothermal Exploration inLatin America. Most of the stream and river samples werecollected by S Arnorsson and R Corrales during a mission ofthe former in September 1985. The isotopic analyses of thewaters were carried out at the IAEA laboratories in Vienna,the geochemical data for thermal waters used here are thosereported by Giggenbach and Corrales (1993).
The major aim of this study was the application of isotopictechniques to obtain information on possible source componentsmaking up the thermal discharges and on the distribution andmovement of the waters within the Miravalles geothermalsystem.
THE ISOTOPIC COMPOSITION OF THERMAL AND NON-THERMAL WATERS
The isotopic composition of samples of thermal and non-thermalwaters collected from the Miravalles Geothermal Field aregiven in Table 1. For non-thermal waters estimated flow ratesare shown, they are u^ed to distinguish rivers (R), with flowsfclOOO kg/s, from streams (Q). For thermal waters, dischargetemperatures, in °C, and chloride contents, in mg/kg, aregiven. On the basis of analytical data reported by Giggenbachand Corrales (1991), thermal waters are also classified interms of their major anion into chloride (c), bicarbonate (b)and sulfate (s) waters. The well discharge compositions areweighted means of separated water and steam samples and,therefore, represent total discharge compositions. They alsorepresent averages of the compositions reported by Giggenbachand Corrales (1991). The samples are arranged with regard totheir geographic distribution from north to south, samplelocations are shown in Fig. 1.
203
TABLE 1The isotopic composition of river (R), stream (Q), thermalspring and geothermal well (P) discharges over the MiravallesGeothermal Field, from north to south, together with estimatedflows, in kg/s and temperatures and chloride contents, Inmg/kg, for thermal waters. Thermal waters are classified interms of their major anion: c- chloride, b- bicarbonate, s-sulfate.
RRRRRQQRQCQLTLQQGBRQ36QQG34RPlP2P5P10Pll
EGQQRQQQBNBBSBBJQROPPQQR
temp .Location ( °C )
GuacalitoLa ChepaSan IsidroLos AngelesLas Huacas -FrijolesLos ChilesHigueronQ de la Chepa 20RaudalesLa Torre 32Los Quesos 18Stream N of GBGuayabal, pool 70Guayabal, riverEl CarmenPH-36, well 140m 67HerrumbreGaveteros, stream 20PH-34, well 280m 115Guayabo River
PMG-1 245deep PMG-2 220wells PMG-5 240
PMG-10 2PMG-11 2
El Gomes 37La FortunaPejeCuipi lapaLas PalmasArenaBlancoBenaçon 50S Bernardo Arriba 46Salitral Bagaces 72S Bernardo Abajo 50EstanqueRio Blanco 45Punto de Palo 25Rio BagacesVilla ViejaRio Tenor io
flowtype
100003000100060001500
5010
2000b
500sb
150ss80s
350sb-ccccc
b70300
3000301050bbcb
10bs6070
30000
02H( °/oo )
-26-
-26-25-27-27-26-28-27-30-29-40-35-24-38-37-24-37-38-34-34-28-29-34-29-31
-38-45-40-30-49-41-38-48-48-34-48-52-50-44-53-52-39
.5
. 3
.0
. 7
.2
.5
.5
.9
.3
.9
.6
.5
. 5
.1
.6
.0
.9
.1
.9
.1
.9
. 1
.8
.0
. 3
.0
.9
.7
.2
.2
.7
.2
.9
.8
.9
.3
.9
.8
.0
.7
.2
.2
a18o("/o«)
-4.-4.-4 .-4.-4 .-4 .-4 .-4.-5.-5.-5.-6.-5.-4.-6.-6.-4.-6.-6.-5.-5.-3.-2 .— T
-3.-3.
-6.-7.-6 .-5.-7.-6._
-7.-7.-3.-7.-7 .-7 .-6.-7 .-7.-5.
7489928870908498043520528643443052085010484886890023
400872425268964612924351596853 -2276
Cl(mg/kg)
--------3-
124-
6607-
701-49-
25703090237028302630
52------8
1552560112
-8137---
204
rversstreamsChloride waters
bicarbonate w.sulfate waters
Fig. l- Sketch map of Miravalles Geothermal Field showingsample locations for thermal and non-thermal waters
The isotopic compositions of the waters listed in Table 1 areplotted in Fig.2. Both river and stream samples follow a trendclose to that indicated by the global meteoric water line, butsomewhat shifted to higher deuterium, or lower oxygen-18,values suggesting that the formation of groundwaters over thisarea may be governed by a "local meteoric water line" with asomewhat steeper slope of 9. Most of the thermal waters, withthe clear exception of the chloride waters, follow a similartrend suggesting that they are largely derived from localgroundwaters.
205
62HWoo)
-30-
-40
-50
-7 -6 -4 6'80(%o) -3
Fig. 2- Plot of <32H versus 0 •*•8 0 for thermal and non-thermalwater discharges over the Miravalles Geothermal FieldFor symbols see Fig. 1 and Table 1.
Comparison of the isotopic composition of all these waterswith their geographic position shows that there is apronounced decrease in heavier isotopes on going from north tosouth, a trend most clearly reflected in the composition ofstreams. Rivers can be assumed to represent the weighted- meanof groundwaters over a much larger catchment area (Fritz,1981). The compositions of river waters, close to those ofnorthern groundwaters, suggest that they are recharged largelyfrom the northern part of the Miravalles area and that most ofthe regional precipitation falls over the northern, "Atlantic"parts of the study area, carried there by predominantlynortherly winds (Vialc et al., 1986).
The chloride waters, as discharged from deep wells and atSalitral Bagaces (SB), are "shifted" to higher oxygen-18contents. The shift of thermal waters to higher oxygen-18contents is generally ascribed to isotopic exchange of thethermal waters with rock during their rise to the surface
206
(Craig, 1963). Accepting this process to be active atMiravalles, the deuterium content of the most shifted water(P2) would point to recharge from northern groundwaters. Thepattern exhibited by individual wells and the discharge fromSalitral Bagaces points to subsequent mixing of a deep parentwater with southern groundwater.
The waters most enriched in deuterium of the area arerepresented by the acid sulfate-chloride waters dischargedfrom a spring at Guayabal (GB) and a nearby shallow well PH-36(36). None of the groundwaters is isotopically heavy enough toqualify as the recharge water suggesting these waters containan as yet undefined component even more enriched even more
1 Qenriched in deuterium. A slight " 0-shift" is also indicatedfor the other shallow well PH-34 (34). The water dischargedthere, however, is a very dilute bicarbonate water.
The large variability in the isotopic composition ofgroundwaters renders the Miravalles area especially suitableto investigate recharge processes and the origin of possiblesource components of the geothermal discharges. In order to beable to establish the isotopi<~ identity of the various groupsof waters more clearly, variations in deuterium and oxygen-18contents as a function of geographical position areinvestigated in more detail.
ISOTOPIC COMPOSITIONS AS A FUNCTION OF GEOGRAPHIC POSITION
Variations in deuterium contents of waters discharged over theMiravalles Geothermal Field are shown in Fig. 3. For the non-thermal waters, two groups are distinguished: riverc withestimated flows fclOOO kg/s, and streams with lower flow rates.This distinction allows probable differences in the size ofthe respective catchment areas to be taken into account. Smallstreams are more likely to represent local groundwaters whilethe waters of rivers usually contain contributions from muchmore distant groundwaters. Generally river waters areisotopically depleted as compared to smaller tributariesbecause of the presence or predominance of isotopically
207
-50-
(km) 30
Fig. 3- Deuterium contents of waters discharged over theMiravalles Geothermal Field from north to south.Reference point for distances is summit of Miravallesvolcano. For symbols see Fig. 1 and Table 1.
depleted, higher altitude precipitation (Fritz, 1981;Giggenbach et al., 1983).
Over the Miravalles area the pattern is reversed, river watersbeing isotopically considerably enriched as compared to mostof the streams. Also, in contrast to the generally observeddepletion in heavier isotopes of groundwaters with increasingaltitude, waters of the Miravalles area become isotopicallyenriched on moving north from the coast uphill to theCordillera. The most likely explanation for such behavior isbased on the assumption that the isotopic composition of thesegroundwaters is governed by a "continental" effect rather thanan "altitude" effect (Eriksson, 1983). In this caseatmospheric moisture moving predominantly from north to south(Viale et al., 1986) would lose its heavier isotopes onapproaching the Cordillera leaving isotopically more and moredepleted moisture to fall on the areas to the south.
208
Both stream and river waters follow distinct trend lines, thedeuterium content of the streams together with that of most ofthe bicarbonate springs decreasing much more rapidly on goingfrom north to south than the river samples. The closecorrespondence in the composition of streams and bicarbonatesprings suggests that the latter are essentially derived frominfiltrating local groundwater. The preservation of highdeuterium contents in river waters collected far to the southof the Cordillera, e.g. Rio Tenorio, indicates that thecomposition of river waters all over the study area is domi-nated by groundwaters forming over the northern parts of theMiravalles area, from essentially "Atlantic" precipitation.
The chloride waters have deuterium contents corresponding tothose of the northern groundwaters. In the case of SalitralBagaces, the waters would have travelled a horizontal distanceof some 25 km without losing their isotopic identity. Suchlong horirontal travel paths for thermal waters are not verycommon, but have been identified isotopically e.g. for the ElTatio geothermal system in Chile (Giggenbach, 1978). Asa;ready pointed out, the deuterium content of the acidsulfate-chloride waters from the Guayabal area (GB and 36) arehigher than those of any other thermal or non-thermal water.The only water likely to be affected by an isotopic "altitude"effect is that discharged at Los Quesos (LQ). Its comparativeisotopic depletion is probably due to admixture of higheraltitude precipitation falling on the higher slopes ofMiravalles volcano.
The overall pattern is repeated with the oxygen-18 contents asshown in Fig. 4, except that chloride waters are much moreenriched than any of the other waters. Again, the two acidsulfate-chloride waters (GB and 34) are isotopically heavierthan any of the potential recharge waters. Further insightinto the relative importance of the various source componentsmaking up the thermal waters may be obtained by taking intoaccount variations in one of the chemical components of the1 pwaters, e.g. by comparing 0 and Cl contents of the thermalwaters, as done in Fig.5.
209
-4-
6180("/oc )
-5-
-6-
-7-
Y
N
1O 0 10 2O ( k m )
Fig . 4- Oxygen-18 content of wa te r s d i scharged over theM i r a v a l l e s G e o t h e r m a l F ie ld f r o m n o r t h to sou th .
0 1000 2000 300O CI (mg/kg)Fig . 5- Plot of oxygen-18 versus ch lo r ide content for the rmal
waters f r o m t h e M i r a v a l l e s G e o t h e r m a l F i e l d .
210
The correlation between chloride and oxygen-18 contents can befully explained in terms of intermixture of three endmember
1 P ncomponents: northern groundwater with 0 of -4.8 /oo.southern groundwater with 180 of -7.5°/oo/ both low inchloride, and a deep, high chloride, "parent" water with 3300mg/kg of Cl and a 180-content of -2.5°/oo All the stream, riverand bicarbonate waters can be assumed to be derived fromgroundwaters present over the Miravalles area. Geothermal welland the Salitral Bagaces discharges are likely to originatefrom a common parent water and to be diluted to varyingdegrees by southern precipation.
The position of the data points for the two acid sulfate-chloride waters from the Guayabal area would also agree withderivation from this common deep Cl water, but throughdilution with northern precipitation. Because of the largedifference in the degree of maturation (Giggenbach, 1988)between the Guayabal and deep well waters, however, it appearsunlikely that they are derived from the same deep sourcecomponents or have been generated under similar conditions.The Guayabal waters are likely to have formed at muchshallower levels. The simple pattern of Fig. 5, correspondingto dilution of a common deep chloride component to form boththe well and Guayabal waters, is likely to be only beaccidental .
SOURCE COMPONENTS OF THERMAL WATERS
Based on an allegedly generally observed close correspondencein the deuterium contents of thermal waters and localgroundwater, Craig (1963) concluded that geothermal watershave a predominantly local meteoric origin, with contributionsfrom any common "magmatic" component being too small to bedetected. The frequently observed enrichment in oxygen-18 for
1 Rhigher temperature waters was ascribed to an 0-shift causedby isotopic exchange between the waters and rock generallyenriched in oxygen-18.
By use of a large set of isotopic data for a wide range ofgeothermal systems along convergent plate boundaries
211
(Giggenbach, unpublished results), however, it becomesapparent that deuterium contents of thermal and localgroundwaters generally do not agree. Deuterium contents ofmost higher temperature geothermal discharges appear to beaffected by an additional, distinct shift to heavier values,the magnitude of this apparent "deuterium shift" increasingwith decreasing deuterium content of the local groundwater.
By plotting the isotopic compositions of pairs of geothermaland local groundwaters for a number of such "andesitic"systems, Giggenbach (1991) concluded that much of the-i P odeviations in both 0 and ^H of thermal waters from those oflocal groundwater may as well be explained in terms of mixingbetween local groundwater and a quite well defined, commonendmember water with a deuterium content close to -20°/oo and anoxygen-18 content of + 10°/oo.
The isotopic composition of this water coincides with that ofa large number of fumarolic condensâtes from andesiticvolcanoes e.g. in Kamchatka (Taran et al., 1989) and Japan(Sakai and Matsubaya, 1977) and many other such centers. Atpresent the origin of this "andesitic" water is still quiteuncertain, but is likely to represent modified seawater,subducted together with marine sediments (Taran et al., 1989)."Magmalic" waters from other tectonic environments, such ashot spots and rifts and ridges along divergent plateboundaries, may well be quite different and similar to that ofthe frequently invoked "juvenile" water with a H content ofabout -SO0 /oc
Accepting admixture of "andesitic" water to be the m a i n causeof isotopic enrichment of the Miravalles Cl waters, possiblerel a t ionships among the various source components may bediscussed by use of Fig. 6. Both the acid sulfate-chloridewaters issuing at Guayabal and the neutral chloride vatersdischarged from the wells are assumed to have formed throughabsorption of "andesitic" vapors into the northerngroundwater, quantitatively predominating over the Miravallesarea. In the case of neutral chloride waters, this process isthought to lead to the formation of an initially acid parent
212
water with a <32H of -25° /ooand 0180 of -2.0 °/oo. Subsequentwater-rock interaction and dilution with now southerngroundwater gives rise to the formation of the range ofneutral chloride waters as encountered in the wells.
The very fact that no additional oxygen-18 shift is rbservedfor the Salitral Bagaces waters, even rafter having travelledfor another 10 to 15 km, suggests that isotopic exchange withrock, under "geothermal" conditions, may be a processaffecting the isotopic composition of thermal waters onlylittle.
The presence of comparatively immature waters within theMiravalles geothermal system is indicated by the incursion ofan acid sulfate water into well PGM-2 after deepening from1200 m to 2000 m in 1984 (Giggenbach and Corrales, 1992). Thesimilarly acid, high sulfate waters of Guayabal, however, aremore likely to have formed at more shallow levels, throughabsorption of volcanic gases higher up within the Miravallesvolcanic system.
Fig. f> contains contour lines corresponding to the fraction xa
of andesitic vapor absorbed to form a geothermal discharge, ifthe above model is correct. Values of xa may also be obtainedfrom
*a = '"g - <V/(fla ~ <V
Where the subscripts a, g and m refer to the respectiveandesitic, geothermal and meteoric waters. Inserting values of-2 %o for <5180g/ -4.8°/oo for <3180m and +10°/oo for fl1?>a/
the fraction of andesitic vapor contributing to the formationof the hypothetical geothermal parent water becomes 0.39 or19%. Subsequent dilution by southern groundwaters reduces thisproportion to about 15%.
For the Guayabal waters a value of xa of 2.5% is obtained.Their S/Cl mol-ratio of 1.5 is close to that of many volcanicvapor discharges (Giggenbach et al., 1990) suggesting thatboth Cl and SO^ in these waters are derived through directabsorption of volcanic vapors rising within the system.
213
-6CH
-5 18,&'öO(°/oo) 10
Fig. 6- Plot of ö H versus 00 illustrating possibleformation of Miravalles thermal waters throughabsorption of "andesitic" vapors into groundwater.Contours xa correspond to fraction of andesitic vaporabsorbed.
Assuming all the Cl present in the well discharges, 3000mg/kg, to have been introduced with the magmatic vapor, its Clcontent would have to be about 20 000 mg/kg or 2°* b.w.. Thisvalue is somewhat above the range of 0.4 to 1.8% b.w. usuallyobserved for HC1 contents in high temperature vapor dischargesfrom active volcanoes (Giggenbach et al., 1990), but of thesame order. While not providing conclusive proof as to thecause of the "oxygen-18" shift observed for the Miravalleschloride waters, the above findings may still be used toderive a hydrological model of the Miravalles geothermalsystem compatible with geochemical evidence.
GEOCHEMICAL MODEL OF MIRAVALLES GEOTHERMAL SYSTEM
A geochemical model describing the origin and distribution ofthe various fluid components within the Miravalles geothermalsystem, compatible with the isotopic composition of the waterdischarges, is shown in Fig. 7. The procedures used in itsconstruction follow closely those described for the Nevado del
214
N
bicarbonate wchloride waterssulfate-chloride wcooling magma
(km)
0 10 15 (km)
Fig. 7- Cross-section through Miravalles geothermal systemalong line N - S of Fig. 1 showing distribution offluids compatible with geochemical evidence. Uppernumbers in boxes are deuterium ratios, lower numbersoxygen-18 values of respective waters.
Ruiz volcanic-magmatic-hydrothermal system (Giggenbach et al.,1990). Permeabilities dre assumed to be isotropir and uniform,the model, therefore, describes only the potentialdistribution of the fluids underground. Actual fluid movementis governed by the generally unknown distribution of faults,fissures and permeable strata.
The overall direction of fluid movement is assumed to bedominated by the influx of the quantitatively most importantgroundwater component, that forming from precipitation fallingto the north of the Cordillera. Vapors released from coolingand crystallising bodies of magma at depth are absorbed intothis deeply percolating groundwater forming initially quiteacid, sulfate rich waters. Ongoing water-rock interactionleads to the gradual conversion of these early primitivewaters to neutral chloride waters (Giggenbach, 1988). Becauseof the general direction of groundwater flow from north tosouth, the halo of neutral chloride waters islikely to be deflected to the south where it mixes to varyingdegrees with minor proportions of groundwater there.
215
Over the northern, "upstream" parts of the rising thermalplume, and within the Miravalles volcanic structure, residencetimes of the fluids are probably too short to permitconversion of the acid sulfate-chloride to neutral chloridewaters and they are able to reach the surface at Guayabal(GB). Interaction of these immature waters with rock atshallow levels, especially after dilution, may give rise toquite neutral bicarbonate-sulfate waters as discharged e.g. atLos Quesos 'LQ), El Gomes (EG) and from well PH-34 (34).
The tongue of neutral chloride waters is obviously able totravel underground for a distance of some 10 km m a i n t a i n i n gits isotopic and chemical identity. Marginal layers of thiswater, still containing dissolved CC>2, interact further withrock leading to the formation of the bicarbonate-chloridewaters as discharged, again in diluted form, from springs atSan Bernardo (BB, BJ) and Rio Blanco (RO). The appearance ofacid sulfate-chlor i de waters in the easternmost well PGM-2,after deepening from 1200 to 2000 m in 1984, suggests thatimmature water reach shallow levels over this part of thefield.
On the basis of the above model, the optimum siting of wellswill have to take into account several conflicting tendencies:drilling close to the center of thermal upflow enhances thelikelihood of encountering higher temperatures, but also thatof incursion of acid waters. Drilling farther "downstream"reduces the risk of acidification but also lowers dischargeenthalpies or temperatures and with them increases the risk ofcalcite deposition (Vaca et al., 1989).
ACKNOWLEDGMENTS
The present investigation was carried out within the frameworkof the IAEA Coordinated Research Program on the "Applicationof Isotopic and Chemical Techniques to Geothermal Explorationin Latin America" (COS/8/002) with financial support from theGovernment of Italy.
216
REFERENCES
Craig H, 1963: The isotopic geochemistry of water and carbonin geothermal areas. In Nuclear Geology on GeothermalAreas, CNR Pisa, 17-53.
Eriksson E, 1983: Stable isotopes and tritium in precipi-tation. In Guidebook on Nuclear Techniques in Hydrology.IAEA Techn. Report 91: 19-33.
Fritz P, 1981: River waters. In Stable Isotope Hydrology -Deuterium and Oxygen-18 in the Water Cycle. IAEA Techn.Report 210: 177-201.
Gardner M C and R Corrales S, 1977: Geochemical investigationsof the Guanacaste Geothermal Project, Costa Rica.Geothermal Resources Council, Trans., 1: 101-102.
Giggenbach W F, 1978: The isotopic composition of waters fromthe El Tatio geothermal field, Northern Chile. Geochim.Cosmochim. Acta: 42, 979-988.
Giggenbach W F, 1988: Geothermal solute equilibria. Derivationof Na-K-Mg-Ca geoindicators. Geochim. Cosmochim. Acta,52: 2749-2765.
Giggenbach W F, 1991: What is the real cause of the oxygen-18shift of waters from geothermal areas along convergentplate boundaries? Geotherm. Resources Council, Trans.,Reno .
Giggenbach W F, Gonfiantini R, Jangi B L and Truesdell A H,1983: Isotopic and chemical composition of Parbati Valleygeothermal discharges, North-West Himalaya, India.Geothermics, 12: 199-222.
217
Giggenbach W F, N Garcia P, A Londono C, L Rodriguez V, NRojas G and M L Calvache V, 1990: The chemistry offumarolic vapor and thermal-spring discharges from theNevado del Ruiz volcanic-magmatic-hydrotherma1 system,Colombia. J Volcanol. Geotherm. Res., 42: 13-39.
Giggenbach W F and R Corrales S, 1991: The isotopic andchemical composition of water and steam discharges fromvolcanic-magmatic-hydrotherma1 systems of the GuanacasteGeothermal Province, Costa Rica. Appl. Geochem., (inpress ) .
Sakai H and Matsubaya 0, 1977: Stable isotopic studies ofJapanese geothermal systems. Geothermics, 5: 97-124.
Taran Y A, Pokrovsky B G and Esikov A D, 1989: Deuterium andoxygen-18 in fumarolic steam and amphiboles from someKamtchatka volcanoes, "andesitic" waters. Dokl. Akad.Nauk. USSR, 304: 440-443.
Vaca L, Alvarado A and Corrales R, 1989: Calcite deposition atMiravalles Geothermal Field, Costa Rica. Geothermics, 18:305-312.
Viale P, Corrales R, Manieri A, Mayra C and Vaca L, 1986:Mineral alteration and fluid characteristics ofMiravalles Geothermal Field - Costa Rica. ExtendedAbstr . , Fifth Int. Symp. Water-Rock Interaction,Reykjavik, Iceland, p. 667-670.
218
MODELO GEOTÉRMICO PRELIMINAR DE ÁREAS VOLCÁNICASDEL ECUADOR, A PARTIR DE ESTUDIOS QUÍMICOS EISOTÓPICOS DE MANIFESTACIONES TERMALES
E. ALMEIDA, G. S ANDO VALInstituto Ecuatoriano de Electrificación,Quito, Ecuador
C. PANICHI, P. NOTO, L. BELLUCCIIstituto Internazionale per le Ricerche Geotermiche,Consiglio Nazionale delle Ricerche,Pisa, Italia
Resumen-Abstract
MODELO GEOTÉRMICO PRELIMINAR DE ÁREAS VOLCÁNICAS DEL ECUADOR, A PARTIR DEESTUDIOS QUÍMICOS E ISOTÓPICOS DE MANIFESTACIONES TERMALES.
fi partir de 1986 se mostrearon y anaii:aron las aguas y gases de ocho áreas geotéraicas, para establecersu composición quiíica e isotópica, En Chachisbiro y Cuicocha, se observan enriqueciaientos de hasta el 5X.en {) y SOS, en Beuteno. La evaporación de una hipotética agua geotersica profunda, desde 2WC hasta lateaperatura en superfice, puede ser responsable de las variaciones isotópicas observadas en Chachisbiro,Por otra parte, en Cuicocha, un proceso de sezcla entre agua subterránea caliente y agua fresca de unalaguna es responsable de los valores observados. En otras áreas, la coiposicién isotópica de las aguastérsales corresponde escencialsente a aguas aeteóncas que caen a diferentes altitudes, desde 2500 nasta4200 a sobre el nivel del sar. Las relaciones Na/K indican teiperaturas del reservono de aproxiaadasenteEW, 210 y EOfl'C, para aguas cloruradas que eiergen en las áreas geoterncas de Chacbíibiro, Cuenca ypapaliacta respectivasente. En la caldera de Chalupas, los datos no peralten hacer ninguna hipótesisrazonable sobre la existencia de jn acdfero profundo de alta teiperatura. Muestras procedentes del -oleanTungurahua son de aguas sulfatadas-acidas, que tienen una coiposicién isotópica concordante con aguascalentadas por gases a teiperaturas de 200'C aproxisadanente. En el área de Chiiborazo, la reducidacantidad de suestras disponibles no peralten dar ninguna inforaación acerca de sus característicastérsales. En Tufií5o, las aguas provienen de aculferos superficiales aodificados por gases calientes, cuyosgeotersfisetros dan teaperaturas de SSO'C.
PRELIMINARY GEOTHERMAL MODEL OF VOLCANIC ÁREAS IN ECUADOR BASED ON CHEMICALAND ISOTOPIC INVESTIGATION OF THERMAL INDICATORS.
Since 1986, the waters and gases of eight geothermal áreas have beensampled and analysed to establish their chemical and isotopic composition. InChachimbiro and Cuicocha 0 enrichment up to 5% and deuterium enrichment upto 20% have been observed. The isotopic variations seen in Chachimbiro couldbe explained by hypothetical deep geothermal water with a temperature rangefrom 240°C to surface temperature. The valúes observed in Cuicocha, on theother hand, are due to mixing of hot groundwater with fresh lagoon water. Inother áreas, the isotopic composition of the thermal waters basicallycorresponds to that of meteoric water falling from various altitudes rangingfrom 2500 to 4200 m above sea level. The Na/K ratios indicate reservoirtemperatures for the chloride waters emerging in the Chachimbiro, Cuenca andPapaliacta geothermal áreas of approximately 240°C, 210QC and 200°C,respectively. From the data on the Chalupas caldera it is impossible toconstruct any reasonable theory regarding the presence of a deep,high-temperature aquifer. Tungurahua volcano samples consist of sulphuric
219
acid water with an isotopic composition similar to waters evaporating at about200°C. The number of samples from the Chimborazo área was too low to justifyany assumptions about the thermal characteristics of the water. The waters inTufiño origínate in surface aquifers modified by hot gases registering 230°Con the geothermometers.
1 INTRODUCCIÓN
En el marco del Programa de Exploración de los Recursos Geotérmicos de laRepública del Ecuador, ejecutado por INECEL, se han efectuado estudiosgeocientificos exploratorios en las áreas de Tufiflo, Chachimbiro, Cuicocha,Papallacta, Chalupas, Tungurahua, Chimborazo y Cuenca, a fin de determinarlas posibilidades de utilización del recurso geotérmico en la generacióneléctrica. Parte importante de estos estudios, constituyen lasinvestigaciones geoquímicas e isotópicas llevadas a cabo con el apoyo de laOrganización Internacional de Energía Atómica (OIEA), a través del Contratode Investigación 3991/IG, suscrito con INECEL. Los resultados finales deestas investigaciones se resumen en este documento.
2 SITUACIÓN GEOLÓGICA REGIONAL
El margen continental activo del Ecuador es caracterizado por la subducciónde la placa Nazca, que transporta a la Cordillera de Carnegie creada por elpaso de la Nazca sobre el punto caliente Galápagos. La subducción de Nazcay Carnegie es muy importante en la evolución de la Cordillera de los Andes,particularmente de su volcanismo. El Ecuador continental se divide en tresregiones: Llanura costanera (Costa), Cordillera de los Andes (Sierra) yCuenca superior amazónica (Oriente).
La Costa esta1 formada por la parte emergida de un prisma acrecionado,constituido por basaltos tolelticos cretácicos que representan cortezaoceánica C13 y sedimentos volcanoclásticos que yacen bajo sedimentospertenecientes a cuencas terciarias y cuaternarias (Figura 1).
La Sierra tiene en su mitad septentrional dos cordilleras paralelas, laOccidental y la Real, separadas por un valle angosto conocido comoDepresión Interandina. La mitad meridional, es una cordillera de menoraltura y sin un valle central. La Occidental es formada por rocasvolcánicas básicas e intermedias de edad cretácica, emplazadas en unambiente submarino (arco volcánico oceánico), y cubiertas de sedimentospelágicos. La Real es un cinturón metamórfico barrowiano, cuyos protolitosson sedimentos de plataforma y pendiente continental metamórfizados en elCretácico y Terciario inferior CE],
El Oriente forma parte de la cuenca amazónica; tiene un basamentoprecámbrico sobre el cual se encuentran potentes depósitos sedimentariosmarinos de plataforma continental, sedimentos lacustres y detríticoscontinentales.los terrenos volcánicos continentales se extienden a lo largo de la Sierra,con un ancho promedio de 80 km. La actividad terciaria fue importante entoda la cordillera, en tanto que la cuaternaria esta' restringida a lamitad septentrional, donde se localizan casi todas las áreas geotérmicasestudiadas (Figura 1).
220
80° 79° 78°
SIMBOL06IA
O
CUENCA SEDIMENTARIA CUATER-NARIA DE LA COSTA
VOLCÁNICO CUATERNARIO YTERCIARIO DE LOS ANDES
BASAMENTO TOLEITICO; SEDI-MENTOS MESOZOICOS Y TER-CIARIOS DE LA COSTA
ARCO VOLCÁNICO MESOZOICO DEJJ LOS ANDES
CUENCA AMAZÓNICA .MESOZOI-CO HASTA CUATERNARIO
BASAMENTO METAMORFICO W-LEOZCHCO DE LOS ANDES
ÁREA GEOTÉRMICA
FIGURA I. ESQUEMA GEOLÓGICO DEL ECUADOR
El volcanismo durante el Cuaternario es muy importante; han sidoidentificados aproximadamente EOO centros de emisión C33 y muchos de ellospresentan una evolución magmática muy desarrollada. Los volcanes másvoluminosos y activos se caracterizan por una compleja alternancia de lavasbásicas e intermedias, domos ácidos y productos piroclástícos decomposiciones dacíticas y riollticas. El volcanismo en la parte meridionalde la Sierra es fundamentalmente terciario C4-, 53 y ha sido poco estudiado.
3 ASPECTOS QUÍMICOS E ISOTÓPICOS
3.1 TUFINO
Se encuentra en la frontera entre Ecuador y Colombia; los volcanes Chiles yCerro Negro se sitúan en su interior. La cámara magmática que alimento' aeste complejo volcánico constituye la fuente de calor [6, 73 del sistemageotérmico estudiado.La mayoría de las aguas analizadas tienen temperaturas comprendidas entrelos 32 y 5E°C, su composición química e isotópica esta1 resumida en laTabla I. El 75% de las muestras son bicarbonáticas; el 12,57. son sulfatadasy corresponden a los manantiales más salinos y calientes, representando lainteracción de aguas superficiales con gases volcánicos CB3. Las restantesmuestras son cloruradas y magnésicas, que se ubican a varios kilómetros dedistancia del área de interés. En la Figura E están las relaciones entreel 'I) y el Deuteno, donde se aprecia que tanto las aguas termales como lasfrías se encuentran sobre la recta meteórica y no se ve un enriquecimientodel '13, lo que no permite evidenciar fenómenos de intercambio agua-roca, departicular interés.
221
- 70
+ T< 30° CQ T > 3 0 ° C
-110
-15 -14 -13 -12¿B0(%0 .V -SMOW)
FIGURA Z. DIAGRAMA DE VARIACIÓN ¿**0/¿0 DE TU FINO
El análisis de los datos permite concluir que todas las manifestacionestermales de Tufíflo, incluso las más calientes, pertenecen a un acuiferosuperficial alimentado por aguas meteóricas de circulación bastanterestringida, según lo indica el contenido de tritio. Estas aguas han sidomodificadas química y térmicamente por el aporte de gases calientes deorigen magmático (1!C del orden de +2,7*/,. hasta +5,6*/.. PDB) C8] y,probablemente mínimas cantidades de vapor.
El único indicio geoquímico sobre la posible existencia de un sistemageotérmica de alta entalpia es constituido por la notoria presencia de gas,cuyos geotermómetros dan temperaturas del orden de los 230 °C.
3.2 CHACHIMBIRO
Se encuentra en un segmento de la Cordillera Occidental, que tiene unagran concentración de aparatos volcánicas y un volcanismo persiste desde elPlioceno. La característica básica, desde el punto de vista de la fuente decalor, es que los centros eruptivos del complejo volcánico de Chachimbirohan sido alimentadas por varias cámaras magmáticas evolucionadas C9].
El reservorio estarla localizado en rocas volcánicas a profundidadesvariables entre los 10OO y EOOOm. Evidencias para su existencia están dadaspor: Zonas de alteración hidrotermal; Fuentes termales, y; Emanaciones degases. Las aguas termales son Cloruradas con temperaturas de ¿"C, caudalesde 70 1/min, salinidad de 7000mg/l. El contenido de 5iOB varia entre 180 yEOOppm; además, tienen una gran cantidad de gas disuelto, fundamentalmenteCOE CIO].
La Figura 3 presenta las relaciones entre el "D y el Deuterio; alli' seevidencian dos grupos de aguas: Aguas frías de circulación somera, con lamisma composición isotópica de las aguas meteóricas, y; Aguas termales quese desvian fuertemente de la recta de las precipitaciones.
222
>o
-40
-50 -
-60 -
-70 -
-80 -
-90 -
-100-
- 110--14
¿>D- 86 0+ 10
SINGLE STAGESTEAM LOSS
A (AGUA PROFUNDA ORIGINAL . T= 240° C; Cl =1 5 g / Kg )B (AGUA PROFUNDA RESIDUAL : T = 60° C; CI = 2.4g/Kg)• AGUAS CLORURADASD AGUAS BICARBONATADAS
-12T
- 6-10 -8
3,V. SMOW)
FIGURA 3. DIAGRAMA DE VARIACIÓN 5*0/80 DE CHACHIMBIRO
-4S'8o
Las aguas termales muestran una evidente evolución química, desde aguasbícarbonatadas hasta aguas cloruradas, en donde el 'ti se incrementa de -11a -6%.; los sólidos disueltos también se incrementan conforme aumentan losisótopos pesados en solución, sugiriendo un mecanismo de enriquecimiento desolutos, debido a una evaporación del agua meteórica original encondiciones termales anómalas, o también, por procesos de mezcla de aguastermales salinas con aguas frías.La evaporación del agua desde una temperatura relativamente alta (p.e.2 0°C), hasta condiciones superficiales (cerca de 60°C), ocasiona unincremento del "Ü en aproximadamente 2,8%. y, un enriquecimiento del clorohasta el 30% de los valores originales. Para el caso de Chachimbiro,solamente este proceso no explica como, partiendo de aguas meteóricas queinicialmente tienen contenidos de "O y cloro más bajos que -10%. y lOOmg/kgrespectivamente, se observe que el "D alcanza un desplazamiento del 57.. yel contenido de cloro llegue a 2250mg/kg. Una explicación puedeencontrarse considerando, adicionalmente, otro proceso en el cual las aguasmeteóricas que circulan por rocas más oxigenadas del reservorio profundo,si la temperatura es más alta que EOO°C Cll, 123, incrementan su contenidode "O en aproximadamente el 3%., por intercambio con la roca. Estos dosprocesos están esquemáticamente indicados en la Figura 3, donde, partiendode un agua meteórica se llega a un agua geotérmica "A" que experimenta unproceso de evaporación en profundidad (single stage steam loss), hastaalcanzar un valor composicional como el de "B". Esta agua residual semezcla con aguas subterráneas someras para dar el conjunto de aguas que seencuentran sobre la linea de mezcla.
En la Figura ^ se indican las variaciones del "O con la altura deemergencia, en las aguas de Chachimbiro y Cuicocha. Para Chachimbiro, esevidente la existencia de dos diferentes grupos: El primero, son aguassubterráneas someras de circulación restringuida, por cuanto las áreas deinfiltración están muy próximas a la zona de emergencia; El segundo grupo
223
4000
3000 -
Eo
< 2000
a n= O 18 %o
LAGUNA DE° CUICOCHA
D CHACHIMBIROO CUICOCHA
- 14 -12—r-8-10
6 "o (%0 , v SMOW)-6
FIGURA 4. DIAGRAMA DE VARIACIÓN <J18 O/ALTITUD DE CHACHIMBIRO Y CUICOCHA
D CHACHIMBIROO CUICOCHA
1000 2000 3000C l ( p p m )
FIGURAS. DIAGRAMA DE VARIACIÓN CI/B DE CHACHIMBIRO Y CUICOCHA
224
400
B ( AGUA ORIGINAL PROFUNDA 240° C }O CHACHIMBIROO CUICOCHA
501000 2000 3000
Cl (ppm)
FIGURA 6. DIAGRAMA DE VARIACIÓN C L / S I O E DE CHACHIMBIRO Y CUICOCHA
(muestras que se desvian a la derecha de la recta), refleja procesos demezcla puestos en evidencia en la Figura 3, asi1 como, en las Figuras 5 y6, donde los contenidos de boro y sílice son confrontados con el cloro.En el diagrama Cl/SiOe existe una correlación positiva entre las aguastermales y frías de tipo bicarbonatado, con las cloruradas, que se desvianhacia la derecha de la recta de dilución, porque tienen altos contenidos decloro con respecto al sílice. Por el contrario, en el diagrama Cl/B, todaslas muestras presentan una correlación lineal. Esta discrepancia puede serexplicada tomando en cuenta que tanto el boro como el cloro son parámetros"conservadores", y, la solubilidad del sílice varía con el enfriamiento delas aguas termales emergentes C13]. Esto implica que, aparte de la mezcla,el enfriamiento también puede considerarse en las aguas cloruradas queemergen a la superficie. La simultánea existencia de los dos mecanismos,explica las composiciones presentadas en las Figuras 5 y 6, donde esinteresante notar que las lineas de mezcla definen un componente profundosimilar al obtenido del modelo isotópico presentado en la Figura 3.
3.3 CUICOCHAIncluye los volcanes Cotacachi y Cuicocha cuyas característicasvulcanológicas permiten inferir una importante fuente de calor. El Cuicochatuvo un estilo eruptivo explosivo C5, l¿f, 15] emitiendo flujospiroclásticos daciticos [16]. La actividad se inicio' en el limitePleistoceno-Holoceno, durante la cual se formo' una caldera, hace 3100 arios[173. El Cotacachi es un estratovolcán andesltico de edad pleistocénica alcual se asocian varios domos daciticos de edades holocénicas.
Las aguas que afloran en el sector son Bicarbonatadas, con temperaturas de45°C, notándose que hacia las cercanías de la caldera existe unsignificativo aumento en el contenido de cloruros CIO].
225
De la relación "Ü/Deuterio (Figura 7) se desprende que el desplazamiento esconsecuencia de la mezcla entre el agua de la laguna de Cuicocha y lasaguas termales de los alrededores. La linea de mezcla corta a la de lasaguas meteóricas en la abscisa -1S'¿., que puede representar el contenido deI!D en las aguas de alimentación de la laguna. La pendiente de esta linea esigual a 4,8 que es concordante con con una evaporación superficial a bajatemperatura C18]. Si este es el caso, un enriquecimiento del 67,. en el "t)ocurre en las aguas del lago hasta alcanzar condiciones estacionarias. Sinembargo, este fuerte enriquecimiento no esta' acompasado de un sensibleincremento en la salinidad total de las aguas evaporadas. En efecto, lasaguas del lago presentan un muy bajo contenido de sal ( 20 Siemens), dondeel ion dominante es el bicarbonato. Esto sugiere que las aguas meteóricasque lo alimentan, son escencialmente constituidas por nieve fundida, lacual esta' presente en el vecina volcán Cotacachi. En realidad, la mezcladel agua de la laguna con las aguas termales ocasionara1 una desigualdilución de las soluciones termales, porque acurre solamente a lo largo defracturas y fallas. Por esta causa, las dos muestras ubicadas en la lineade mezcla (Figura 7) contienen cada una el 70 y el 30'/, del agua del lago y,los sólidos disueltos totales son de 3800 y 500mg/kg respectivamente,sugiriendo con esto, que estas dos aguas termales representan dosreservorios distintos.En la Figura 4, se aprecia a un grupo de aguas que se desplaza a laizquierda de la recta, esto se interpreta como aculferos con largacirculación, parque las áreas de alimentación están a alturas mucho máselevadas. Particularmente el lago de Cuicocha, ubicado a mayor altura quelas fuentes termales.
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LAGUNA DECUICOCHA
LINEA DE MEZCLA
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FIGURA 7. DIAGRAMA DE VARIACIÓN <5~'8 O/ f D DE CUICOCHA
226
3.4 PAPALLACTA
Esta área tiene manantiales de composición clorurada sódica con unimportante componente sulfato calcico, una elevada salinidad C193 ytemperaturas de 66°C. Estas aguas afloran en una zona de fallamiento, en elcontacto entre el basamento metamórfico de la Cordillera y el volcánicoterciario. Una compleja situación tectónica y volcánica, dificultaestablecer hipótesis sobre el origen de la fuente de calor, a nivel dereconocimiento.Si bien los datos disponibles son pocos, lo que condiciona fuertemente lainterpretación, desde el punto de vista térmico la situación pareceinteresante. La Figura 8 presenta la relación "D/Deuterio de las tresvertientes muestreadas; una de ellas esta' enriquecida en "D con respectoa las otras dos, que se ubican en la linea de las aguas meteóricas. Por lotanto, suponemos que un posible mecanismo de intercambio con las rocas pudotener lugar; además, las características geomorfo lógicas del área y larelación "D/altura indican que esta agua tiene una zona de alimentación600m más elevada que las otras dos. Todo esto, permite asumir que esta aguaexperimento' condiciones de alta temperatura en profundidad, lo que parececonfirmarse al aplicar los geotermómetros químicos reportados, másadelante.
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TUNGURAHUA D EBCHWBORAZO ^
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3.5 CHALUPAS
Se trata de una caldera (16 x 14)k<n,estratovolcán Chalupas, luego de que éstecóbicos CEO] de materiales riollticos C163;
formada por el colapso delemitió' por lo menos 100 kmposteriormente, el volcanismo
se reactivo' en el interior de la caldera. Las particulares características
227
del sistema de alimentación magmatica de este complejo volcánico, activodesde hace un millón de aftos, son buenas evidencias de la existencia de unafuente de calor.
En la Figura 9, las muestras relativas al área de Chalupas, caen sobre larecta de las aguas meteóncas en un amplio intervalo de composición. Estopermite suponer la existencia de una fuerte circulación superficial sinencontrarse condiciones anómalas de temperatura, esto confirma el modelogeovulcanológico de INECEL CEO] en el cual se indica que la caldera esta'rellenada por materiales impermeables (piroclástos y sedimentos lagunares)que constituyen un efectivo sello al sistema geotérmico. Las variaciones enel contenido isotópico corresponden a diversas cotas de infiltración,situadas entre los 3500 y 4c?00m de altura.
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FIGURAS DIAGRAMA DE VARIACIÓN s'80/,5 D EN CHALUPAS
3.6 TUNGURAHUA
En los flancos del activo volcán Tungurahua, existen abundantesmanifestaciones de aguas bicarbonatadas acidas y sulfatadas, con temperatu-ras hasta de 53°C [21]. Tres muestras fueron analizadas (EUI-42, 43 y M dela Tabla I), la correlación entre el "D y el Deuterio es una recta dependiente +1,8 (Figura 8) sugiriéndonos que las aguas pudieron sersometidas a procesos de evaporación-condensación a una temperatura deaproximadamente EOO°C C18].
La EUI-^4 es agua subterránea local que durante su descenso por el interiordel cono, interacciona con gases volcánicos calientes (S0e , I-JS y C0¡, ),dando como resultado un fuerte incremento de HC03 y SQ en la solución y, unsimultáneo incremento de la temperatura del liquido hasta alcanzarprobablemente los 200°C. Bajo estas condiciones, se produce la evaporaciónparcial de la solución, formándose vapor que se condensa en la parte altadel volcán, originando un aculfero termal con una composición química e
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isotópica similar a la de la muestra EUI-42. Por otra parte, las aguasresiduales de la evaporación circulan hacia la base del cono, originandoaguas termales similares a la muestra EUI-*f3, que tiene al sulfato comoion dominante en la solución, y presenta además, un alta concentración delos isótopos pesadas.
3.7 CHIMBORAZQ
Es el estratovolcán más alto del Ecuador, su actividad se inicio' en elPleistoceno y fue prácticamente continua hasta el Holoceno. Lainterpretación petrológica de los productos emitidos en sus tres etapasevolutivas, sugiere la presencia de una cámara magmática en donde el magmabasáltico-andesítico de las etapas iniciales, evoluciono' hasta un productofinal de composición dacltica- nolítica C16Ü. Durante su evolución, elmagma transfirió' calor provocando una anomalía térmica, cuya primeraevidencia en superficie son fuentes termales con temperaturas de 46°C.
La única muestra que ha sido analizada puede ser considerada como aguasubterránea cuyo origen esta' en los deshielos del Volcán Chimborazo, aalturas superiores a los 4500m. La desviación que se observa en la FiguraB, respecto a la recta de las aguas meteóricas no encuentra, en estemomento, una explicación satisfactoria debido a la falta de otros datosanal iticos.
3.8 CUENCA
Esta área es conocida por la existencia de fuentes termales con unaelevadlsima salinidad, alta conductividad y caudales del orden de los EO1/s CIO, 193. Estas aguas tienen las mayores temperaturas (73,3°C)registradas en el país y, precipitan travertinos que son explotados comoroca ornamental. Esta zona termal se encuentra en una cuenca sedimentariacretácica y los materiales volcánicos más próximos son de edad terciaria.
Las manifestaciones aflorantes son de dos clases: Aguas cloruradas contemperaturas mayores que 48°C, concentradas en un área de aproximadamenteun kilómetro cuadrado y que son aprovechadas como balnearios (Baños deCuenca), y; Aguas bicarbonatadas frías (17 a £2°C), diseminadas en unazona relativamente amplia, varios kilómetros alrededor de los balnearios.En esta zona marginal existe un manantial frío de tipo clorurado que tieneel menor contenido de "ÍD y una elevada salinidad (TDS - 8,£g/kg).
Las aguas termales tienen una zona de alimentación situada aproximadamentea una elevación de 3EOOm, esto es, 500m sobre la altura de los balnearios.En el área de Baftos se muestrearon 11 vertientes cuya temperatura deemergencia varia entre 48 y 73,3°C. No obstante tal diferencia térmica, lacomposición isotópica de los diversos manantiales es muy homogénea (Figuras10 y 11), situándose sobre o muy próximos a la linea de las precipita-ciones; al mismo tiempo, el contenido de ion cloruro se mantiene constante.Todo esto permite suponer que el agua profunda alcanzo' condiciones deequilibrio químico con las rocas de un reservarlo somero, en el tope de lapotente cuenca sedimentaria.
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18,FIGURA 10. DIAGRAMA DE VARI ACIÓN & O / ¿ D EN CUENCA
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FIGURA II. DIAGRAMA DE VARIACIÓN 6'% Y Cl CON LA TEMPERATURAEN CUENCA
4 EVALUACIONES GEOTERMOMETRICAS
La Figura IB presenta un diagrama triangular donde se relacionan lasconcentraciones relativas de K, Na y Mg en todas las aguas analizadas. Estediagrama ha sido modificado de un original propuesto por Giggenbach CE23,con el fin de ampliar el área correspondiente a la esquina del magnesio, enla cual generalmente están localizadas las aguas inmaduras. Muchas de las
233
Na/5
200°CHACHIMBIROCUICOCHACHALUPASCUENCACHIMBORAZOPAPALLACTATUNGURAHUA
\T Mg
FIGURA 12. CONCENTRACIONES RELATIVAS No.K, Mg(mfl/kg) DETODAS LAS AGUAS ESTUDIADAS
muestras de agua caen entre las dos lineas que describen la temperaturaK/Mg en el rango de 40 a 60°C, indicando con esto que este par iónico esincapaz de reflejar las condiciones termales profundas. Por otra parte, larelación Na/K parece más útil para evaluar las temperaturas profundas, peroesta1 restringuida a pocas muestras. Realmente, la Figura 1E indica quemuchas de las muestras tienen temperaturas Na/K mas altas que E50°C pero,no parecen ser reales si se toma en cuenta las características isotópicasya discutidas.
Solamente pocas muestras de Chachimbiro, Papallacta, Chimborazo y Cuencaparecen admitir la aplicación del geotermómetro Na/K, ellas son aguascloruradas y parecen ser maduras o parcialmente equilibradas con rocas delos acuiferos (segán la definición dada por Giggenbach CEED).Consecuentemente, temperaturas de E*tO, EOO, 100 y EOO'C pueden serobtenidas, respectivamente, en las áreas indicadas.
En cuanto se refiere a Tufiflo, las manifestaciones termales, incluso lasmás calientes, son aguas superficiales modificadas por el aporte de gasescalientes, no existiendo una evidencia segura de la contribución de uncomponente liquida profundo. La evaluación de la termalidad, porconsiguiente, es posible únicamente con ayuda de la geotermometria degases. Las temperaturas obtenidas de esta manera, dan un máximo de EEO aE30°C CE3,
5 CONCLUSIONES
Los indicadores químicos e isotópicos presentados, ratifican el interésgeotérmico de las áreas de Tufiflo, Chachimbiro y Cuenca. Además,constituyen argumentos positivos que justifican la iniciación de
234
investigaciones geocientlficas detalladas en Papallacta y Chimborazo. Conrespecto a Chalupas, los métodos químicos e isotópicos no han sido de muchautilidad, debido a las características de las aguas muestreadas.
AGRADECIMIENTOSLos autores agradecen a INECEL y al IIRG por las facilidades para elaborarel presente trabajo, elaborado en el marco del "Programa Coordinado deInvestigaciones para América Latina, sobre la Aplicación de TécnicasGeoquímicas e Isotópicas en la Exploración Geotérmica", financiado por elGobierno de Italia. Especial reconocimiento a la OIEA y, particularmente,al Dr. Roberto Gonfiantini quien brindo' su total apoyo a las gestionestécnicas del Contrato de Investigación 3991/IG mediante el cual serealizaron las investigaciones de las áreas ecuatorianas.
REFERENCIAS
[13 FEININGER, T. "La Geología Histórica del Cretácico y Paleógeneo de laCosta Ecuatoriana" Politécnica, Monografía de Geología. Vol. V, No.2, (1980) 7-45 p.
C23 FEININGER, T. "El Basamento Metamórfico del Ecuador" Politécnica,Monografía de Geología 3, Vol. VIII, No. 2, (1983) 37-48p.
[33 ALMEIDA, E. "El Riesgo Volcánico en el Ecuador Continental" MemoriasSegundas Jornadas Nacionales de Geología, Minas y Petróleo.Universidad Central del Ecuador, (1988), 57-60 p.
[43 HALL, M., CALLE, J. "Control Geocronológico de los PrincipalesEventos Tectónico-Magmáticos del Ecuador" Politécnica, Monografía deGeología 2, Vol. VI, No. 4, (1981), 7-36 p.
[53 BARBERI, F., COLTELLI, M., FERRARA, G., INNOCENTI, F., NAVARRO, M.,SANTACROCE, R. "Plio-Quaternary Volcanism in Ecuador" Geol. Magm. 135(1), (1988), 1-14 p.
C63 NAVARRO, J., ALMEIDA, E., AVALA, J. "Geovolcanologla del Norte delEcuador y, en Particular, del Área de Tufiflo" OLADE-INECEL, Inf. Int.(1982), 98 p.
[73 GEOTÉRMICA ITALIANA. "Aprovechamiento de los Recursos Geotérmicos dela Zona de Tufiflo, Ecuador Septentrional, Informe Geovulcanológico"OLADE-INECEL, Inf. Int., Quito. (1982), 110 p.
[8] ENEL, Unita' Nazionale Geotérmica, "Indagine Isotópica nella Regionedi Tuf iflo-Cerro Negro, Repubblica dell'Ecuador, Sintesi dei DatiRaccolti ed Interpretazione Preliminare" IAEA-INECEL, Pisa, Diciembre1986.
[93 ALMEIDA, E. "Alternativas para el Desarrollo Geotermoeléctrico en laRepública del Ecuador" Inf. Int. INECEL, Proyecto Geotérmico. Quito,(1990), 49 p.
C103 SANDOVAL, G. "Investigación Geoquímica e Isotópica en la Región deCuenca-Chalupas-Chachimbiro en la República del Ecuador, Síntesis deDatos Obtenidos e Interpretación Preliminar" INECEL-OIEA, Inf. Int.,Quito, (1988), 8 p.
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Cll] PANICHI, C., GONFIANTINI, R. "Geothermal Waters" ín: Stable IsatapeHydrology: Deuterium and Oxigen-18 in the Water Cycle. IAEA, TecnícalReports No. 210, Vienna, (1981).
[123 GIGGENBACH, W., GDNFIANTINI, R., PANICHI, C. "Geothermal Systems" ín:Guidebook on Nuclear Techníques ín Hydrology. IAEA, Tecnical ReportsNo. 91, Vienna, (1983).
[13] TRUESDELL, A., FOURNIER, R. "Procedures for Estimating theTemperature of Hot Water Component ín a Mixed Water Usíng a Plot ofDissolved Silíca Versus Entalphy" J. Res., U. S. Geol. Survey, V. 5,(1977), p. 49.
[14] HALL, M. "El Volcanismo en el Ecuador" Inst. Pan. Geografía eHistoria. I.G.M., Quito, (1977), 120 p.
[15] CALVACHE, M., BARREIRO, J., ORTIZ, S. , CORONEL, V., CREUSOT, A.,GONZÁLEZ, V., JARAMILLO, S. "Estudio Geovulcanológico del Área deCuicocha-Cotacachi" Informe Final del Primer Curso Latinoamericano deGeovulcanologla Aplicada a la Exploración Geotérmica, Quito, OLADE,(1983), 50 p.
[16] COLTELLI, M. "II Vulcanismo del le Ande Ecuadoriane" Tesis, Corso dil.aurea in Scíenze Geologiche, Uníversita* Degli Studí di Pisa,(1984).
[17] von HILLEBRANDT, Ch. "Estudio Geovulcanológíco del ComplejoVolcánico Cuicocha-Cotacachi y sus Aplicaciones, Provincia deImbabura" Tesis, Escuela Politécnica Nacional, Quito. (1989).
[18] TRUESDELL, A., NATHENSON, M. "The Effects of Subsurface Boilíng andDílutíon on the Isotopic Compositíon of Yellowstone Thermal Waters"J. Geoph. Res. V. 82, No. 26, (1977), p. 3694.
[19] AQUATER. "Proyecto de Investigación Geotérmica de la República delEcuador, Estudio de Reconocimiento" Informe preparado para OLADE eINECEL, San Lorenzo ín Campo, Italia, (1980).
[20] INECEL. "Estudio de Exploración de los Recursos Geotérmicos enChalupas, Primera Fase Prefactibi1idad". Informe interno preparadopor el Proyecto Geotérmico, (1983).
[213 INECEL. "Estudio Geológico de las Fuentes Termales de Palíctahua,Provincia de Chimborazo". Informe interno preparado por el ProyectoGeotérmico, (19B4).
[22] GIGGENBACH, R. "Geothermal Solute Equilibria. Derivation ofNa-K-Mg-Ca Geoindicators". Geochím. Cosmochim. Acta, 52: 2749-2765.
[23] AQUATER. "Proyecto Geotérmico Binacional Tufiflo-Chiles-Cerro Negro,Estudio de Factibi1idad" Informe interno preparado para OLADE.(1987).
[24] BARBERI, F., PATELLA, D., SCANDIFFIO, G. "Proyecto GeotérmicoBinacional Tuf iflo-Chí les-Cerro Negro, Informe de la Reunión Final dela Junta Asesora" Informe interno preparado para OLADE. (1988).
236
AVANCE DE LAS PRUEBAS DE RADIOTRAZADO EN EL CAMPOGEOTÉRMICO DE AHUACHAFAN, EL SALVADOR
W.J. McCABEInstituto of Nuclear Sciences,Department of Scientific and Industrial Research,Petone, Nueva Zelandia
E. MAYEN, P. HERNÁNDEZCentro de Investigaciones Geotérmicas,San Salvador, El Salvador
Resumen-Abstract
AVANCE DE LAS PRUEBAS DE RADIOTRAZADO EN EL CAMPO GEOTÉRMICO DE AHUACHAFAN,EL SALVADOR.
Con el fin de encontrar las rutas hidrológicas de las aguas geotérmicas y localizar tan-to áreas productoras, como áreas idóneas para realizar el proceso de reinyección de las aguasresiduales, cinco pruebas con el trazador radioactivo 1-131, se han realizado en el Campo Geo_térmico de Ahuachapán (S.O. de la República), desde diciembre de 1987 a marzo de 1990. En laprimera experiencia, el trazador fue inyectado en el pozo AH-5 y únicamente fue detectado enel pozo AH-1; en la segunda prueba, fue inyectado en el AH-2, no lográndose obtener respues-ta radioactiva, la tercera inyección fue realizada en el AH-29, obteniéndose respuesta en elAH-1 y escasamente en los pozos AH-20 y AH-22. Todas las pruebas anteriormente mencionadasfueron llevadas a cabo a reinyección estática, con agua residual proveniente del AH-1. Poste-riormente se hizo una cuarta inyección, en el mismo AH-29 a reinyección constante por tresmeses, y se obtuvo respuesta significativa en los pozos AH-1, AH-5, AH-20 y AH-22. Una quintaprueba fue iniciada inyectando el 1-131 en el pozo AH-2, a reinyección constante con agua delAH-1 por 3 meses pero problemas con el detector de centelleos líquido, obligaron a suspender-la a 10 días de inicio.
En todas las pruebas fueron recolectadas muestras de aguas de la mayoría de pozos delcampo, tanto aguas residuales en superficie, como muestras a profundidad.
Las muestras fueron llevadas al laboratorio y procesadas químicamente por método gravi-métrico, separando el compuesto radioactivo en forma de ioduro de plata y cuyo rendimientofue comparado con un segundo análisis usando el método del electrodo específico para iodo.Posteriormente se realizó el conteo del iodo radioactivo por el método instrumental de cente-lleo líquido.
PROGRESS IN RADIOTRACER TESTS IN THE AHUACHAFAN GEOTHERMAL ÁREA, ELSALVADOR.
Five tests were carried out with the radioactive tracer 131j ^n theAhuachapán geothermal área, in the SW of the country, over the periodDecember 1987 to March 1990, to lócate the hydrological courses of geothermalwaters and to find both productive áreas and áreas suitable for residual waterreinjection. In the first experitnent, the tracer was injected into the AH-5well and detected only in the AH-1 well. In the second test, the tracer wasinjected into AH-2, but no radioactive response was obtained anywhere. Thethird injection was into AH-29 and a response was obtained in AH-1, with aslight response detected in the AH-20 and AH-22 wells. All the above testswere carried out with static reinjection of residual water from AH-1. Afourth injection was subsequently made, also into AH-29, using constantreinjection over a period of three months, and this gave a significantresponse in the AH-1, AH-5, AH-20 and AH-22 wells. A fifth test wasstarted. 131j was ^o be injected into the AH-2 well with constantreinjection of AH-1 water for three months, but problems with the liquidscintillation detector forced the test to be abandoned after ten days.
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In all the tests, samples of both residual surface water, and deepwater were collected from most of the wells in the área.
The samples were taken to the laboratory and subjected to gravimetriccheroical processing. The radioactive constituent was separated out in theform of silver iodide and the yield compared to a second analysis using theiodine-specific electrode method. Afterwards, a count was made of theradioactive iodine using a liquid scintillation detector.
INTRODUCCIÓN
El Campo Geotérmico de Ahuachapán, El Salvador, es un campo "líquido dominante", con temperaturas máxima de 240°C, y que inició operaciones en 1975, conuna unidad de 30 MWe. En 1976 y 1980 fueron instaladas dos unidades más (30 y35 MWe respectivamente) para un total de 95 MWe. Posee 14 pozos productores(600 - 1200 mts) de un total de 32 perforados, con presiones de separación de6-7 Kg/cm2 (150-160°C) y porcentaje promedio de vapor de 20% en peso. El aguaresidual ha sido eliminada por dos nétodos: Por reinyección al reservorio apresión de separación (suspendida temporalmente desde 1982) y por descarga su-perficial por gravedad al Océano Pacífico a través de un canal de concreto de80 Km de longitud.
Diversos estudios han sido realizados desde su inicio, con el fin de opti-mizar la explotación del yacimiento y prolongar la vida útil del mismo. Estu-dios recientes (1) han revelado la conveniencia de expansión a la zona Sur delcampo y de la reinyección de las aguas residuales como acciones a corto y me-diano plazo, que mantengan y amplíen la explotación del recurso. Como parte delo anterior, y con la colaboración del Organismo Internacional de Energía Ató-mica (OIEA) , se han implantado experimentos de radiotrazado y estudios de isó-topos ambientales en el área, desde 1986. El 1-131 ha sido el elemento radioac-tivo escogido como trazador en los experimentos realizados al presente, en ba-se a experiencias exitosas en otros campos geotérmicos y en pruebas de cortaduración, ya que su tiempo de vida media es de 8 días. (2, 3)
PRIMERA PRUEBA RADIOTRAZADOCon el objetivo de corroborar una prueba previa llevada a cabo en 1971
con tritio antes de iniciada la explotación del campo, y de chequear el siste-ma de tratamiento de muestras y análisis en el laboratorio de isótopos inestables del Centro de Investigaciones Geotérmicas de CEL, el 11 de diciembre 1987se ejecuta la primera inyección de material radioactivo, 1-131, con una con-centración de 0.8 curies (31 GBq); este fue introducido en el pozo AH-5 conun volumen de agua residual de 140 m^ proveniente del pozo AH-1. (Ver Figura1). La recuperación radioactiva obtenida en el pozo AH-1 fue más del 25% enlos 76 días monitoreados, obtuvo su máxima concentración a los 20 días de in-yectado y comenzó a detectarse a los dos días de iniciada la prueba. Fueronsignificativas las recuperaciones detectadas en los pozos AH-20 y AH-26. Nose detecto en ninguno de los demás pozos en el campo. El sistema de inyeccióndel trazador es mostrado en la Figura 2.
Comparando con los resultados obtenidos de la prueba en 1971, aún cuandoen esta oportunidad fue recuperado en un 10% y detectada una concentración pi-co a los 3 días, parece ser que es consistente con la prueba actual, teniendoen cuentra las diferentes condiciones del campo antes y después de su explota-ción. En la Figura 3 se muestran las dos curvas, donde se puede apreciar dosconcentraciones pico o trayectorias de llegada en la prueba reciente y algunaposibilidad de una tercera alrededor del día 50. La prueba con tritio dio unasimilar dimensión de la trayectoria del flujo, pero no dio una resolución su-ficiente para mostrar una segunda trayectoria.
Aparentemente existe un flujo preferencial entre el pozo AH-5 y el AH-1con una velocidad de 100 m/día y aproximadamente un tercio del agua desde lavecindad del AH-5 es descargada al AH-1. Esta buena conexión podría estar aso-ciada a una falla o contacto plano; ya que el AH-1 es uno de los pozos mas profundos en la parte principal del campo, esto sugiere una paralelismo con otros
238
312
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309
SIMBOLOGIA
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campos como Wairakei N.Z., en donde se ha demostrado, después de muchas prue-bas con trazadores, que agua ligeramente enfriada (160-200°C), entrando al campo desde arriba a la zona principal de producción a través de fallas, se mue-ve a gran velocidad a nivel de zona de contacto con el basamento. Este movi-miento podr£a ser ayudado por fallas abiertas que coincidan con ese plano, lo-grándose de esa forma que sólo los pozos que penetran a la formación del basa-mento sean detectados, no así los pozos menos profundos cercanos al pozo inyec-tor, en donde el trazador es descargado hasta que ha sido mezclado con el cuer-po de agua varios días mas tarde. Probablemente la segunda curva del AH-1 po-dría representar el trazador mezclado en el reservorio.
SEGUNDA PRUEBA RADIOTRAZADO
Con el ob]eto de comprobar la influencia del sistema de reinyección utili-zado en el campo de 1975 a 1982, se realizó una segunda inyección (9/Jul/88)en el pozo AH-2 con una concentración de 0.6 curies (21 GBq) y 216 mts^ deagua residual proveniente del AH-1; fueron monitoreados por 56 días todos lospozos del campo, tanto muestras a profundidad como en superficie. A pesar dela profundidad del AH-2 {1200 m) , comparable con alguno de los pozos cercanoscomo el AH-29 (1200 m) y el AH-1 {1205 m), no se obtuvo ninguna señal radioac-tiva en estos, ni en los restantes con respecto al rumbo tomado por el agua den-tro del reservorio, por lo que probablemente, se haya encausado hacia un reser-vorio mas profundo. Debido a lo anterior, esta prueba puede considerarse posi-tiva, y si en la prueba programada a reinyección constante se obtiene similarrespuesta, podría considerarse una zona idónea de reinyección al reservoriogeotérmico.
TERCERA PRUEBA RADIOTRAZADOEsta prueba se realizó en el pozo AH-29 el día 12 de marzo de 1989, con el
objeto de corroborar también el grado de incidencia de la reinyección en elcampo, ya que desde 1975 hasta 1982 se mantuvo la reinyección de agua residualen el AH-29, produciéndose cambios termodinámicos en pozos adyacente al mismo,
240
lo que llevo a suspenderla. En esta oportunidad se inyectaron 2.0 curies (72.6GBq) de 1-131 con 738 mts3 de agua residual del pozo AH-1, monitoreándose elcampo por 45 días y obteniéndose señal radioactiva solamente en el AH-1y some-ramente significativa en el pozo AH-20 y AH-22. Las Figuras 4 y 5 ilustran adiferente escala (normal y semilog), los resultados de esta prueba.
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FIG. 4. Prueba de radiotrazado en el pozo AH-29, Abril de 1989.
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241
CUARTA PRUEBA RADIOTRAZADOEsta inyección del 1-131 se llevó a cabo el 29 de junio de 1989 en el mis-
mo pozo que se utilizó para la tercera prueba (AH-29), solamente que se reali-zó a reinyección constante con un flujo de agua de 49 Kg/seg durante el perío-do del monitoreo, el cual tuvo una duración de 67 días. En esta oportunidad seintrodujo una concentración de 1.7 curies (62 GBq) y fue detectado en 4 pozos,los cuales en orden de aparición fueron: El AH-5 (apto en esta oportunidad paramonitorearlo) y el AH-20 se comenzaron a detectar al primer día y los pozosAH-1 y AH-22 al cuarto día, variando solamente en la concentración o fracciónde radioactividad detectada, de acuerdo a la cercanía desde el pozo inyectoral pozo monitoreado. Debido a acciones operativas propias del campo, los pozosAH-5 y AH-22 fueron monitoreados sólo por 30 días. Las Figuras 6 y 7 ilustran,a diferente escala (normal y semilog), los resultados de esta prueba.
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242
QUINTA PRUEBA RADIOTRAZADOEsta correspondió al mismo esquema que la segunda, sólo que a reinyección
constante del pozo AH-1 (38 Kg/seg) y tuvo que ser suspendida a los 10 días deiniciado por desperfectos del equipo contador de centelleos liquido, utilizadopara detectar el radiotrazador. La concentración del 1-131 inyectado el 7 demarzo de 1990 fue de 2.0 curies (72.6 GBq) y en una muestra a profundidad delpozo AH-29 colectada al tercer día de la prueba, indicó ya la presencia del1-131 en ese punto.
PROTECCIÓN RADIOLÓGICA DURANTE LOS EXPERIMENTOS
El procedimiento para inyectar el trazador es simple y sencillo. Esta di-señado de tal forma se obtenga una mínima dosis de radiación al operador. Trans-ferir el frasco con el 1-131 del contenedor al sistema de inyección toma nor-malmente de 10 a 20 segundos. Con una cantidad de 2 curies (74 GBq) de 1-131(valor típico utilizado) resultará en una dosis equivalente efectiva menor de3 mrem (0.03 mSv), la cual es mucho menor que la dosis ambiental anual de 200mrem (2 mSv).
Para todas las pruebas, se realizan las siguientes medidas de seguridad rjadiológica:a) El sistema de inyección es chequeado visualmente a condición de la prueba
antes de realizarse para detectar fugas y defectos en válvulas y acceso-rios .
b) Material plástico es colocado debajo del sistema de inyección.c) El personal encargado del experimento se protege completamente con gaba-
chas y accesorios de caucho y plástico.d) El frasco con el elemento radioactivo es manejado con largas pinzas espe-
ciales, y colocado internamente en una válvula de compuerta, recubierta deplomo, en el sistema de inyección.
e) Las dosis de radiación se miden y registran adecuadamente durante la inyec-ción del radiotrazador.
f) Se construye una fosa de 1 mt. de profundidad cercana al sistema de inyec-ción, como prevención y desecho del material descartable utilizado.
SEPARACIÓN DE IODO DE LAS AGUAS GEOTERMALES PARA EL CONTEO RADIOACTIVO
La siguiente técnica gravimétrica es utilizada para recuperar el iodo sinprecipitación de sílice o cloruros: El sulfhídrico es oxidado con permanganatode potasio acidificado, sulfito es adicionado para reducir el iodato a ioduro,y ácido fluorhídrico es agregado para complejar y prevenir la precipitación dela sílice. Esta solución se filtra a través de una membrana. Posteriormente seadiciona nitrato de plata para precipitación de los cloruros. Debido a la reía,tiva solubilidad de los haluros de plata, el iodo es cuantitativamente precip_itado, mientras sólo se forma una pequeña cantidad de cloruro de plata. Esta so-lución es filtrada nuevamente a través de una membrana previamente tarada y loscloruros de plata fijados, son removidos con amoníaco diluido.
DETERMINACIÓN DEL IODURO EN AGUAS GEOTERMALES CON ELECTRODO DE ION ESPECIFICO
El electrodo para ioduro es sensitivo al sulfhídrico y también a los bro-muros, pero en menor capacidad. Por lo tanto, es necesario oxidar el sulfhí-drico de las aguas geotermales antes de la determinación de ioduro. Los bromu-ros no pueden ser usados como agentes oxidantes sin introducir un apreciable"blanco reactivo". El permanganato de potasio alcalino oxida el sulfhídrico asulfato pero el ioduro existente es también oxidado a iodato. El iodato tieneque ser entonces reducido a ioduro nuevamente con la adición de ácido y sulfi-to de sodio.
Para evitar problemas con el electrodo, es aconsejable mantenerlos fuerade la solución mientras hay un excedente de agente oxidante y ninguna forma-ción de ioduro.
243
CONTEO DE CENTELLEO LIQUIDO DEL IODO SEPARADO DE LAS AGUAS GEOTERMALES
Es la técnica mas sensitiva para la detección y cuantificación de radioac-tividad. Es aplicable a todas las formas de medición de emisiones con desint£gración nuclear (Alfa, Beta, etc.). La técnica analítica esta definida por laincorporación de un elemento radioactivo distribuido uniformemente en un mediolíquido capaz de convertir la energía cinética de las emisiones nucleares enemisiones de fotones. El elemento radioactivo colocado en un frasco con la so_lución centellante, encerrado en la oscuridad, permite que la intensidad delfotón sea observado en la región ultravioleta del espectro de energía electro_magnética. Esta señal es ampliada y detectada después de procesos que creanun pulso eléctrico representativo de los fotones. El registro de cada pulsodurante el tiempo de medición, provee una indicación del número de eventos cen-tellantes que ocurren en ese tiempo.
CONCLUSIONESLos resultados obtenidos al presente en Ahuachapán, han demostrado la uti-
lidad de esta técnica de avance, como una herramienta extremadamente poderosaque conlleva a poseer un conocimiento hidrológico de las condiciones de opercíción de los campos geotérmicos, en un tiempo relativamente corto, lo que ayu-da a planificar estrategias inmediatas de explotación-reinyección del recurso.
REFERENCIAS
(1) Aunzo, Z.P., Bodavarsson, G.S., Laky, C., Lippman, M.J., Steingrimsson,B., Truesdell, A.H., and Witherspoon, P.A., 1989. The Ahuachapán Geo-thermal Field, El Salvador: Reservoir Analysis, Lawrence Berkeley Labo-ratory, Report LBL-26612, U.S.A.
(2) McCabe, W.J., 1990. Artificial Tracers in Geothermal Hydrology. Instituteof Nuclear Sciences, Department Science and Industrial Research, NewZealand.
(3) McCabe, W.J., Barry, B.J., Manning, M.R., 1983. Radioactive Tracers inGeothermal Underground Water Flow Studies. Geothermics, Vol. 12, No.2/3,pp 83-110.
244
ISOTOPIC AND CHEMICAL COMPOSITION OFWATER AND GAS DISCHARGES FROM THEZUNIL GEOTHERMAL SYSTEM, GUATEMALA
W.F. GIGGENBACHDivisión of Chemistry,Department of Scientific and Industrial Research,Petone, New Zealand
D. PAÑI AGUA DE GUDIEL, A.R. ROLDAN MANZOInstituto Nacional de Electrificación,Guatemala City, Guatemala
Resumen-Abstract
LA COMPOSICIÓN ISOTÓPICA Y QUÍMICA DE LOS CAUDALES DE AGUA Y GAS PROCEDENTESDEL SISTEMA GEOTÉRMICO DE ZUNIL, GUATEMALA.
Las diferencias en la composición química e isotópica de los caudales deagua y vapor sugieren que el sistema geotérmico de Zunil es un típico sistemavolcánico magmático-hidrotérmico estrechamente relacionado con el complejovolcánico de Cerro Zunil - Domo El Azufral que se encuentra al SE de la zonatérmica principal. Los vapores magmáticos originales llegan a la superficiedel depósito de azufre de Azúfrales con mínimas alteraciones. Su interaccióncon aguas subterráneas locales en niveles comparativamente poco profundos ori-gina la formación de aguas acidas sulfatadas que se descargan sobre las lade-ras de la estructura volcánica. El movimiento de aguas primitivas más profun-das hacia el N y el O conduce a su neutralización y conversión en aguas acidu-ladas con CO , con predominio de Cl, tal como se encuentran en los pozosprofundos. La proporción del agua originalmente magmática en las aguas pro-fundas con Cl es de ~20%. La posterior interacción agua-roca transforma lamayor parte del CO en disolución en HCO , y las aguas bicarbonatadas re-sultantes se descargan a través de manantiales situados a lo largo del río Sá-mala. Los vapores que se desprenden de las aguas profundas calientes sirvende alimento a fumarolas y depósitos vapocalentados. A medida que disminuye ladistancia con las zonas ascendentes principales de los fluidos térmicos rela-cionados con el complejo de Cerro Zunil, puede esperarse que aumenten las tem-peraturas profundas del agua. Al mismo tiempo, sin embargo, existe un peligrocreciente de que lleguen a los pozos fluidos ácidos y oxidantes inmaduros deorigen magmático.
245
ISOTOPIC AND CHEMICAL COMPOSITION OF WATER AND GAS DISCHARGES FROM THE ZUNILGEOTHERMAL SYSTEM, GUATEMALA.
Variations in the chemical and isotopic composition of water and steam
discharges suggest that the Zunil Geothermal System is a typical volcanic magmatic-
hydrothermal system closely associated with the Cerro Zunil - Domo El Azufral
volcanic complex to the SE of the main thermal area. The originally magmatic vapors
reach the surface least altered at the Azufrales sulfur deposit. Their interaction with
local groundwater at comparatively shallow levels gives rise to the formation of acid
sulfate waters discharged over the flanks of the volcanic structure. Movement of
deeper primitive waters to the N and W leads to their neutralisation and conversion
to CÛ2 charged, predominantly Cl waters as encountered in the deep wells. The
proportion of originally magmatic water in the deep Cl waters is --20%. Further
water-rock interaction converts most of the dissolved CC>2 to HŒ>3, the resulting
bicarbonate waters are discharged from springs along the Samalâ River. Vapors
released from the deep hot waters feed fumaroles and steam-heated pools. Deep
water temperatures can be expected to increase with decreasing distance to the major
upflow zones of thermal fluids associated with the Cerro Zunil complex. At the same
time, however, there is increasing danger of the incursion into wells of immature acid
and oxidising fluids of magmatic origin.
INTRODUCTION
The Zunil Geothermal Field lies some 200 km west of Guatemala City. On the basisr\
of geoscientific surveys in 1973 and 1977, a 4 krrr area was cosen for the drilling of
six deep exploration wells during 1980 to 1981 to a maximum depth of 1310 m, the
highest temperature encountered was 288°C. According to these early investigations,
the heat source of the field was thought to lie to the west of the Samalâ River
(Caicedo and Palma, 1990). Initial production testing indicated rapid pressure
drawdown due to restricted permeabilities (Bethancourt and Dominco, 1982). In a
246
recent series of papers, Adams et al. (1990a) evaluate the geochemistry and
hydrology of the Zunil Field and agree that a plume of high temperature water
originates in the western part of the existing well field to travel south and east and to
mix with shallow steam-heated water there; Adams et al. (19905) describe another set
of hydrogeochemical investigations and suggest that the reservoir at Zunil is
geochemically inhomogeneous and that the recharge to the field comes primarily from
the north and east. Menzies et al. (1990) report the results of an integrated well test
program and conclude that steam output from the presently drilled wells could sustain
a 15 MW power station, however, additional wells would have to be drilled to
maintain production. Foley et al. (1990) discuss the geology and geophysics of the
Zunil geothermal system and suggest that fluid production is controlled by the
intersection of NW and NE trending faults.
The present investigation was carried out within the framework of the IAEA
Coordinated Research Program on the Application of Isotopic and Chemical
Techniques to Geothermal Exploration in Latin America. The major aim of this study
is the development of an internally consistent hydrological model of the system
possibly useful in the selection of more favorable drilling targets. The model is based
largely on the isotopic and chemical composition of water and steam discharges.
CHEMICAL COMPOSITION OF WATERS
The chemical composition of samples from well and spring discharges collected during
the present investigation are given in Table 1. The results agree closely with those
reported by INDE (1978) for earlier much more extensive surveys. The three well
samples were taken from the weirbox, but are corrected for steam loss and therefore
represent total discharge compositions. Sample locations are shown in Fig. 1. AR is
the composition of a solution resulting from the "dissolution" of 10 g of average
crustal rock in 1 kg of water. For all neutral to alkaline waters Al contents were < .05
247
Table 1.- The chemical composition of well and spring discharges from the Zunil
Geothermal Field, in mg/kg.
pH Li Na K Rb Cs Mg Ca B H(:O3 SiO2 SO4 Cl réf.
wells, weirbox samplesZ3 8.1Z6 8.4ZU 7.8
Chloridecl 9.0C2 8.3
8.70 9338.10 10286.31
springs2.706.05
1092
545788
231.0212.0101.0
51.133.9
2.33 21.89 20.55
-0.39 1
.02 .012
.01 .0402.26 .070
0.3.35 0.2
151130
748
40.045.050.8
26.234.2
5115741
9632
951 31889 61580 105
404 210396 115
181017001740
7281236
aaa
ca
Bicarbonate watersBl 9.2B2 7.0B3 7.0B4 8.4B5 7.8B6 8.7B7 8.7B8 6.1
sulfateSI 2.1S2 3.1S3 2.1S4 2.0
surfaceRl 7.0R2 7.1R3 6.1
solutionAR
0.130.180.280.370.560.560.570.91
water0.120.080.050.07
waters<.01<.01<.01
7965166199286372258157
438089134
98
16
of average0.30 240
7.410.012.218.637.236.627.319.0
14.57.7
30.932.3
4.43.35.9
crustal210.0
0.02 .0.02 .0.03 .0.05 .0.07 .0.07 .0.08 .0.09 .
0.04 .0.04 .0.11 .0.23 .
0.01 .«c.Ol <0.01 <
rock1.50 0
003 11.6012 5.4027 5.9037 18.1030 36.2022 40.5025 45.2019 30.8
009 18.9009 45.0004 14.6010 28.3
002 5.6.01 3.8.01 3.6
(10 g/kg.04 230.0
2211172542414318
761044372
987
0.71.32.83.44.74.74.62.5
0.2<.l1.81.7
0.3<.l0.2
79140259340463491501503
AI7
425939
HCO3746938
160 64132 22138 103161 129194 210196 194200 193146 235
242 117292 633209 1600287 2060
2852 5622 18
315710111418016316871
151078
151520
abbaaaab
bbaa
bbb
of water)420 0.12 2 d
a.- DSIRb.- pH, HCO3, SO4, Cl by INDE
C.- INDE (1984)d.- Taylor (1964)
248
NW
chloride watebicarbonate w.sulfate waters
fumarolesproposed dri l l sites
Fig. l - Sketch map of Zunil Geothermal Field showing positions of collection points
for water and steam samples, the inferred flow of deep fluids (arrows) and the
suggested position of drillsites.
mg/kg. An initial classification of the waters, on the basis of relative Cl, SO^ and
HCC>3 contents is carried out in Fig. 2. The samples belong to three distinct groups:
The first group consists of chloride waters discharged from the three wells (Z3, Z6,
Zll), the only natural high chloride spring Z-20 (Cl), and an abandoned exploration
well, ZP-6 (C2),
The second group covers all the neutral bicarbonate springs along Rio Samalà (Bl to
B8) and also includes the much less mineralised waters of rivers entering (RI, R2)
and leaving the thermal area.
249
neutraliseddeep
/water
HCO:
Fig. 2 - Relative Cl, 804 and HCO-j contents of waters from the Zunil Geothermal
System.
The third group is made up of acid, high sulfate waters. Samples SI and S2 represent
boiling, evidently steam-heated pools. Waters S3 and S4, discharged from over the
slopes of Dome El Azufral, have considerably higher sulfate contents, but are well
below boiling.
There appears to be very little mixing among the three groups of waters. Cl and C2
are to a minor degree affected by possible admixture of a high sulfate water. The
waters from the central group of bicarbonate springs, B3 to B7, form a tight
cluster,those farther to the north, Bl and B2, show deviations to higher and lower
804 contents respectively. The trend described by these data points may be
interpreted in terms of mixing of sulfate waters with a hypothetical bicarbonate-
chloride water possibly resulting from direct partial or complete conversion of the
CC»2 in the deep Cl waters to HCO3.
250
B8 at the southern end of the thermal area has lower Cl contents. Together with the
other HCOj springs it may delineate another trend corresponding to mixing of deep
Cl waters, as represented by the wells, with a low Cl, sulfate-bicarbonate water
possibly formed through underground absorption of CC^ rich vapors into local
groundwater. The composition of the river waters, R2 and R3, is likely to reflect
admixture of minor proportions of HCO^ waters. The existence of three distinct
groups of waters is likely to reflect three distinct environments of water-rock
interaction and three distinct degrees of attainment of water-rock equilibrium.
The interaction of fluids within hydrothermal systems with close volcanic-magmatic
associations can be evaluated in terms of two endmember processes: initial rock
dissolution adding cationic components in proportions close to those in the original
rock, and eventual rock equilibration with minerals thermodynamically stable over the
major water-rock equilibration zones within the system (Giggenbach, 1988). An initial
evaluation of the Zunil waters in terms of these processes is carried out in Fig. 3 on
the basis of relative Na, K, Mg and Ca contents. The two deep well discharges (Z3,
Z6) plot close to the rock equilibrium line at temperatures close to those measured,
the more shallow well Zll at a correspondingly lower temperature. The water
discharged from the abandoned exploration well reflects water-rock equilibrium at
considerably lower temperatures, ~18(to that from the only natural high C] spring C1
the effects of admixture of the more shallow bicarbonate waters. Extrapolation onto
the full equilibrium line suggests a temperature close to that of Zll.
Assuming relative Na, K contents of the HCOj springs to reflect those of deeper
equilibration, temperatures of 220 to 240°C are indicated. Their highly increased Mg
contents point to extensive interaction with rock at lower temperatures (Giggenbach,
1988). The data points for both river waters and acid sulfate springs approach the
composition expected for rock dissolution. Again all three groups are well separated
suggesting distinctly different formation conditions.
251
l t
0-8
0-6-
Na
0-4 -
0 -2 -
10 Mg
R 3 rivers
S3rock
HCO3 - waters acid ËJ dissolutionwaters
wa ter-rockequilibrium
10K
0-2 0-4 0-6 0-8
Ça
Fig. 3 - Plot of 10cMg/(10cMg + cCa) versus 10cK/(10cK + cNa) (in mg/kg) for
water discharges from the Zunil geothermal Field.
Other techniques to evaluate water-rock equilibration temperatures are based on
dissolved silica and relative K, Mg contents. Corresponding temperatures may be
obtained by use of the relationships (Giggenbach, 1988)
and
ts = (1000/(4.55 - log csi02)) - 273
tkm = (44107(14 - log(c^/cMg)) - 273
(1)
(2)
where the temperatures t are in °C and Cj are the concentrations of SiO2, K and Mg
in mg/kg. In Fig. 4 a line is shown for ts = t^m. For the three well discharges, both
temperatures are very similar, again close to those measured suggesting attainment of
full equilibrium among the waters and a mineral assemblage containing Na and K
feldspars, chlorite, a CaA^-silicate and chalcedony.
252
50 tfcn, (°C) 100 150 200 250 300
U
2.!
2.0-
1.5-
-300
50
0 1 2 3 4 5 6 Lkm
Fig. 4 - Evaluation of silica, ts, and K-Mg, tj^, temperatures .
The two surface samples of Cl waters, Cl and C2, plot above this line possibly
indicating equilibration with a more soluble polymorph of silica or quenching of
higher chalcedony equilibration temperatures, 240°C. The differences between ts and
tfcm are even more pronounced in the case of the HCC«3 and 804 waters. Their data
points plot close to the line representing the most soluble silica polymorph,
amorphous silica. In this case, equilibration temperatures for both silica and K-Mg are
close to those measured in the surface pools (—70°).
Many of the solution components depicted in Figs 2 and 3, such as HCO^, 804, Mg
and Ca, are greatly affected by shallow processes. In order to obtain information on
deeper processes and to establish possible genetic links among the various types of
waters, relationships among the more "conservative" constituents such as Cl, B, Li, Rb
and Cs are investigated in Figs. 5 and 6. Relative Cl, B and Li contents confirm the
253
Cl/10
.01
02
03
.05
B/CI
.10
B 1O LlFig. 5 - Relative Cl, B and Li contents of water dischatges from the Zunil Geothermal Field.
Li/10
Rb/4 CsFig. 6 - Relative Li, Rb and Cs contents in waters from the Zunil Geothermal Field.
254
close relationship between Cl and HCOß waters as already proposed by Bethancourt
and Dominco (1982) an the basis of B/C1 ratios. The two groups of sulfate waters
occupy distinct positions. The low B content of the two obviously steam heated pools
SI and S2 is likely to reflect the low B content of the comparatively low temperature
steam injected. The position of the acid waters S3 and S4 suggests interaction with a
much more B enriched, probably much higher temperature vapor at greater depth.
There is no obvious genetic relationship between the 804 and the Cl-HCO^ waters.
The overall pattern is confirmed by reative Li, Rb and Cs contents shown in Fig. 6.
Again, 804 waters occupy positions approaching that of dissolved rock suggesting
much of their solute contents to be derived from straightforward rock dissolution. The
Cl, HCC>3 waters desribe a trend corresponding to removal of Cs from the deep Cl
waters at close to constant Li/Rb ratios. The most likely process giving rise to this
pattern is incorporation of Cs into secondary zeolites at lower temperatures (Goguel,
1983). The degree of Cs removal, therefore, may reflect variations in the distance
travelled by the waters. Accepting this explanation, Waters B3 and B4 have travelled
the shortest distance or are closest to the major flow path of the thermal waters, Bl
and B8 are likely to represent waters having moved farthest or having resided
underground longest.
According to this evaluation, the natural upflow of thermal waters is centered on the
area represented by springs B3 and B4, with the waters approachig the valley floor
either from the NW or SE, but probably not from the SW or NE. Relative Cl, B, Li,
Rb and Cs contents of the Zunil waters point to the existence of a common parent
water for both Cl and HCOß waters. Information on the origin of this deep water may
possibly be obtained from their isotopic compositions.
255
ISOTOPIC COMPOSITION OF WATERS AND STEAM
The isotopic composition together with sampling dates and temperatures, tritium and
Cl and SÛ4 contents of the samples listed in Table 1 are given in Table 2. The
isotopic composition of steam discharges, together with the 6^S and mole fraction of
F^S are given in Table 4. Isotopic relationships among the samples are best discussed
on the basis of Figs. 7A and B.
Table 2.- Isotopic compositions (%o) and tritium (TU), Cl and 804 contents (mg/kg) of
well and spring discharges from the Zunil Geothermal Field.
INDE date temp. 5180 62H 3H Cl SO4 ref
wells (total discharge)Z3Z6Zll
ZCQ-3ZCQ-6Z-ll
090908
.85
.85
.87
260280250
-8-8-7
.74
.36
.25
-79-75-70
.3 1.3
.9 1.3
.1
11401070992
203860
aab
chloride springsClC2
Z-20ZP-6
bicarbonateBlB2B3B4B5B6B7B8
Z-23Z-29Z-4Z-9Z-17Z-10Z-13Z-15
0111.80.88
9393
-8-7
.20
.22-
-741.1
.2700
1236202115
cb
waters0806060707070706
.87
.86
.86
.87
.87
.87
.87
.86
4050636270678764
-11-10-10-11-11-11-11-11
.89
.38
.75
.39
.30
.89
.54
.05
-85-77-79-77-83-84-84-82
.2 1.1
.0 1.2
.7 0.0
.5
.3 1.1
.1 0.3
.7
.4 3.2
31571014
18016316871
6422103129210194193235
baabbbba
sulfate watersSIS2S3S4
ZF-38Z-36Z-19Z-31
06060808
.86
.86
.87
.87
91905674
-7-7-9-10
.25
.58
.38
.02
-70-67-75-80
.6
.4
.8
.7
151078
11763316002060
aabb
surface watersRlR2R3R4
ZR-1AZR-2AZR-16rain
06060606
.86
.86
.86
.86
181618-
-11-12-12-8
.64
.23
.22
.47
-84-85-85-55
.7
.7
.4
.3 4.8
151520-
-5618-
aaaa
a.- IAEA b.- DSIR c.- Fournier and Hanshaw (1981)
256
-60-
-80-
&2H
-80-
-100'
-14 -12 -10 S180(%0) -8
Fig. 7 - The isotopic composition of waters (A) and steam condensâtes (B) from the
Zunil Geothermal Field. In Fig. 7A a line is shown representing the
composition of residual waters after single step steam separation at a given
temperature, in Fig. 7B the compositional area of steam having separated
from the deep chloride, DW, or the more shallow bicarbonate (SW) waters
or their mixtures.
Data points for the HCO^ waters plot close to the meteoric water line suggesting that
they are largely made up of local groundwater. By assuming the river waters to be
most representative of this local groundwater, its composition is represented by point
LG. Most of the bicarbonale waters, however, are isotopically considerably "heavier"
possibly pointing to the existence of a distinct type of shallow thermal water, its
isotopic composition, SW, is taken to correspond to the mean of all bicarbonate
waters.
257
The two deep well discharges, Z3 and Z6, show increased 18O contents. Such 18O-
shifts are generally explained in terms of water-rock isotope exchange (Craig, 1963).
Recent findings on water and vapor discharges from hydrothermal and volcanic
systems along convergent plate boundaries, however, suggested that much of this
"oxygen shift" may be due to admixture of a so-called "andesitic" water with 6180 Qcf\
+ 10±2%o and 6ZH of -20±10%o . In this case their should also be some "deuterium
shift". Comparing the isotopic compositions of the two deep well discharges and that
of the range of possible groundwaters as represented by the bicarbonate springs and
the river waters, there appears to be indeed a ^H enrichment in the well discharges.
The isotopic composition of the deep Cl water, DW, is assumed to correspond to the
mean of the two well waters. Before discussing possible admixture of a magmatic
water further, an attempt is made to interpret the isotopic composition of the
remainder of the samples.
Again the two groups of acid sulfate waters occupy distinct positions. The two steam-
heated pools, SI and S2, show clearly the effects of isotopic enrichment due to surface
evaporation (Giggenbach and Stewart, 1982). The much higher 804 contents of S3
and S4 are not accompanied by higher isotopic enrichment suggesting that they are
probaby formed through absorption of S-rich vapors undergound. The composition of
the two lower temperature Cl waters, C2 and Z11, would be in agreement with
possible isotopic enrichment due to vapor loss as indicated by the line marked "steam
separation".
The isotopic composition of fumarolic vapors and the effects of vapor separation
processes are discussed by use of Fig. 7B. Two of the fumaroles were re-sampled
about one year apart, their isotopic compositions are quite different. For both Paxmux
(PX) and Fumarola Negra (FN), the 1988 samples appear to be isotopically conside-
rably depleted compared to the 1987 samples. All five data points including that of
the fumarolic area of Las Fresas to the west, however, fit into an area delineated by
258
the theoretical composition of vapors possibly produced from the deep Cl water, DW,
or the shallow water (SW), representing the HCO^ waters, or their mixtures. The
vapor at Las Fresas is most likely separated from the deep Cl water at a temperature
of around 200 °C. The compositions of the 1987 samples from Paxmux and Fumarola
Negra suggest derivation from an intermediate water, between DW and SW, the 1988
samples from SW only at quite low temperatures of about 140°C. The shift in isotopic
composition for these two vents from 1987 to 1988 may reflect differences in rainfall
giving rise to differences in the underground distribution and temperatures of the feed
waters.
The isotopic compositions of the vapor from the "dry" well ZCQ-5 (Z5) and from the
Azufrales area on Domo El Azufral (Fig. 1), are close to that of D W and therefore
show also both an 1°O and ^H shift. The similar isotopic compositions of the Azufral
vapor and of the well discharges suggests that they are derived from a common, high
temperature parent water and that the body of this parent water extends from the
main drilling area, to the west of Samalâ River, to well underneath Domo el Azufral.
According to Fig. 7A, the isotopic composition of samples Zll and C2 may be
explained in terms of isotopic enrichment due to underground vapor loss. A check as
to the validity of this conclusion can be made by use of Fig. 8 taking into account
variations in Cl contents with those in the isotopic composition of the waters. The Cl
content of the deep water is again assumed to be that of the mean of the total••}
discharge compositions of the two deep wells of 1100 mg/kg. In the case of ZH
contents, the position of sample C2 corresponds to that of the residual water after
steam separation from DW at temperatures close to 210°C. The increased ^H content
at a reduced Cl content of sample Zll suggests that its composition is due to a
process other than simple steam loss from DW.
This conclusion is supported by the position of data points in Fig. 8B. None of the
more shallow Cl waters lies along the line representing steam separation from DW.
259
-60-
4OO 80O 120O Cl (mg/kg)
Fig. 8 - Deuterium and oxygen-18 contents versus Cl contents. Lines indicatind thé
effects of dilution and steam loss on deep water DW are shown.
1SAll three show considerable enrichment in 10O at Cl contents too low to be
compatible with simple single step underground steam separation. Their compositions
are likely to reflect, in addition to steam separation, the effects of surface evaporation
or mixing with shallow waters, other than local groundwater LG, but resembling SI
and S2. A similar mixing process is likely to be responsible for the variability in the
isotopic composition of the bicarbonate waters.
As pointed out above, the increased 1°O and ^H contents of the deep well waters
with respect to the isotopic composition of local groundwater LG, may be explained in1 8therms of the admixture of a magmatic, in this case "andesitic" vapor with 610O of
260
around +10%o and 52H of about -20%o (Giggenbach, 1991). The derivation of the Zunil
waters from an "andesitic" water is evaluated by use of Fig. 9. Again assuming the
composition of local groundwater to be represented by LG, any admixture of an
"andesitic" water is described in terms of three lines representing the likely range of
compositions. The data point for DW falls within the expected range, as do those for
Zll and C2, and even those of the highly immature sulfate waters S3 and S4. The
isotopic compositions of all these waters would therefore be compatible with
formation through absorption of an andesitic vapor into local groundwater.
Fig. 9 also contains lines representing the possible fractions xa of andesitic water
involved in the formation of a thermal water. They may also be calculated by use of
the relationship
- 6m)
where the subscripts refer to the isotopic composition of the andesitic water (a), of
the thermal discharge sampled (d) and of the local meteoric water (m). For DW a
value of close to 0.2 is obtained implying that 20% of the water is of andesitic,
62H
-5O-
-100-
-10 -5 0 &18O Woo) 10
Fig. 9 - Derivation of Zunil geothermal waters through mixing of local groundwater,
LG, with "andesitic" water.
261
magmatic origin (Giggenbach, 1991). Assuming all the Cl to have been introduced
with the andesitic vapor, its Cl content is 1100/0.2 = 5500 mg/kg or 0.55% by weight.
ISOTOPIC COMPOSITION OF SULFUR SPECIES
The isotopic composition of sulfur species present in geothermal discharges can be
expected to provide additional information on the origin of the fluids and on the
conditions over the deeper parts of the system. The S contents of H^S in steam
samples are given in Table 3, the ™s and *°O contents in dissolved sulfate in Table
4. Possible correlations are discussed by use of Fig. 10.
Table 3.- Isotopic composition (in %o) and mol-fraction (Xj_j § in pmol/mol) and 6-^S
(in %o) of H2§ in steam condensâtes from the Zunil Geothermal Field.
date
25C2LFPXPXFNFNAZAZ
ZCQ-5ZP-6Las FresasPaxmux
it
Fum. Negran
Azuf rales"
091108081108110811
.85
.88
.87
.87
.88
.87
.88
.87
.88
°C
1709394939394939181
6180
-8.
-11-12-14-12-14
-9
H2O
66-.5.3.0.9.1-.1
62H
-75-
-83-82-94-94-97-
-83
H20
.0
.6
.9
.2
.0
.4
.7
XH2S
9028257260273178196802
1143
634SR
--1.-0.-0.-5.+ 0.-1.-0.""" J. •
2S
68411817
Table 4.- Sulfur-34 and oxygen-18 content of dissolved SO^ together with oxygen-18
content (%o) and 804 and Cl contents (mg/kg) of associated waters.
ZllB4B5B6B7S3S4
date
08.8707.8707.8707.8707.8708.8708.87
ST°C
144627067875674
so4 ci(mg/kg)
60129210194193
16002060
9924
18016316878
634SH2S S04
+3.6 +15.8+7.6+8.6+9.7+8.9+0.9+0.7
6180SO4 H2O
+ 4.3+4.0+4.4+6.1+5.9+ 13.0+9.5
-7.3-11.4-11.3-11.9-11.5-9.4
-10.0
262
<518oS04
-10518oHO(°/o«,)
-5 -5 0
Fig. 10 - Oxygen-18 contents in dissolved sulfate and waters (A) and sulfur-34
contents in dissolved sulfate and I^S (B) of Zunil geothermal fluid
discharges.
The isotherms shown in Fig. 10A are based on the temperature dependent
fractionation of 1°O between water and sulfate as determined by Lloyd (1968) and
Mizutani and Rafter (1969) according to
AS04-H20 * looolnaS04-H20 = (3 °65 S00^2) - 4-9 (4)
where T is in K. For the two acid waters, S3 and S4, attainment of equilibrium is
indicated at temperatures close to those measured, a process greatly facilitated
by the low pH of the waters. The trend observed for the bicarbonate springs suggests
an increase in 1"O equilibration temperatures from just below 100° for the two
southern samples, B6 and B7, to 110 to 120°C for the two more northern samples, B4
and B5. This minor trend may reflect an actual difference in equilibrium
temperatures, but also differences in the residence time of the waters underground.
263
For the shallow well Zll a temperature of 155° is suggested if the total discharge
1°O content is used, 180° if that of the separated water is used. Both are, for
unknown reasons, below that measured of 250° (Table 2). Residence times of the
waters within the Zunil geothermal system are obviously long enough to allow 1°O in
sulfate to adjust to quite low temperatures.
A much "slower" process is 34S exchange between SO4 and H2S. Accepting the
isotopic composition of H2S within much of the Zunil system to be close to the range
of about -2 to 0%o, as observed for the fumarole discharges, apparent equilibration
temperatures may be evaluated by use of
AS04-H2S ~ 100° lnS04-H2S = (6 04° °00^) + 2-16 (4)
as reported by Robinson (1973).
Both the two acid sulfate waters were collected from springs on the slopes of Domo
El Azufral. The most closely related discharge of H2S is likely to be that from the
Azufral fumaroles. Their 6 S values are within the range observed for the other
vapor condensâtes. By use of a value of -l%o for H2S, exceedingly high temperatures of
>1000°C are obtained for S3 and S4. The very low isotopic fractionation suggests that
formation of sulfate from a sulfur containing vapor proceeded under conditions
preventing attainment of equilibrium even for these acid waters. Such conditions are
likely to prevail over parts of the system were the originally magmatic, or "andesitic",
vapors first interact with groundwater. These vapors would contain much of their
sulfur in the form of SO2. In this case the acid sulfate waters form through
disproportionation of SO2 according to
4 SO2 + 4 H2O = 3 H2SO4 + H2S (5)
If the H2S and H2SO4 are separated shortly after their formation, the ™s content of
the sulfate may still reflect that of the original magmatic gas of around 0 to + l%o.
264
Sulfur-34 equilibration temperatures for the bicarbonate waters are also unrealistically
high, the shift to higher ^S values, however, would point to some equilibration.
The composition of sample Zll suggests reasonable equilibration temperatures of
350° if a value of -l%o is accepted for the l S, 500° if equilibrium with coexisting Ir^S
(+3.6%o, Table 4) is assumed. Both temperatures are very high but may reflect the
occurrence of such temperatures deeper within the Zunil hydrothermal system.
The 34s content of 804 in the two acid waters S3 and S4 points to their formation
under conditions preventing even minor re-equilibration with I^S. Such conditions
are likely to occur if the zones of conversion of magmatic to hydrothermal fluids lie at
shallow levels. The isotopic compositions of sulfur species in fluids from the Zunil
geothermal field, therefore, strongly suggest that initial interaction of magmatic vapors
with groundwater takes place within Domo el Azufral structure. On moving away
from these zones the initially highly immature waters are converted to neutral chloride
waters as encountered in deep wells and lateron to more dilute bicarbonate waters
discharged from surface springs.
CHEMICAL COMPOSITION OF VAPOR DISCHARGES
The chemical compositions of steam samples from the Zunil Geothermal Field are
given in Table 5 together with typical analyses from other fields in Guatemala for
comparison. The samples were collected and analysed by use of procedures described
by Giggenbach and Goguel (1989). For the 1987 samples from Paxmux and Fumarola
Negra (Table 3) only data for the gases absorbed into the alkaline condensate are
given.
An initial assessment of the possible origin of the gases is carried out in Fig. 11 on the
basis of relative N2, He and Ar contents. The samples from Zunil, representing a
wide variety of steam discharges from wells (Z3, Z5, Z6, Zll) and fumaroles (LF, PX,
FN, AZ) still cover quite a limited range halfway between the composition of
265
K) TableOsOs
wellsZ3Z5Z6Zll
5.- Composition of vapor discharc
°C
260280280250
ys
.301.0.34.43
H
997997997996
2°
700200200900
co22192269126822994
H2S
67905643
jes fr
NH3
5.24.05.13.1
om the
He
.0060
.0059
.0056
.0040
Zunil Ge
Ne
.00018
.00014
.00039
.00040
otherma
H2
13.1012.3011.9012.31
1 Fiel
Ar
0.140.080.250.28
a, in mol
°2 N2
<.2 23<.2 11<.3 36<.3 46
/mol .
CH4
0.580.390.671.02
fumarolesC2LFPXPXFNFNAZAZ
9394939494939181
1.01.01.01.01.01.01.01.0
998995990989996994994992
710800800120200500200505
123037978942
102153620512048436165
28257260273178196802
1143
6.51.30.3<.l0.3<.l0.30.6
.0061
.0206-
.0534-
.0248
.0296
.0353
.00051
.00118-
.00330-
.00121
.00116
.00053
1.3140.65-
3.60-
35.400.460.38
0.300.76-
2.94-
1.100.700.60
0.6 33<.2 101-
<.9 389-0.8 139<.3 151<.9 186
0.442.77-
20.00-6.980.460.98
San Marcos1.0SM 93
TecuamburroTA 95 1.0AmatitlânLC 95MoyutaMC 97
998 700 1150 83 0.6 .0051 .00013 0.35 0.90 <.l 60 0.20
956 000 36740 5764 0.7 .1280 .01230 0.44 6.60 <.4 1480 3.52
1.0 944 900 54300 110 7.7 .3700 .00055 14.30 0.11 606 19.3
1.0 994 400 5387 44 <.l .0235 .00039 6.90 0.45 <.6 162 0.05
N2/100
air
air saturatedwater
10He
Fig. 11 - Relative N2, He and Ar contents in gases from the Zunil Geothermal System.
meteoric gases (air, air saturated water) and "andesitic" gases as defined on the basis
of a large number of volcanic gases from around the world (Giggenbach, unpublished
results). The similarity in relative N£, He and Ar contents supports a common origin
of the gases within the Zunil system. The compositions of the gases from the San
Marcos (SM), Tecuamburro (TA) and Moyuta (MC) geothermal fields follow this
trend, typical of geothermal systems along convergent plate boundaries, that from
Amatitlan (LA) shows a somewhat increased He content.
Relative contents of the three quantitatively most important constituents of
geothermal vapors, O, CC>2 and S are shown in Fig. 12. Except for Paxmux,
data points for the Zunil field lie along a line indicating large variations in S
contents at close to constant CC>2 contents of about 0.3 mol-%. Well discharges and
C2, SM and MC show the lowest S content, the two low altitude fumaroles FN and
PX occupy intermediate positions, the highest relative S content is found for the
267
H2O/100
0-1
"/„CO
30H2S5 CO2 /H2S
Fig. 12 - Relative H^O, CC^ and H2S contents in vapor discharges from Zunil.
sulfur depositing vent at Azufrales. The high I-^S content there corresponds to that of
similar S-depositing vents at Tecuamburro (TA). The repeat samples from Zunil
occupy very similar positions suggesting no major changes in the chemistry of these
gases over the period from August 1987 to November 1988.
The major causes of variations in CC^/I^S ratios of geothermal steam discharges are
differences in the solubility of the two gases, I-^S being three times as soluble as CC>2
(Giggenbach, 1980). Vapors boiling off from a deep liquid phase, therefore, are
initially enriched in CC^, the residual solution becoming enriched in IH^S. The wells
then are likely to tap little degassed water having travelled deeply without much
opportunity to lose gas. The fumaroles discharge vapors from probably more shallow
bodies of already degassed water. The validity of this assumption may be checked by
including a gas with an even higher solubilty contrast, such as
268
Because of its very low solubility, CH^ contents can be expected to be highest in
"early" vapors, very low in vapors from already degassed bodies of water. Relative
CH4, CC>2 and I^S contents are plotted in Fig. 13 together with lines representing
the composition of vapors and residual fluids formed on removal of the fraction of
steam, y, at 150° and 300°C from a an original deep water with a composition close
that of the wells Z3, Z6 and Zll. The theoretical lines were calculated for closed
system, single step steam separation. The effects of temperature on the relative
position of the "early" and "late" steam separation lines are obviously quite small.
Separation of even very minor fractions of vapor, however, leads to marked shifts in
relative gas contents, especially at lower temperatures. Contrary to the findings
suggested by Fig. 12, PX and FN appear to represent very "early" vapor carrying much
of the little soluble CH^ while the vapor from Z5 is possibly derived from a body of
5000CH4250
300
LMC
350'
400'
\10
30HoS30 20 10 5 C02/H2S
Fig. 13 - Relative CH^ CC>2 and H^S contents of Zunil vapor discharges. Lines are
shown illustrating the effects of vapor separation from the deep water DW, as
a function of y, the fraction of steam removed, in %, at 150° and 300°C.
269
water having lost a few percent of vapor. The composition of vapors from the
Azufrales fumarole may indicate derivation from an extensively degassed deep water.
The above evaluation, of course, assumes that all the vapor samples are derived from
a common body of deep water with a uniform composition. There are however a
series of other processes which could have affected relative CH^ CC>2 and S
contents. One of the most obvious is re-equilibration of CH4/CÜ2 ratios in response
to variations in temperature. Variations in this ratio are indicated in Fig. 13 in terms
of isotherms calculated by use of
tMC = (46257(10.4 + log(xCH4/xCo2))) -273 (6)
where tjyjc is the temperature for equilibrium of CH4 and CO2 with crustal rock
(Giggenbach and Goguel, 1989). According to this interpretation, the two gases in the
three wells (Z3, Z6 and Zll) and in C2 equilibrated at about 340°. The apparently
higher temperature derived for Z5 is probably due to some earlier underground steam
loss, while the temperatures of 360 to 380° for the steam discharged from vents on
top of Domo El Azufral (AZ) may be indeed those in deeper zones. The lower
temperatures of 270 to 300° indicated for PX, FN and LF may reflect equilibration in
more shallow, cooler zones of the system.
Additional gas phase equilibration temperatures may be obtained by use of recently
derived relationships (Giggenbach and Goguel, 1989)
log(xH2/xAr) = °-014t - 2-5 (7)
and
log(xco /XAT) = 0.0277t - 7.53 + 2048/(t + 273) (8)
where Xj are again the mole fractions as given in Table 5 and t is the temperature in
°C. In Fig. 14 a line is shown representing equilibrium for both H2 and CC>2 in a
single liquid and a single vapor phase. Tie lines represent equilibrium mixtures of
liquid and fluid.
270
-HA
1 '
0-
-1-
equilibrium vaporLHA=
LCA=log(xc02/xAr)
1 2 3 4 5 . 6LCA
Fig. 14 - Evaluation of temperatures of H2 and CC>2 equilibration with rock. Lines are
shown for attainment of equilibrium of the gases dissolved in a single liquid
and in a single vapor phase.
All the data points for the wells and for some of the steam vents (LF, FN) plot close
to the full equilibrium line at temperatures between 250 and 300°C. For the more
outlying vapor discharges (C2, PX), H2-Ar temperatures are below CO2-Ar
temperatures possibly reflecting the different rates of equilibration of the two gases
with H2 being considerably "faster" than CO2 (Giggenbach, 1987). Apparent H2/Ar
and CO2/Ar temperatures for the fumaroles are lower than those of the deep wells
suggesting long residence times at lower temperatures, in agreement with CH4/CO2
temperatures (Fig. 13).
Table 6 gives the hydrocarbon contents xj of the Guatemalan thermal vapor
discharges. The mole fractions given may be converted to total discharge mol-fractions
Xj, in nmol/mol, according to
X =i = vs XCH4 xi
where ys and XCH are the steam fractions and CH4 contents given in Table 5.
(9)
271
Table 6.- Hydrocarbon contents of geothermal gases in Guatemala, in mmol/mol of
the hydrocarbon fraction.
met ete eta pre pra ibu nbu ipa npa ben toi ebe pmx o-x
Z3 969 4.0 8.8 1.4 1.5 0.2 1.2 0.8 1.5 3.9 1.5 0.9 1.0 0.8Z5 980 1.4 4.6 0.4 0.8 0.3 0.8 1.0 1.2 4.0 0.9 0.7 0.9 0.7Z6 978 1.1 8.5 0.8 1.2 0.2 0.6 0.4 0.7 3.7 0.5 0.3 0.4 0.4Zll 535 <.5 5.2 <.5 0.7 <.5 <.5 <. 5 <.5 3.8 42 135 219 57C2 349 1.1 7.2 <.5 1.5 <.5 <.5 <.5 <.5 6.5 70 225 215 59LF 988 <.3 2.7 0.4 <.3 <.3 <.3 <.3 <.3 1.3 0.5 1.4 2.5 0.8PX 993 .02 4.3 .01 1.0 0.2 0.2 .05 .05 1.0 .08 .03 .07 .02FN 990 < . 0 2 4.1 < .02 0.9 0.2 .25 .06 .14 2.1 .40 .78 .93 .31AZ 970 <.5 3.1 <.5 <.5 <.5 <.5 <.5 <.5 2.0 1.7 7.8 11 3.9AZ 841 <1 <.5 <.5 <.5 <.5 <.5 <.5 <.5 3.1 17 42 55 17San MarcosSM 950 0.5 3.2 0.7 1.1 <.8 <.8 1.3 1.5 1.4 1.5 4.2 2.7 2.5TecuamburroTA 707 2.9 2.5 0.4 0.3 <.2 <.2 <.2 <.2 4.8 39 90 108 36AmatitlanLC 940 <.5 30.8 <.5 7.6 0.9 1.9 0.5 0.5 13.6 1.4 1.8 1.3 1.2MoyutaMC 910 3.2 5.6 <.5 2.1 0.7 <.5 <.5 <.5 38.3 7.9 9.3 16 4.0
met: methane pre: propene nbu: n-butane ben: benzene pmx: p+m-xyleneete: ethene pra: propane ipa: i-pentane toi: toluene o-x: o-xyleneeta: ethane ibu: i-butane npa: n-pentane ebe: ethyl benzene
Hydrocarbons in geothermal vapor discharges are largely derived from the
decomposition of sedimentary organic material subjected to increased, up to magmatic
temperatures. The presence of unsaturated and aromatic hydrocarbons in the Zunil
well discharges points to the existence of very high temperatures (<500°C) of
interaction with sedimentary material (Giggenbach, unpubl. results).
Because of their higher solubility in water, aromatic hydrocarbons tend to accumulate
in residual waters, again allowing the differentiation between "early" and "late" vapors.
The very low contents of aromatic hydrocarbons in PX and FN, and possibly LF,
agrees with their position in the "early" vapor part of Fig. 13. Similar low contents in
the deep wells, Z3, Z5, Z6, would also classify them as "early" vapors. The extremely
272
high contents of aromatic hydrocarbons in the more shallow Cl water discharges Zll
and C2 can only be explained in terms of possible enrichment from percolating vapors
without affecting much relative CH^ Œ>2 and IH^S contents. The comparatively high
contents in samples from Azufral, AZ, agrees with their possible "late" nature as
already indicated by Fig. 13. Generally speaking, the hydrocarbon contents of all the
Guatemalan vapors are those expected for geothermal systems closely associated with
volcanic-magmatic systems.
The chemistry of gases associated with fluid discharges fom the Zunil Geothermal
Field suggests an essentially magmatic, or more specific "andesitic" origin. Gas
equilibration temperatures are close to those measured in the wells or higher pointing
to the possible presence of higher temperature zones within the system.
GEOCHEMICAL MODEL OF THE ZUNIL GEOTHERMAL SYSTEM
The evolution of fluids with close volcanic-magmatic associations generally proceeds
along the sequence magmatic vapor - acid chloride - msulfate waters - neutral
chloride water - bicarbonate water. Accepting this general pattern also for Zunil, both
chemical and isotope evidence clearly show that the first step, the conversion of
magmatic vapors to acid waters occurs within the andesitic structures of Domo El
Azufral and Cerro Zunil, to the SW of the discharge area. The next step, the
conversion of the initial acid waters takes place during further movement of the
waters to the north and west.
The driving force behind the upward movement of thermal waters is the buoyancy
caused by the density difference between hot thermal and surrounding groundwater.
The level to which the waters are able to rise is determined by the static groundwater
level. In the case of Zunil it is likely to be defined by the level od the Samalâ river.
Any water rising within the El Azufral - Cerro Zunil Complex will spread out to reach
the surface at the nearest hydrological low, the bed of the Samalâ River. Steam may
273
boil off from the spreading tongues of hot water as they approach the surface to
reach the surface as fumaroles or steaming ground. Vapors generally rise close to
verticallty thus marking the epicenters of deeper boiling zones.
Relative HCG^/Cl ratios of the bicarbonate waters correspond closely to CC^/Cl
ratios in the waters supplying the deep wells. The HCC>3 waters, representing most of
the natural thermal water discharge from the Zunil Geothermal Field, then are most
likely to have formed through virtually quantitative conversion of CC>2 to HCOß by
interaction with rock preceding or following about tenfold dilution with groundwater
The incresased sulfate content of these waters may be due to admixture of some of
the immature acid sulfate waters (S3, S4) at shallow levels or incomplete "maturation"
of the more primitive, deep parent water.
The extent of the deep body of high temperature waters can be expected to be
delineated by the occurrence of hot springs and vapor discharges. To the NW the
limit of the field is likely to be marked by the Los Vahos vapor discharges, to the N
by the Almolongo bicarbonate springs. The inferred flow paths of the waters
compatible with isotopic and chemical evidence is marked in Fig. 1 by arrows, the
likely distribution of the waters underground is shown in Fig. 15.
The model was constructed by assuming uniform and isotropic permeability conditions
underground and therefore is likely to represent only the potential distributuion of
fluids within the system. The actual distribution is largely governed by generally
unknown geological factors such as the nature and distribution of igneous and
sedimentary rocks and the presence and rate of formation of faults and fissures
facilitating fluid movement. Isotherms were drawn by assuming conductive heat
transfer and a thermal gradient of 50°/km outside the actual geothermal system. The
hydrothermal system is assumed to be associated with a body of cooling magma
beneath the El Azufral - Cerro Zunil complex. On the basis of models drived for the
White Island (Giggenbach, 1987) and Nevado del Ruiz (Giggenbach et al., 1990)
274
NW
LLANO DEL FINAL
LosVohos
C CANDÊLARIA
chloride waters -=liquid+vaporsulfate waters-2-
10 ( k m ) 5 0
Fig. 15 - Cross-section through the Zunil geothermal system along line N W - SE of
Fig. 1. The distribution of fluids is that indicated by the geochemistry of the
fluids discharged.
volcanoes, the magmatic system is assumed to be surrounded by an extensive two-
phase, vapor-brine envelope. It provides the environment for initial conversion of
magmatic fluids to acid brines. Over the upper slopes of Domo El Azufral, the vapors
released from deeper primitive waters reach the surface leading to the formation of
extensive patches of steaming ground, the deposition of elemental sulfur and the
formation of the acid sulfate waters S3 and S4. At greater distance, continuing
intertaction with rock leads to the neutralisation of the acid waters, removal of most
of the magmatic sulfur in the form of alteration minerals such as sulfates (alunite,
anhydrite) and sulfides (pyrite). Much of the neutral Cl water appears to remain
stored underground. A small fraction was discharged naturally from spring Z-20 (Cl)
or is discharged in diluted form from the bicarbonate springs.
According to Fig. 15, the major upflow zones of thermal waters lie to the east of
Samalâ River. Because of permeability problems encountered over the present
production area to the west, exploratory drilling to the east should be considered
275
seriously. The choice of drillsite is largely a function of accessibility and depth to the
production zones. Another limiting factor which has to be taken into account when
drilling close to Domo El Azufral, is the possible ingression of immature, acid
waters both at shallow and deep levels. The potential for such ingression is likely to
increase with decreasing distance to the Azufral Dome. The position of drillsites
representing possibly the best compromise are indicated in Fig. 1.
ACKNOWLEDGMENT
The present investigation was carried out within the framework of the IAEA
Coordinated Research Program on the "Application of Isotopic and Chemical
Techniques to Geothermal Exploration in Latin America" (GUA/8/009-2) with
financial support from the Government of Italy.
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278
INVESTIGACIONES GEOQUÍMICAS REALIZADAS ENLOS CAMPOS GEOTÉRMICOS DE ZUNIL Y AMATITLAN,GUATEMALA
A.R. ROLDAN MANZOUnidad de Desarrollo Geotérmico,Instituto Nacional de Electrificación,Ciudad de Guatemala, Guatemala
Resumen-Abstract
INVESTIGACIONES GEOQUÍMICAS REALIZADAS EN LOS CAMPOS GEOTÉRMICOS DE ZUNIL YAMATITLAN, GUATEMALA.
Los estudios geotérmicos en Guatemala se iniciaron en 1971 en elcampo geotérmico de Moyuta. En 1981 OLADE realizó un estudioregional determinando trece zonas de interés geotérmico. Zunil yAmatitlán, analizados en este estudio, fueron clasificados dentrodel grupo A o de prioridad uno.
Las aguas de Zunil se clasifican según tres tipos: cloruro-sódicas,neutras o bicarbonatadas y acidas. Análisis de gases indican origenmeteorice y magmático, con recarga local, y la existencia de dosacuíferos diferentes. Análisis isotópicos evidencian origenprofundo y un tiempo de residencia para el agua de aproximadamente200 años. Geotermómetros de aguas, gases e isótopos proporcionantemperaturas entre 180° y 400°C. Por medio de prospecciones demercurio y radón se identificaron zonas de alta permeabilidad.Las aguas de Amatitlán se clasifican según dos grupos:cloruro-sódicas y bicarbonáticas. Análisis isotópicos sugierenorigen profundo y un tiempo de residencia mayor de 100 años.Análisis de gases indican origen magmático. Geotermómetros decationes proporcionan temperaturas de 240° a 245°C, mientras que lastemperaturas de equilibrio para gases van de 220° a 300°C. Estudiode radón evidencia anomalías con orientación NO-SE las cualesindican un rasgo estructural profundo.
GEOCHEMICAL INVESTIGATIONS IN ZUNIL AND AMATITLAN GEOTHERMAL FÍELOSGUATEMALA.
Geothermic studies in Guatemala began in 1971 in the Moytageothermal field. In 1981 OLADE realized a regional study, finding13 áreas as geothermal prospects. Zunil and Amatitlán, object ofthis study, were classified in group A (priority one).The Zunil waters are classified in three groups: sodium-chloride,bicarbonate or neutral and acid. Gas analysis shows magmatic andmeteoric origin with local recharge and two different reservoirs.Isotopic analysis yields deep origin and a residence time ofaproximately 200 years. Water, gas and isotopic geothermometers
279
yield temperatures between 180°C and 400°C. Mercury and Radonsurveys show high permeable zones.
The Amatitlan waters are classified in two groups: sodium-chlorideand bicarbonate. Isotopic analysis suggests deep origin and aresidence time greater than 100 years. Gas analysis yields magmaticorigin. Cation geothermometers yield temperatures from 240°C to245°C and gas equilibrium temperatures ranging from 220° to 300°C.Radon surveys show anomalies NW-SE oriented which could correspondto a deeper subvolcanic structure.
1. INTRODUCCION
Dentro de la bûsqueda de fuentes alternas de energia paracontrarrestar los elevados precios del petroleo, en Guatemala, seiniciaron las investigaciones geotérmicas en el ano de 1971 a travesdel Proyecto Minero de las Naciones Unidas conjuntamente con laDireccion General de Mineria e Hidrocarburos. Posteriormente, elInstitute Nacional de Electrificaciön (INDE) ha sido el responsablede la investigaciôn geotérmica en el pais; los primeros estudios seefectuaron en el ârea de Moyuta, de 1971 a 1976. En el ârea deZunil, las investigaciones se iniciaron en 1973, con la colaboraciönde Japan International Cooperation Agency, llegando a concluir en1976 que Zunil es un ârea de alto potencial para el desarrollo de laenergia geotérmica (JICA, 1977).
En 1981 OLADE efectuô un estudio régional en el ârea limitada alnorte por la falla del Motagua, al sur por la costa del Pacifico, aloeste por la frontera con Mexico y al este por la frontera con ElSalvador. Como resultado de este estudio se definieron trece zonasde interés geotérmico clasificadas en très diferentes grupos. Loscampos de Zunil y Amatitlân, objeto de este estudio, fueronclasificados dentro del grupo A o de prioridad uno. (Figura 1).
2. CAMPO GEOTERMICO DE ZUNIL
El campo geotérmico de Zunil esta situado al oeste de la ciudad deGuatemala a aproximadamente 218 km por carretera asfaltada, a una
280
00
FIG. 1. Localization de los campos geotérmicos de Zunil y Amatitlän.
altura de 2000 ra.s.n.m. La temperatura ambiente varía entre 15° y20°C, la precipitación promedio anual es de 2000 mm, mientras que laevapotranspiración es de 750 mm por año.El campo ha sido dividido en dos áreas: Zunil 1, donde se instalaráuna planta generadora de 15 MW y Zunil 2 donde actualmente seefectúan estudios de prefactibilidad. (Figura 2).
NW
O AGUAS CLORUROSODICASO AGUAS BICARBONATADASO AGUAS SULFATADASO FUMAROLAS
FIG. 2 Locahzación de muestras seleccionadas en elcampo geotérmico de Zunil y delimitación de las áreasde Zunil 1 y Zunil 2.
2.1 GEOLOGÍA
Zunil se encuentra localizado en el borde sur de la depresión deQuezaltenango, considerada una caldera de edad terciaria precedidapor un volcanismo efusivo del tipo andesítico dacítico. Se tratade un graben de unos 3 kms de ancho con dirección noreste-suroesteformado por un sistema de fallas a lo largo del cual se localizanlos domos Cerro Candelaria, Cerro Quemado y el Volcán Zunil.
El basamento es de naturaleza granítica y granodiorítica de edadcretácica, sobreyaciendo discordantemente un fuerte espesor de lavasandesíticas y piroxénicas de edad terciaria, tobas y brechastobáceas, tobas riolíticas, tobas soldadas, lahares, flujosriodacíticos y flujos basálticos con espesor entre 800 y 1000 m.(Figura 3).
282
COMPLEJO DOMICO DEL CERROQUEMADO
GRABEN DE ZUÑÍ LCOMPLEJO OE ROCASVOLCÁNICAS TERCIARIAS
RIODACITAS DOMO LA PEDRERA. ANDESITAS DOMO EL AZUFRAL. P5RH ROCAS METAMORFICAS,
ROCAS PIROCLASTICAS RECIÉN- RTCpn COMPLEJO ROCAS VOLCANI-TES. IS&ICAS TERCIARIAS.
PTP,LAVAS RECENTES•ÍHDEL DOMO ZUNIL.ANDESITAS Y LAVAS DEL DOMOCERRO QUEMADO. ROCAS INTRUSIVAS.
FIG. 3. Geología del área geotérmica de Zunil (Tobías, 1977).
2.2 GEOQUÍMICA DE FLUIDOS
Como producto de las primeras investigaciones, se localizaron 70manantiales los cuales fueron químicamente analizados y 20 de ellosson analizados mensualmente hasta la fecha. Seis pozos productoreshan sido perforados desde 1980 y sus fluidos químicamenteanalizados. Las aguas de Zunil se clasifican en tres tipos (Tabla 1y Figura 4) cloruro-sódicas (con alto contenido de Na y Cl )neutras o bicarbonatadas (con alto contenido de HCO ) y acidas(con alto contenido de SE~) (Giggenbach 1988, Bethancourt 1978,JICA 1977). Se considera que las aguas del tipo ácido provienen delcalentamiento de las aguas superficiales por vapor de origenprofundo, rico en sulfates y que las aguas bicarbonatadas son elproducto de aguas superficiales calentadas por conducción, mientrasque las aguas cloruro sódicas tienen un origen profundo.
283
COMPOSICIOH QUÍMICA DE
I N D E «o. °C pH
Pozos (ve r tedero)
«1 ÍCQ-3 92 8.1« 2CQ-6 92 8.4K3 Í-1I 144 7.8
Li
8.708.106.31
Ka
93310281092
FOJOS 1
I
231.0212.0101.0
T A B L A
I HARAHTIALES
(G iggenbach
Bb
2.331.890.55
Cs
2.022.012.36
1
DEL CAMPO GEOTEBMICO DE
1988)
«9
.012
.040
.070
Ca
151130
B
40.045.050.8
A l
--
0.51
2 0 N I L
HCO,
51157
41
(ig/kg)
SiOj
951889580
S04
3161
105
Cl
181017001740
reí
AAA
( U D a n t i a l e s c l o r u r o sódicos
Cl J-20 89 9.0
Aguas b i c a r b o n a t a d a s
Bl 1-23 40 9.2B2 2 -29 50 7.0B3 Z-4 63 7.0B4 S-9 62 8.4B5 !-17 70 7.8B6 2-10 67 8.7B7 M3 87 8.7B8 2-15 64 6.1
Aguas s u p e r f i c i a l e s
El IB-1A 18 7.082 Z E - 2 A 16 7.1B3 IB-16 18 6.1
Aguas sulfatadas
SI Z P - 3 8 91 2.1S2 2-36 90 3.1S3 Z-19 56 2.1S4 2-31 74 2.0
AR Soca proiedio
B e f e r e c c i a s :A. DS1RB. pB, f l C O j , SO,, ClC . I N D E
2.70
0.130.180.280.370.560 .560.570.91
(.01(.01(.01
0.120 .060.050.07
3.00
I N D E
545
7965
166199286372258157
98
16
438089
134
2400
51.1
7.410.012.218.637.236.627.319.0
4.43.35.9
14.57.7
30.932.3
2100
-
0 .020 .020.030.050.070.070.080.09
0.01(.010.01
0 .040.040.110 .23
15.0
-
.003
.012.027.037.030.022.025.019
.002( . 0 0 2< . 0 0 2
.009
.009
.004
.010
.500
0.3
11.65.45.9
18.136 .24 0 . 545.230.8
5.63.83.6
18.945.014.628.3
2300
7
2211172542414318
987
76104
4372
4200
26.2
0.71.32.83.44.74.74.62.5
0.3(0.10.2
0.2(0.11.81.7
-
-
( . 0 50.09< . 0 5( .05(.05( .05(.05( . 0 5
0.080.064.40
6.7042 .059.039.0
-
96
79140259340463491501503
746938
----
-
404
160132138161194196200146
285222
2 4 2292209287
-
210
6422
103129210194193235
-5618
117633
16002 0 6 0
-
728
3157
101114180163168
71
151520
1510
78
-
C
ABBáAAAB
BBB
B8AA
-
284
AGUA PROFUNDANEUTRALIZADACOMPLETAMENTE
lADA
HC03
FIG. 4. Diagrama CI-HCO3-S04 para las aguas de Zunil(Giggenbach, 1988).
Varias zonas de fumarolización activa se localizan en el campogeotérmico de Zunil y son denominadas: Fumarola Grande o Zona delGeiser, Fumarola Negra, Fumarolas de Paxmux, Los Vahos, AlteraciónHidrotermal La Cascada y Fumarolas de las Fresas (Tobías 1971,Figura 2). Alteración hidrotermal se observa en las áreasadyacentes a las fumarolas, donde la roca original se hatransformado produciendo minerales arcillosos y oxidacionesferromagnesianas, se observa especialmente caolinización ysilicificación; depósitos de azufre nativo se encuentran en al áreade Las Fresas, mientras que en la Fumarola Negra se ha localizadomercurio.
Análisis de gases muestran un bajo contenido de gases nocondensables (0.85% aproximadamente) principalmente CO y H S.
£* ¿i
Las concentraciones relativas de nitrógeno, helio y argón indicanorigen tanto meteorice como magmático indicando la existencia de dosacuíferos diferentes (Figura 5 y Tabla 2). Esta tesis fue propuestainicialmente por OLADE (1982) considerando que existe un acuíferosuperficial producto de la dilución de aguas provenientes de unacuífero profundo con elevada temperatura.
285
T A B L i 2
CONPOSICIO» DEL VAPOR DEL CAKPO GEOTÉRMICO
(liol/iol o ppi por voluien!(Giggenbach 1988)
!326!5111LFPIPíAZ
HDE Do.
2CQ-3ICO.-62CQ-52-11
FRESASPAIMDÍF.IEGEAAiDFRAL
°C Is
138 0.123 0.- 1.144 0.94 1.93 1.94 1.91 1.
3034004300000000
M
997700997200997200996900995800990800996200994200
C02
21922682269129943797894236204843
H2S
67569043257260178802
DE
«B,
55431000
.2.1
.0
.1
.3
.3
.3
.3
IDIIL
He
0.00600.00560.00590.00400.0206
--
0.0296
¡32625211LFPIPNAI
IKDE Do.
2CQ-32CQ-62CQ-5Z-ll
FRESASPAIHOÍF. NEGRAA20FRAL
He
.00018
.00039
.00014
.00040
.00118--
.00116
Bj Ar Oj
1311124040
0
.10 0.
.90 0.
.30 0.
.65 0.
.65 0.--.46 0.
14250828
0.20.30.20.3
76 0.2
70
--
0.3
«a
23361146
101--
151
00012
0
CH,
.58
.67
.39
.02
.77--.46
R
-4.-4.-4.-4.-4.
--
-6.
•
8892919139
33
OH*
AIRE SATURADO CONAGUA SUBTERRÁNEA
FIG. 5. Diagrama N2-Ar-He para los gases de Zunil(Giggenbach, 1988).
286
Análisis isotópicos fueron efectuados inicialmente para determinarel origen de la recarga en el campo de Zunil (Fournier y Hanshaw1981) concluyendo que la recarga es de origen local. Análisisposteriores (Giggenbach 1986) confirman la tesis de Fournier yHanshaw, y en base a los mismos, Giggenbach propone una hidrologíatentativa del campo geotérmico de Zunil, la cual se muestra en laFigura 6. Un modelo diferente fue desarrollado por West Jec (1990)en base a los estudios de prefactibilidad efectuados en el área deZunil 2 (Ver figura 7). Los resultados isotópicos muestran un
18enriquecimiento de O entre 2.2 y 5%o (Giggenbach 1988, Cordón yMarida & M K Ferguson 1990), lo cual evidencia un origen profundoluego de un equilibrio agua-roca; asi mismo puede observarse laexistencia de aguas calentadas por vapor (Tabla 3 y Figura 8). Eltiempo de residencia ha sido calculado en base al contenido detritio (1.3 Unidades de Tritio en promedio) utilizando la ecuaciónde Pearson y Truesdell en aproximadamente 200 años (Cordón y Marida& M K Ferguson 1990.
2.3 GEOTERMOMETRIA
Diversos geotermómetros han sido aplicados en el campo geotérmico deZunil los cuales revelan temperaturas dentro del rango de 180° a400°C. Utilizando los geotermómetros de cationes, se considera laexistencia de dos acuíferos (OLADE 1982) como fuera mencionado en elpárrafo 2.2; uno profundo a temperatura de aproximadamente 260°C, yotro más superficial, producto de la dilución del primero, a unatemperatura de 210°C. Estas temperaturas han sido confirmadas enlos pozos exploratorios y se sugiere que el acuífero puede presentarmayor potencial a mayor profundidad. Geotermómetros de gasesaplicados a muestras provenientes de los pozos (Giggenbach 1986)confirman la existencia de altas temperaturas en profundidad(superiores a 200°C),zlas cuales se han obtenido además aplicandogeotermómetros isotópicos, (West-Jec 1990) y cuyos resultadosaparecen en la Tabla 4. Los valores más altos se obtienen con losisótopos de carbono, cuyos valores corresponden a niveles muyprofundos, revelando que la temperatura se incrementa dentro delbasamento.
287
NW
LLANO UL. PIÑAL
C CANDELARIA
chlondewctershauíd-t vaporsul'ate waters
lO(k-n)
Hidrología tentativa del sistema geotérmico de Zunil, compatible con la
composición química e isotópica de los fluidos descargados (Giggenbach,
1988)
chlonde waters
J bicarbonate
sulfate wate
/ fumorolesproposed dnlls
O 1 2 3 km
Trayectorias de flujo de las aguas propuestas por W.F. Giggenbach (1988)
FIG. 6. Modelo hidrogeoquímico del campo geotérmico de Zunilpropuesto por Giggenbach (1988).
288
NOAltura (m)
3,000 -,
VolcanCerro Quemado
SEAltura (m s.n.m)
Azufróles Volca'n de ZunllFuentes Gaoramos
- 3.000
- 2,000
- 1,000
otro«)~400°C Qiniolltnongo-Conl»l(7)T
pora Loa Sí»i«m<ji tí» Zunll-lAlmolonga y Funiorola Ntgro-Grond»
Flujo de fluidossubterráneos
> Fluío de agua frfa
* Flujo de ogua callenteprofunda del tipo Cl
-— Flujo de vopor
~~~ Flujo de aguo calentado poi
SOAltura
3.000 -
2,000
1,000 -
Aguas Amargas rFumarolo Grande del tipo ClI Fumarola Negro
Zona de mezcla —\—————— —
Agua callenteAgua caliente del tipo HCO,del tipo SO,
Agua co'ivnhd*l tipo Cl C-HCOj)
t ~*
Coi» mogmaHco» (C02 y olrot)
Ar«a d* recargaparo lot >lit«mo»d« lo Fumarolo N»oro yrumorólo Crond*
Flujo de fluidossubterráneos
: Flujo de aguo fna
: Flujo de aguo callenteprofunda del upo Cl
- — -*• : Flujo de vapor
: Flujo de aguacalentada por vapor
FIG. 7. Modelo hidrogeoquímico del campo geotérmico de Zunilpropuesto por WEST-JEC (1990).
289
T A B L A 3
COMPOSICIOH ISOTÓPICA I COHTEHIDO TOTAL DE Cl 1 SO, DEPOSOS í HASAIIT1ALBS DEL CAHPO GEOTÉRMICO DE SOSU
(Giggeabacb 1988)
ISDE 8o. 1,
Pozos
«1N2K3
ÍCC-3 0.63JCQ-6 0.63¡-11 0.5?
°C
9292144
"00/00
-8.-8.-7.
743625
2fi0/00
-79.-75.-70.
3B Cl S04 REPo/oo ig/kg ig/kg
391
1.3 11401.3 1070- 992
203860
AAB
Manantiales cloruro sódicos
ClAguas
BlB2B3B4B5B6B7B8AguasBlR2R3R4
Aguas
SIS2S3S4
¡-20 93 -8.20 - 1.1 700 202 C
bicarbo Datadas
J-23¡-29¡-4¡-9¡-17¡-10¡-13¡-15
4050636270678764
-11.-10.-10.-11.-11-li-11
59387539308954
-11.05
-85.-77.-79.-77.-83.-84.-84.-82.
20753174
1.1 311.2 570.0 101
41.1 1800.3 163
1653.2 71
6422103129210194193233
BAABBBBA
superficiales
¡R-1A!R-2AZR-16
LLOVÍA
sulfatadasZP-38Z-36H9¡-31
181618-
91905674
-11.64-12.23-12- 8
- 7- 7- 9-10
22.47
.25
.58
.38
.02
-84-85
77
-85.4-55.3
-70.6-67.4-75.8-80.7
151520
4.8
151078
-5618-
117633
16002060
AAAA
AABB
Referencias:
A.- IAEA 8.- DSIR c.- Pournier and Bañaba* 119811
(mg/Kg) -
-90
-12
FIG. 8. Relación de deuterio-oxígeno 18 (A) ysulfato-oxi'geno 18 (B) para las aguas de Zunil(Giggenbach, 1988).
T
TEMPERATURASES ISÓTOPOSEN EL ÁREA
A B L A 4GEOQUÍMICAS BR °C BASADASY GASES (HjS-Bj-CH.-COj)
DE ¡ORIL. (NEST JEC 1990)
LOCALIZACIOS CARBOR OIIGÍHO HIDROGEHOCO
ZCQ-3AzúfralesPuentes GeorginasChuitziquináPuentes GeórgicasLos VahosLos VahosAzufralitoLas PresasPuiarola SegraPanui
2-C8« HjO-CO, Bj-CB4;HjO-B2
383 272(304) — 175320 174344 14311451 —339 282378 140(157) —... 267208 263358 125(205) —288 164(206) —399 157(186) 163 146364 138
GASES
329249227...291...—188249334248
290
2.4 PROSPECCIONES DE MERCURIO Y RADON
Tanto el mercurio como el radón están relacionados con procesos deactividad geotermal profunda (Tobías 1987, Koga 1989) y laprospección de los mismos es ampliamente utilizada para determinarzonas de ascenso de fluidos hidrotermales al localizar fallas yfracturas activas (Gutierres y López 1983, West Jec 1990).
La primera prospección de mercurio en el área de Zunil fue efectuadaen muestras de agua y arcillas por Japan International CooperationAgency (1973), encontrando valores altos de mercurio (33 a 690 ppm)principalmente en el área de la Fumarola Negra. Se reportan ademásvalores relativamente altos en la zona de Fuentes Georginas. En elárea de Zunil 1, el consultor Cordón y Marida & MK Ferguson (1989)efectuó un estudio de mercurio en suelos, analizando 47 muestras a66 cm de profundidad. Los valores encontrados varían de 55 ppb a3.42% con los valores más altos entre los pozos ZCQ3, ZCQ5 y ZCQ6,incluyendo áreas de fumarolización activa; además, la distribuciónde los valores altos de mercurio indica la existencia de fallaspermeables con orientación NE y NO. (Ver Figura 9). Finalmente,
00o
• SSO LOCALIZACION DE MUESTRAS CON CONCENTRACIONES DE MERCURIO EN PARTES POR BILLÓN,EXCEPTO CUANDO SE INDICA.
^~- FALLA MODIFICADA, DE FOLEY (1,989)', DISCONTINUA DONDE SE INFIERE,00 POZO PRODUCTIVO.
FIG. 9. Isoconcentraciones de Hg en el área de Zunil 1 (Cordón y Mérida, 1990).
291
N>\OK>
Ceri(o El Galápagoii
Las Majadas
—~, vabor Pa|omaiuyup~\\^ = g ^ ^ ^\
Tuncenlro Fuenles Georgmas
x 2700mLa Estanciade la Cruz
FIG. 10. Anomalías de mercurio y radón en el campo geotérmico de Zunil (WEST-JEC, 1990)
como parte del estudio de prefactibilidad del área de Zunil 2, lafirma consultora West Jec realizó prospecciones de mercurio y radón,habiéndose analizado mercurio en suelo, mercurio en el aire delsuelo y radón, torón y radón total en el aire del suelo. Losresultados indican que las anomalías de mercurio están orientadas endirección ESE-ONO y que las mismas coinciden con las anomalías deradón y con una zona de baja resistividad determinada por el estudioCSMAT. Las zonas anómalas encontradas en este estudio puedenobservarse en la Figura 10.
3. CAMPO GEOTÉRMICO DE AMATITLAN
El campo geotérmico de Amatitlán se localiza aproximadamente a unos28 kms al sur de la ciudad de Guatemala dentro de los municipiosAmatitlán, San Vicente Pacaya y Villa Canales (Figura 1). Lasalturas dentro del área varían de 1,188 m.s.n.m. (nivel del lago delmismo nombre) hasta mayores de 1,800 m.s.n.m. (Laguna de Calderas).El lago Amatitlán recibe el agua proveniente de la cuenca del ríoVillalobos y descarga hacia el Pacífico por medio del ríoMichatoya. La temperatura media anual es de 22°C, se tiene unaprecipitación promedio de 1,400 mm y una evapotranspiraciónpotencial de 800 mm.
3.1 GEOLOGÍA
Geológicamente, Amatitlán se ubica en el extremo sur del graben deGuatemala dentro de una caldera de origen cuaternario antiguo, dondeactualmente se encuentra el lago del mismo nombre. El áreageotérmica se localiza en la parte norte del complejo dómico delPacaya, la cual está limitada al oeste por la falla de Mixco, aleste por la falla de Santa Catarina Pinula y al norte por la fallade Amatitlán. Un sistema de fallas con orientación norte-surformado por las fallas Mal Paso, Laguna Seca, Los Humitos y otrasmenores atraviesa el campo. Estratigráficamente la zona tiene lavasandesíticas, andesitas basálticas, lahares y lavas intermedias del
293
período terciario; sobreyacen a ellas lavas andesíticas, rocasdómicas dacíticas y riodacíticas, lavas básicas del Pacaya ypiroclásticos del período cuaternario. El basamento, cretácicosuperior y de naturaleza granítica, se encuentra discordantementebajo las vulcanitas terciarias. Los domos El Limón y La Mariposa,ubicados al suroeste del lago, son del tipo riodacítico y sugierenla formación de cámaras magmáticas como probable fuente de calor delcampo geotérmico. En el extremo sur de la caldera se encuentra elvolcán Pacaya, actualmente en actividad, el cual está formado porbasaltos diferenciados. Cercano al Pacaya hacia el noreste, selocaliza el Cerro Grande de composición andesítico-dacítica. (Tobías1987, Figuras 11 y 12).
COTAIm 3 n m )2500
2000
I50O
1000
500
O
-500
-1000
DEPRESIÓN VOLCANO-TECTÓNICA DE A M A T I T L A N
C«rro C h i n o
Fallo deAmotitlOfl
Kg(?>
Tv o Kg ( basamento granítico)
Qr-d
Qp
O DISTANCIA 15 Kms
LEYENDA
Qc £o°o° | Aluvión y Cokivlo'n
Qb Lovat basált ico» de I« M volcan de Pocoya
ft 11 > ' I Lavas ¿¿micas nodaclticasI1' "j f doclticas
f < < i Productos plroclo'sticos (pómez,I* * ' bloques de lava, paleosuelos )
!••.•.• I Lavas andejítlcoi dell.'.'.'.'l velcon ancestral
a.
Qc
Ta
Sedlm«nto» lacustresestratif icado*
Conglomerado arenoso(paleo- a luvldn)
I———] Lava* terc iar las andeelt lcos
Tv IIyI*vl T» rc |ar 'o volcánico Indlferenclado
«g I *«***! 6ron"°> granodlor l tb ,col lzas (? )
Fumare la
Manantial cal iente
FIG. 11. Geología del área geotérmica de Amatitlán sección norte-sur (Tobías, 1987).
294
COTA( m a. n m } NW SOUTHWESTERN OF EL LIMÓN ANO MARIPOSA DOMES
--7^r---rr-¿6"~-Ji^2*o o o o -Jsp oVo o oO S-D O O- O^O O - O
(Basament* Rock
Rocas básales (Basaments Rock»)
600
AM-3POZO EXPLORATORIO DEDIÁMETRO REDUCIDOExp loro*ory well of «molíd lameter .
LEYENDA
Qa
Qr-d P(l7l
y terrazas aluvialesI Aluvión and diluvial terrecesI Roca* domicas (Rlodacitas ydeci ta)I Domic rocks( Rhyodoclte and docite)
DISTANCIA
<Qs I— — 1 Sedimentos lacustresI—-'-^l Lacustnan sediments
Qp T*~>—=1 Pom«« de rlodoclto y d o c i t ol> * 1 Dócil» and rhyodocl te pumlce
Q» I* *,
Qc
To
|o o oí Conglomerado arenosoI » I Sondy conglomérate
Lavas andesltlca* terciarlasT e r t l a r y andesite*
SKm
A M A T I T L A NInst Nacional de Electrificado^
Unidad de Desarrollo GeotérmicoSECCIÓN GEOLÓGICA NW-SE
volcánica* cuaternariasI Quoternary vo lcante r o c k »
( a n d e s i t e )
|v v y] Volcánico terciario Indif erenciadoI * * I Ter t ia ry volcanlc undlferenclated
-80- Isotemperatura °C Isotemperature
FIG. 12. Geología del área geotérmica de Amatitlán sección noroeste-sureste (Tobías, 1987).
3.2 GEOQUÍMICA DE FLUIDOS
La primera localización y muestreo de manantiales en el áreageotérmica de Amatitlán se efectuó en 1977, luego se seleccionaronlos manantiales considerados representativos y estos se hanmuestreado sistemáticamente hasta la fecha (Figura 13). En base asu composición química los manantiales han sido clasificados en doscategorías; (ELC 1989, Giggenbach 1986, Tobías 1987, Figura 14)aguas cloruro sódicas localizadas a la orilla sur del lago Amatitláncon valores hasta de 1450 ppm de Cl y temperaturas mayores de90°C y aguas bicarbonáticas alrededor del domo El Limón y a lo largodel valle del río Michatoya, con temperaturas entre 40° y 70°C, ycontenido de HCO de 750 ppm. El manantial AM-8, con la másalta concentración de cloruros, se considera representativo de aguastermales de origen profundo. Al resto de manantiales se les
295
tCiudad dt Guottmala a 23 Km
voLcan dt Pacaya O 5 4 km
FIG. 13. Localización de manantiales seleccionados enel área geotérmica de Amatitlán (Giggenbach, 1986).
cu
IOSO.
Q INOE
HCOj
FIG. 14. Diagrama de CI-HCO3-S04 para las aguas de Amatitlán(Giggenbach, 1986).
clasifica como derivados de este por dilución con aguassuperficiales. OLADE (1982) considera que los dos grupos de aguasprovienen de acuíferos diferentes; el grupo clorurado sódico con unatemperatura en profundidad de aproximadamente 200°C y el grupobicarbonático, mas superficial, con temperatura aproximada de 100°C.
296
Supone además, la existencia de un acuífero más caliente a mayorprofundidad el cual es mencionado también por ELC (1989) quienesconsideran que el fluido original puede llegar a 300°C.
Análisis isotópicos efectuados en manantiales seleccionados del áreasugieren una simple relación entre las aguas clorurado sódicas,bicarbonatadas y superficiales, de tal manera que todos losmanantiales corresponden a mezclas de agua de origen profundo conagua superficial (Giggenbach 1986, ELC 1989, Tabla 5 y figuras 15 y
1816). El enriquecimiento en el contenido de O (aproximadamente1.8% ) indica una considerable interacción agua-roca a temperaturasmayores de 300°C. Por otro lado, si se considera el agua del lagocomo representante del contenido actual de tritio, las aguascloruro-sódicas tienen más de 100 años.
Áreas fumarolizadas se localizan desde la orilla sur del lago hastaLaguna Calderas. Básicamente son áreas calientes formadas pormaterial areno-arcilloso de color gris claro a blanco, provenientesde alteración de piroclásticos o lavas neutras a acidas. Han sidoclasificadas en tres sistemas (Tobías 1987); uno alrededor del
T i B L A 5
COMPOSICIÓN ISOTÓPICA DE AGOAS DEL CAMPOGBOTBRHICO DE AKATITtA» iCigseabach 19861
INDE do.
AK-1AN-7AH-8AM-19AH-24AÜ-32AH-44AH-57AH-67AH-69AH-7ÜAL-2AL-4
re.
72979150646584572050202225
<l«0 0/00
-8.68-6.44-6.22-8.88-8.88-8.49-6.63-7.26-9.36-7.07-7.86-1.61-2.92
Í3B o/oo
-61.2-54.6-53.3-64.2-64.0-60.2-52.9-57.5-62.9-49.4-52.5-19.2-26.4
JE (T.O.)
0.00.60.90.00.40.40.51.61.8
13.211.49.43.9
Cl-dg/kg)
16612501420158105306
1160235
2245
55
101
297
Dilución con aguo• ubt*rran«o " jov«n
CL(mg/Kg)
FIG. 15. Relaciones tritio-cloruros, deuterio-clorurosy oxígeno 18-cloruros para las aguas de Amatitlán(Giggenbach, 1986).
I9OO
Precipitación d«bala altitud
FIG. 16. Relación deuterio-oxígeno 18 para las aguas de Amatitlán(Giggenbach, 1986).
298
cráter de Los Humitos, otro en los domos El Limón y La Mariposa y untercero en el extremo sur-oeste del lago. Dos muestras de gasesfueron analizadas por Giggenbach (1986); una cercana al lagoAmatitlán (ALA) y la otra cercana a la Laguna Calderas (ALC) Tabla6). El contenido de gases para ambas es muy diferente; 0.9 mmol/molpara el Lago Amatitlán y 55.1 mmol/mol para la Laguna Calderas. Seconsidera que el bajo contenido de gases en la muestra de Amatitlánes debido a una alta desgasificación del agua. La composiciónrelativa de los gases helio, nitrógeno y argón en ambas muestassugiere un origen magmático común con una dilución por nitrógeno yargón provenientes de aguas superficiales (Figura 17). Por otrolado, el contenido relativo de hidrocarburos confirma este origenmagmático (Figura 18, Tabla 6).
3.3 GEOTERMOMETRIA
Geotermómetros de sílice presentan valores de aproximadamente 200°Ctanto para las aguas bicarbonatadas como para las aguas cloruradosódicas. Geotermómetros de cationes por su parte, indican valoresde 240° a 245°C. El uso del diagrama sodio-potasio-magnesio(Giggenbach 1986) revela que las únicas muestras que han alcanzadoequilibrio agua-roca en profundidad son aquellas con valores altosde cloruros, intersectando la línea de equilibrio a una temperaturaaproximada de 230°C (Figura 19). Por otro lado temperaturas deequilibrio de gases presentan valores de 220° a 300°C (ELC 1989,Giggenbach 1986). En base a los resultados obtenidos, Giggenbachpropone un modelo hidrogeoquímico el cual se presenta en la Figura20.
299
10 He
AIRE
AIRE SATURADO CONAGUA SUBTERRÁNEA
FIG. 17. Diagrama nitrógeno-argón-helio paralos gases de Amatitlán (Giggenbach, 1986).
ETANO/IO
PROPAKO BENCENO
FIG. 18. Diagrama etano-benceno-propano paralos gases de Amatitlán (Giggenbach, 1986).
300
T A B L A 6
COMPOSICIOS QDIHICA DE GASES DEL CAHPOGEOTÉRMICO DE AHATITLAK (ppt)
(Giggenbach 1986)
T°C C02 IB, Be
AlCALA
9595
55.1 9860.9 961
2.04.5
0.145.17
.0068
.0061.00001.00073
.26
.05
Ar 02 CH4 «"ClCOjl let ete eta
ALC .02 1.0ALA .40 1.2
11 0.3525 3.42
-2.8-6.7
940998
(.5U
30.80.4
pre pra ibu be nbu ipa opa pe benz
ALCALA
(.5 7.60.1
0.9 (.50.1
1.9U
0.5U
0.5 13.60.7
tol ebe 1-1 p-z o-i
ALC 1.4 1.8 1.3 0.6 0.6ALA 0.8 0.9 0.6 0.3 0.3
let: letano be: buteno tol: toluenoete: eteno nbu: n-butano ebe: etilbencenoeta: eta no ipa: isopentano i: nlenopre: propeoc npa: n-pentano ALC: Laguna de Calderaspra: propano pe: penteno ALA: Lago de Aiatitlánibu: isobutano benz: benceno
301
FIG. 19. Diagrama sodio-magnesio-potasio paralos aguas de Amatitlán (Giggenbach, 1986).
FIG. 20. Modelo hidrogeoqutmico propuesto por Giggenbachpara al área geotérmica de Amatitlán (Giggenbach, 1986).
3.4 PROSPECCIÓN DE RADON
Con el objeto de detectar fallas y fracturas tectónicamente activas,en 1985 se procedió a efectuar una prospección de radón en el campogeotérmico de Amatitlán, cubriendo un área aproximada de 100kilómetros cuadrados, mediante el sistema de cápsulas con películasdetectoras. Ciento dos cápsulas se colocaron cada 500 ó 1000 m en
302
c•o£ i* ii!•o o" £i 58 E
I!5 5 1f f 2
¿j iT u. Q
So
S
£ro
<D•aoo
oO)C3)OQ.E(Oo
"55•oc
<3
1O•o
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ojd
303
ocho líneas con dirección NO-SE, separadas 1 km cada una. El tiempode exposición fue de treinta días y posteriormente se efectuó elanálisis calculando el número de trazas por centímetro cuadrado y
2por hora (T/cm -hr) . Se considera que la emisión de radón haciala superficie se efectúa a través del sistema de fallas norte-sur.El análisis de los resultados evidencia tres anomalías en el rango
2de 20 a 40 T/cm -hr, entre el domo El Limón y la falla LosHumitos. Una cuarta anomalía se localiza superficialmente asociadaa la falla de Mixco (Figura 21). Las tres anomalías presentan unaalineación NO-SE, desplazadas ligeramente hacia el sur de lasmanifestaciones geotermales; el eje de alineación puede correspondercon un rasgo estructural profundo subvolcánico relacionadodirectamente con el reservorio geotérmico principal (Tobías 1987).
REFERENCIAS
Bethancourt, Hugo R. 1978. Proyecto Zunil. Estudio deFactibilidad Preliminar. Informe de Factibilidad Preliminar.Instituto Nacional de Electrificación. Guatemala.
Cordón y Mérida & MK Ferguson Company. 1990. PlantaGeotermoeléctrica de 15 MW. Proyecto Zunil I, Quezaltenango.Estudios Geocientíficos No. 1. Informe final.
ELC Electroconsult. 1982. Primera Unidad Geotérmica. Campo deZunil, Quezaltenango. Informe de Factibilidad Preliminar.Milano, Italia.
ELC Electroconsult. 1989. Área Geotérmica de Amatitlán. Estudiode Prefactibilidad. Informe de Geoquímica de los Fluidos.Milano, Italia.
Fournier, Robert & Hanshaw, Bruce B. 1981. Oxygen and HydrogenIsotopes in Thermal Waters at Zunil, Guatemala, reportsubmitted to the Instituto Nacional de Electrificación (INDE).Guatemala.
304
Giggenbach, Werner F. 1986. The Isotopic and ChemicalComposition of Water and Steam Discharges from the Lago deAmatitlán, San Marcos and Zunil Geothermal Fields, Guatemala.Chemistry División, DSIR. Private Bag. Petone, New Zealand.
Giggenbach, Werner F. 1988. Isotopic and Chemical Compositionof Discharges from Zunil, Tecuamburro and Moyuta GeothermalFields, Guatemala. IAEA Report. Chemistry División, DSIR.Private Bag. Petone, New Zealand.
Goff, Fraser., Adams, Andrew., Meeker, Kimberley., Truesdell,Alfred., Roldan Manzo, Alfredo Rene., Janik, Cathy. 1989.Hydrogeochemical Exploration of Tecuamburro Volcano Región,Guatemala.
Gutiérrez L.C.A. y López, Arturo. 1983. Concentracionessuperficiales de Radón en el Campo de Los Azufres, Mich.Informe 32-83. Comisión Federal de Electricidad. México,
Japan International Cooperation Agency. 1977. Republic ofGuatemala. Report on Geothermal Power Development Project.
OLÁDE. 1982. Estudio de Reconocimiento de los RecursosGeotérmicos de Guatemala. Informe Final.
Roldan Manzo, Alfredo Rene. 1989. An Outline of GeothermalDevelopment in Guatemala. Report presented in the 20thInternational Group Training Course on Geothermal Energy, heldat Kyushu University, Japan.
Tobías, Edgar. 1977. Proyecto Zunil. Estudio de FactibilidadPreliminar. Informe Preliminar Geológico y Vulcanológico.Instituto Nacional de Electrificación (INDE). Guatemala.
Tobías, Edgar. 1987. Proyecto Geotérmico Amatitlán. Estudio dePrefactibilidad. Instituto Nacional de Electrificación (INDE).Guatemala.
305
CARACTERÍSTICAS GEOQUÍMICAS E ISOTÓPICAS DELOS FLUIDOS PRODUCIDOS POR LOS POZOS DELOS HUMEROS, PUEBLA, MÉXICO
E. TELLO HINOJOS AGerencia de Proyectos Geotermoeléctricos,Comisión Federal de Electricidad,Morelia, México
Resumen-Abstract
CARACTERÍSTICAS GEOQUÍMICAS E ISOTÓPICAS DE LOS FLUIDOS PRODUCIDOS POR LOSPOZOS DE LOS HUMEROS, PUEBLA, MÉXICO.
Los análisis químicos e isotópicos de los fluidos producidospor los pozos del campo geotérmico de LosHumeros,Puebla.,fueron estudiados con el objeto de conocer laquímica del yacimiento y los procesos que ocurren en él. Deacuerdo con las características químicas de los fluidos seencontró que la salmuera producida por los pozos productoreses una agua de baja salinidad,cuyo carácter geoquímico esvariable y depende del tipo de pozo y la zona donde produzca.Los pozos más someros presentaron un carácter bicarbonatado-sódico. Mientras que los pozos más profundos presentan un tipogeoquímico clorurado-sódico.También se encontró que de acuerdocon los datos de exceso de vapor el pozo Hl produce en unazona de líquido dominante, mientras que el resto de los pozosproducen en una zona de dos fases. Debido a lo anterior parael pozo Hl la composición química en la descarga total esigual que a condiciones de yacimiento, para el resto de lospozos no se puede decir lo mismo. Ya que, son pozos quepresentan un exceso de vapor alto, y la composición a descargatotal no es la misma que a condiciones de yacimiento cuando sepresentan estas características.
De acuerdo con su composición isotópica los pozos deHumeros presentan un corrimiento de oxígeno-18 característicode fluidos de origen geotérmico que se han equilibrado con laroca a altas temperaturas. Los manantiales aledaños al campoen su mayoría se ubican en la linea meteórica. La muestraproveniente de Alchichica presenta una composición isotópicatípica de una agua que ha sido modificada por procesos deevaporación a temperatura ambiente.
GEOCHEMICAL AND ISOTOPIC CHARACTERISTICS OF THE FLUIDS GENERATED IN LOSHUMEROS WELLS, PUEBLA, MÉXICO.
Chemical and isotopic analyses of the fluids produced by wells in thegeothermal área of Los Humeros, Puebla, were carried out to establish thechemical composition of the field and the processes taking place in it. Thechemical characteristics of the fluids showed that the brine produced by thewells is a low-saline water, whose geochemical character varíes according towell type and production zone. Shallower wells produced sodium bicarbonate-type water, whereas the deeper wells exhibited a sodium chloride geochemical
307
type. It was also found, according to vapour excess data, that the Hl well isin a zone where liquid is dominant, while the remaining wells are in adual-phase zone. As a result, the chemical composition of the Hl well totaldischarge is the same as that of the field as a whole, whereas that statementcannot be made for the other wells. Thus there are wells which produce a highvapour excess, and the total discharge composition is accordingly not the sameas for the field as a whole.
The isotopía composition of the Humeros wells shows an ^0 shiftwhich is characteristic of fluids of geothermal origin that have reachedequilibrium with the rock at high temperatures. The springs adjoining theárea are mostly located on the meteoric line. The Alchichica sample has anisotopic composition typical of a water which has been modified by evaporationprocesses at ambient temperature.
1. INTRODUCCIÓN
El presente trabajo fue realizado dentro del marco delprograma de investigaciones sobre la aplicación de técnicasisotópicas y geoquímicas en la exploración geotérmica enAmérica Latina.
El campo geotérmico de Los Humeros, Puebla se encuentraubicado en la porción Centro-Oriente del Estado de Puebla, enlímites con la porción Centro-Occidente del Estado de Veracruza 53 Km al noroeste de la ciudad de Jalapa,Ver.
El ob je to del presente estudio es r ea l i za r unacaracterización geoquímica e isotópica de los fluidos produci-dos por los pozos de Los Humeros. Además de incluir manant-iales localizados en la periferia del campo, con el fin de versi existe alguna interacción de estos con fluidos geotérmicos.
Para la realización de este trabajo se seleccionaron losdatos químicos de los pozos que hasta el momento han sidomuest reados . Por tanto , aquí se contemplaron los datosquímicos de pozos que se perforaron hasta diciembre de 1989.
32 pozos han sido perforados a partir de 1981 a 1990 delos cuales 22 son productores, 4 están contemplados comoinyectores, 4 fueron fallidos y 2 están en calentamiento(Figura 1).
2. COMPOSICIÓN QUÍMICA DEL AGUA SEPARADA DE LOS POZOS Y AGUADE MANANTIALES
Con el fin de caracterizar el tipo de agua que producenlos pozos se utilizó la gráfica 2, la cual relaciona el conte-nido relativo en peso de SO4, Cl, y HCO3 ( Giggenbach, W.,1989). Los pozos H6, H7, H9, H10, Hll, H12, H13, H17, H 2 0 , H23 ,H24, H27, H28, H30, H32 Y H33 presentan un carácter clorurado-sódico. Este tipo geoquímico es característico de agua prove-niente de un yacimiento geotérmico . Normalmente en un yacimi-ento geotérmico el equilibrio agua-roca se lleva a cabo aprofundidad y a altas temperaturas. Ahora bien, el agua de lospozos H6, H10, Hll, H12, H23 , H 2 4 , H 2 7 , H28 , H30 , H32 y H33
308
2I80C)500 661500 663500 665 500
2178000
217*000
2174000
2172000
I
H-21ffi
H-22
' H-9
# "V '
Oo
H-16
H-29®
^ H-328*-®
H-8
H-5
H-20©
H-27
£M *
y?'* ^ i ""1-»"»'A? H-26 _W H~s
v © T *•
H-25® -
^<'"»,-<
C^c, ^
2180000
2178000
2176000
2174000
2172000
2170000
FIGURA IrLOCALIZACION DE LOS POZOS DE HUMEROS, PUE.
caen en la zona de aguas equilibradas con la roca. En la tabla1 están referidos los resultados del análisis químico del aguaseparada de los pozos que la producen.
Los pozos Hl, H16 y H18 presentan un carácter geoquímicobicarbonatado-sódico. Estas características son adquiridasdebido a que están produciendo en la parte más somera delyacimiento. Esta parte del yacimiento corresponde o se compor-ta como una zona de condensación ( Figura 2 ).
309
TABLA 1.- COMPOSICION OU/MICA DEL AGUA SEPARADA DE LOS POZOS DE LOS HUMEROS.PUEBLA.LA CONCENTRACION DE LOS SOLUTOS ESTA DADA EN mg/1 Y LA CONDUCTIVIDADELECTRIC A ESTA ENjmhos/cm. LA ENTALPIA TOTAL ESTA EN kJ Ikg.
NoDE
POZOH-1H-6H-7H-8H-9H-10H-11H-1 2H-1 3H-1 5H-1 6H-17H-1 8H-20H-23H-24H-27H-28H-30H-31H-32H-33
FECHA
1210891210891910891210892610891907891110891210891210897068913108913108913108960989
210189180589130489180789140389121089131089261089
ENTALPIADE
MEZCLA1385237825872168266226622636259616712115249826621747262820642491226017312662248926372662
PH
8.27.96.68.06.86.04.87.18.15.28.97.68.07.37.67.67.48.45.37.75.77.2
C.B.
12001025750
1150775
10103500440
1650550
2100635
1645525
10601500107524001925455
1275900
A/a
282. 0227.0147.0282.0170.0141.9203.0180.0350.0120.0586.0112.0123.093.0
146.0285.0
75.0533.4112.0112.0107.0180.0
K
46.041.022.046.026.019.127.032.066.015.032.019.023.016.010.046.0
6.028.114.421.021.027.0
Ça
1.780.802.202.200.761.807.100.504.001.200.900.840.921.206.001.501.700.600.801.005.301.70
Mg
0.050.050.040.070.020.020.400.040.070.070.050.030.040.020.060.110.070.010.080.050.180.04
Si02
9111142885988940
9091023949502551519229441163406
152970914911
Cl
99.6252.7
76.996.879.4
265.1982.7133.4393.1
9.899.3
158.8112.379.5
193.6324.6252.5556.0498.4
14.4329.9178.0
SO4
110.81.7
73.299.64.0
12.433.3
1.771.5
131.9142.0
75.342.819.4
107.727.321.031.813.90.6
15.85.7
HC03
207.539.834.3
100.382.080.528.249.387.710.0
463.841.5
396.670.62.4
58.210.3
110.4259.6
26.39.5
33.2
B
247253
2410503
2648533117161743267142220184118469
73422187227
5963119216281495
Li
0.850.750.300.570.760.340.830.581.370.400.850.400.370.310.200.900.101.540.230.420.240.40
NOTA: C.E. CONDUCTIVIDAD ELECTRIC ALOS ANALISIS FUERON REALIZADOS POR CFE
FIGURA 2.. CONTENIDO RELATIVO DE CI.S04
Y HCOi EN PESO (mg/Kg). LARAZON HCOi/CI ESTA EN RAZON
PESO.
SO 4
OK)
PO ZO S
M A N A N T I A L E S
0-25
% CI
HCOï/CI
1.0
4-0
60____% HCO»____80HCOi
Cuando los pozos están produciendo en una zona decondensación normalmente se detectan bajas concentraciones decloruros. Sin embargo, se detectan altos contenidos de bicar-bonatos y sulfatos. Esto se debe a que el vapor que transportaconsigo CO2 y H2S condensa en el acuífero (zona decondensación). Bajo estas condiciones la reacción con la rocaproduce soluciones bicarbonatadas-sulfatadas con pH ácido y/oalcalino.
El pozo H15 presenta un carácter sulfatado-ácido, el cualtambién es adquirido cuando el pozo está produciendo en laparte más somera del yacimiento, la cual corresponde a unazona de condensación. Esto hace que al interaccionar el H2S deorigen geotérmico con el agua de la zona de condensación partedel H2S, es disuelto en la fase líquida enriqueciéndose éstaen sulfatos. Como se puede ver en la figura 2 el pozo H15 caeen la zona indicativa de aguas calentadas por vapor. Esto solopuede ocurrir en la parte más somera de un yacimientogeotérmico o bien en manantiales superficiales. Debido a loanterior se puede decir que también este pozo produce en unazona de condensación.
Con respecto a los manantiales aledaños al campo, loscuales están referidos en la tabla 2, en su mayoría son deltipo bicarbonatado- sódico. Este carácter geoquímico sugiereque se trata de aguas de reciente infiltración que han inter-accionado con roca de origen volcánico a bajas temperaturas(Figura 2). Cabe mencionar que solo el agua proveniente deArteziano (M7) presenta un carácter clorurado-sódicocaracterístico de agua de origen geotérmico ( Figura 2).
Los geotermómetros de solutos iónicos tales como Na/K(Ellis and Manon, 1964) y de Na-K-Ca ( Fournier and Truesdell,1973) son una poderosa herramienta para evaluar las condi-ciones profundas de sistemas geotérmicos. Normalmente elproblema que se presenta en la utilización de estosgeotermómetros es el que se deriva del uso indebido a muestrasque no se les puede aplicar. Una selección o depuración ini-cial para determinar si es aplicable uno de estosgeotermómetros está basado en el valor del pH y los contenidosrelativos de Cl, SO4 y HCO3 ( Figura 2 ).
Recientemente ( Giggenbach, W. 1986 y Ginggenbach, W.1988 ) desarrolló una técnica que da una indicación automáticasobre la aplicabilidad de geoindicadores de solutos iónicos.Esta basada esencialmente en la dependencia de la temperaturade las dos siguientes reacciones:
K-Feldespato + Na = Na-Feldespato + K ( 1 )2.8 K-Feldespato +1.6 H20 + Mg =0.8 K-Mica +0.2 Clorita +5.4 Sílice + 2 K ( 2 )
312
TABLA 2- COMPOSICIÓN QUÍMICA E ISOTÓPICA DEL AGUA DE MANANTIALES Y NORIAS DE LOS HUMEROS.PUE.LA CONCENTRACIÓN DE LOS SOLUTOS ESTA DADA EN mg/l Y LA CONDUCTIVIDAD ELÉCTRICA ESTAEN/ffnhos/cm.
No
1234567891011121314151617181920212224
FECHA
2701883001882018810288
2701882701882501882501882601882501882501882501882601882601882901882601882901883028830288
2901881028830288
280188
NOMBREDEL
MANANTIALHUICHOTITA
ATOLUCAMIXQUIAPAN
CHACHARSOTÓLA
HUIZILPOPOCAARTEZIANO
S.N. PIZARROTEXCALPITZAHUAC
BARRIENTOSSANTIAGUITOSAN ROQUEHUIZILTEPEC
GONZAGATZOCUILA
MATLAHUACALATEMOXIZAMAZAPA
EL TESOROCHIGNAUTLA
S.J. XIUTETELCOZOATZINGO
TRES OCOTES
PH
4.57.07.58.17.67.69.08.58.18.58.37.88.28.77.48.27.67.37.67.57.56.67.1
C.E.
90 J7010513512091
131005150
3403504001345404001345401061371000
1451089555
TIPO
MMMMMNNNPNPNNMMMMMMMMMM
Na
5.04.36.68.65.66.6
3160833.0
30.427.039.642.017.647.6
6.08.37.09.6
83.014.09.06.04.3
K
3.65.01.71.43.23.2
300.095.0
1.45.33.92.14.64.24.22.51.72.59.21.41.03.22.5
Ca
17.012.09.2
19.57.15.6
26.915.621.820.232.418.828.025.0
7.076.020.017.038.011.611.94.03.4
Mg
0.90.71.51.01.30.9
19.378.45.68.3
12.28.73.31.11.72.81.02.2
27.61.11.41.00.5
S¡02
45.023.037.723.047.540.24.0
13.432.828.023.023.030.532.852.313.442.645.049.622.225.246.537.5
Cl
1.32.72.02.74.14.1
2364622.0
0.04.1
15.211.14.88.22.78.21.35.5
70.55.52.74.12.7
S04
0.00.00.00.08.93.9
1974899.0
5.88.8
18.37.82.89.42.9
34.80.00.03.50.00.00.80.0
HC03
31.524.246.129.1
9.724.213791128
128.6114.1179.0182.0152.9
0.036.4
223.338.853.4
371.450.929.124.316.9
B
0.000.000.000.000.000.00
37.803.340.080.170.080.040.000.000.000.000.000.081.270.000.000.000.00
DEUTERIO
-68.5-64.8-67.1-72.2-82.6-80.4-48.4-75.5-74.2-71.4-70.8-72.2-86.4-88.4-57.1-75.1-68.3-68.5-77.3-68.2-71.4-53.3-67.9
OXI-18
-10.6-9.8
-10.2-11.0-12.0-12.1
-5.3-10.2-11.0-10.5-10.3-10.8-12.3-11.0
-8.8-11.1-10.6-10.6-11.2-10.6-10.1
-8.6-10.2
TRITIO
9.78.30.78.4
11.7
3.50.84.02.6
U)
( confinación Tabla 2 ~ página 2 de 2)
No
25262728293031323334353637
FECHA
202882028820288102882028810288
280188280188260188260188270188270188
11287
NOMBREDEL
MANANTIALCOATLAMINGO
EL RANCHOEL ATRIO
LA BARRANCAAHUACATLAN
LA PASADALA CUEVAACUACO
HUITZILAPANXALTIPANAPACALZACATENOSIETE AGUASALCHICHICA
PH
7.36.76.97.77.77.17.88.28.18.17.77.6
C.E.
76107180100776570
2001647406558
TIPO
MMNMMMMPNNMM
Na
5.04.64.66.04.63.64.67.69.0
73.04.64.0
K
2.52.56.02.13.22.52.52.13.5
15.02.82.1
Ca
4.87.3
12.57.06.74.3
12.521.4
9.669.33.02.9
Mg
0.71.51.71.40.90.61.01.51.94.40.50.4
SiO2
34.431.319.231.358.740.431.346.543.525.231.319.2
Cl
17.920.7
4.12.72.72.71.33.54.14.11.32.8
SO4
0.00.00.00.00.00.40.00.11.0
28.30.02.8
HCO3
31.543.782.538.831.516.928.771.962.20.0
76.724.0
B
0.000.000.100.080.080.000.000.000.080.000.000.08
DEUTERIO
-67.5-52.7-62.0-66.8-45.9-53.7-65.5-73.2-75.0-66.3-66.3-65.5-12.3
OXI-18
-10.5-7.9-9.6
-10.3-8.1-8.6-9.8
-10.9-10.6-10.1-10.3-10.3
0.7
TRITIO
4.2
2.52.5
NOTA : (M) MANANTIAL, (N) NORIA, (P) POZO ARTEZIANO, (C. E.) CONDUCTIVIDAD ELÉCTRICA
EL MUESTREO PARA ANÁLISIS ISOTÓPICO FUE REALIZADO DE OCTUBRE A DICIEMBRE DE 1987
EL ANÁLISIS QUÍMICO FUE REALIZADO POR CFE. DEUTERIO YOXIGENO-18 LO ANALIZO EL HE Y EL TRITIO
FUE ANALIZADO EN EL LABORATORIO DEL OIEA EN VIENA.
Ambas reacciones involucran minerales en equilibrio totalque se espera obtener después de la recristalizaciónisoqulmica de una roca de la corteza de composición promedio,bajo condiciones de interés geotérmico. La reacción (2)responde más rápido y entonces se obtiene la últimatemperatura de reequilibración dando generalmente temperaturasbajas. Combinando los dos subsistemas, se obtiene un métodoque permite obtener el grado de equilibrio agua-roca, ydeterminar si es factible aplicar los demás geotermómetros. Enla figura 3 está graficado el contenido relativo de Na, K y Mgpara el agua de los pozos y manantiales de Humeros,Puebla. Losanálisis de agua de los pozos, se sitúan en la zona deequilibrio parcial, a excepción del H16 que está en equilibrioa una temperatura de 200°C. El pozo H28 el cual se localizamás arriba de la linea teórica indica que este pozo presentapérdida de vapor e incremento del contenido absoluto delsoluto (Giggenbach, W. 1989).
Las aguas que corresponden a manantiales están todasdesplazadas hacia la esquina del Mg. Esto indica que se tratade aguas de reciente infiltración bicarbonatadas-sódicas quese han equilibrado con roca de origen volcánico a temperaturasinferiores a 100°C. Solamente la muestra proveniente de Arte-ziano (M7) se ubica en la zona de equilibrio parcial con unatemperatura del geotermómetro K/Mg de 156°C.
3. EVOLUCIÓN DE LA COMPOSICIÓN QUÍMICA DEL AGUA DEL YACIMIENTOUn muestreo sistemático de los fluidos producidos por los
pozos de Los Humeros se ha llevado a cabo por CFE desde 1982 ala fecha. Para conocer la historia de los solutos en el yaci-miento se seleccionó a los pozos Hl, H6, H8 y H13 los cualesproducen una mezcla vapor-agua.
En las figuras 4 y 5 esta graficado el contenido de cloru-ros a descarga total y a condiciones de yacimiento contratiempo para los pozos Hl, H6, H8 y H13 desde su inicio hastadiciembre de 1989. Para el caso del pozo Hl (Figura 4), sepuede observar que tanto a descarga total como a condicionesde yacimiento los cloruros han permanecido constantes conrespecto al tiempo.
Para el pozo H6 (Figura 4) , se observa que laconcentración de cloruros tanto en la descarga total como acondiciones de yacimiento también ha permanecido constante conrespecto al tiempo.Para el pozo H8 se puede decir que alprincipio se detectaron los valores mayores de cloruros en elyacimiento y en la descarga total. Sin embargo, a partir de1986 a la fecha el comportamiento a sido asintótico.El H13 quees otro de los pozos que producen una mezcla vapor-agua, lacomposición química a descarga total y a condiciones de yaci-miento se ha incrementado con respecto al tiempo. Esto se debea que este pozo estuvo produciendo en la zona decondensación y ahora produce en la zona de dos fases de ahíque los cloruros se incrementen con respecto al tiempo.
315
FIGURA 3 - CONTENIDO RELATIVO DE Na, K Y MgEN PESO ( m g / K g )
No/1000
A P O Z O S
0 MANANTIALES
K/IOO
Lo* Mum«ros
110
100
90
80
70
60
50
40
30
20
10
O82
f** *
H-l
Cl de Yocím.
Cl en la Mezc la—1——————i———
84 86
TIEMPO (AMOS)
88 90
300
á
280 -
260-
240 -
220 -
200-
180 -
140 -
120 -
100-
80-
60-
40 -
20 -
0-
B
g
D Cl.de Yac im
+ Cl en la Mezcla
D
+
nn. ° D•*• n
86t
87
H-6
.+ +*
88
TIEMPO (AROS )
i89 90
Figura 4.— Cloruros contra tiempo
317
Los Humeros
*"N
|V-'
w§23u
-£V)oK3¥9
zuu -190 -180 -170 -160 -150 -140 -130 -120 -110 -100 -90 -80 -70 -60 -50 -40 -30 -20 -10 -
H-8
B
Dn
D
«
2k D n CL, Cpn'"1 n E
+ °g gana130 ntftijf1
•f* * +*t- + -H- "*"++ +
* + + +
0- ——— ———— • , , , • - - - - r- ——— -- ———— i85 87 89
TIEMPO (AÑOS)
280 -
260 -
240 -
220 -
200 -
180 -
160 -
140 -
120 -
100 -
80 -
60 -
40 -
20 -
H-13
P n
n n n
a
•*• + + +. .Jf*B
^ +__
+
+^
+
U Cl de Y a c i m .
+ C 1 en la M e z c l a
" 1 L 1 1 1 í 1 1 1 t 1 1 1 1 1 !
88.5 88.7 88.9 89.1 89.3 89.5 89.7 89.9
TIEMPO (AÑOS)
Figura 5.— Cloruros contra tiempo
318
4. COMPORTAMIENTO DINÁMICO DEL YACIMIENTOLa composición química del agua de los pozos puede cambi-
are Estas variaciones se pueden deber a reducciones en lapresión de yacimiento ó bien a fenómenos de mezcla, dilución,evaporación, condensación, desgasificación etc.
Por varios años los pozos productores de Los Humeros hansido muestreados para observar los cambios ocurridos en sucomposición química. Además la variación en las temperaturasdel agua profunda durante la producción puede ser monitoreadapor medio de geotermómetros químicos.En las figuras 6 y 7 estágraficada la temperatura de los geotermómetros de sílice yNa/K para los pozos Hl, H6, H8, y H13 con respecto al tiempo.Se puede observar en estas figuras que los dos geotermómetrosconcuerdan muy razonablemente y además han permanecido con-stantes con respecto al tiempo para estos pozos.
La solubilidad de la sílice y la razón de Na/K al variarla temperatura pueden ser usados para determinar que mecanismoes el que causa cambios en la composición química. Por ejem-plo, cuando la temperatura disminuye a causa de una ebulliciónrápida o a la influencia de agua fría puede no afectar inme-diatamente la razón Na/K. Sin embargo, una mezcla de dos aguaspreviamente equilibradas a diferentes temperaturas podríaafectar ambas razones SiOo y Na/K (Ellis, A. J. andManon,W.A.J., 1977).
En las figuras 8 y 9 se ilustran los cambios que ocurrenen la concentración de cloruros y la temperatura de la sílicepara el pozo Hl durante el periodo 1982-1988. Además en estasfiguras se incluye un patrón de lineas las cuales cada unaindica un proceso diferente el cual puede ocurrir en el yaci-miento. Por tanto, se puede decir que para el pozo Hl loscambios son regulares y variables y se observa que estoscambios están dominados por una serie de procesos complejos deebullición, evaporación, condensación, dilución y en el perio-do de 1986 a 1988 predominan los procesos de pérdida y ganan-cia de vapor. Para el pozo H6 los procesos que predominandurante el tiempo de observación, que comprende de 1986 a 1988fueron: condensación de vapor, ebullición, ganancia y pérdidaconductiva,dilución por aguas frías y aguas a 150°C ( Figura10 ). En el caso de del pozo H8 se siguió el mismo procedimi-ento encontrándose que los cambios en la concentración decloruros de yacimiento y temperaturas de sílice han sidograndes los cuales han ido haciéndose menos variables conrespecto al tiempo. Los procesos predominantes son:condensación de vapor, dilución por aguas a 150°C, ebullición,evaporación, ganancia y pérdida conductiva.
5. EXCESO DE VAPOR Y COMPOSICIÓN EN LA DESCARGA TOTAL Y ACONDICIONES DE YACIMIENTO
Si la entalpia de descarga de los pozos excede la de unlíquido saturado a la temperatura de reservorio, se consideraque existe un exceso de vapor. Las entalpias de descarga delos pozos del campo geotérmico de Los Humeros varían de 1385 a2662 kJ/kg. Una alta entalpia de descarga refleja dos fases
319
400 -
300 -
200 -
100 -
Los Humeros
n
+
H-i
n
82i
84l
86
TIEMPO (AÑOS)
i88 90
500
a.
400 -
300 -
200 -
100 -D T Nal K
T S¡0
-\—————i—————i—————r86 87 68
TIEMPO (AROS)
H-6
n p D
i—————r~89 90
figura 6. Geotermómetros contra tiempo
320
500
4OO -
300 -
2% 200-
100 -
85
Los Humeros
n n
—i——————87
TIEMPO (ANOS)
H-8
—r~89
500
g
a
400 -
300 -
200 -
100 -D T N a l K
+ T S Í0 2
i i i i88.4 88.6 88.8
n
n n n
H-13
i i i i i i i i i i89 89.2 89.4 89.6 89.8 90
TIEMPO (AROS)
Figura 7. Geotermómetros contra tiempo
321
u)NJ
CT
E
c/jOCEDcuo_jo
90
80
70
60
50 -
40
P E R I O D 0 2 4 - I O - 8 3 o 16-10-84E V A P O R A C I Ó N
0(""""!lg Conductivo
Cond * f t * o c t on
260 270 280 290 300
GEOTERMOME""RO DE S Í L I C E (°C )
90
80
0>E
ÜJ
cooce
70
CEO-J 60O
50
40
P E R I O D O 16-10-840 12-08-85E V A P O R A C I Ó N
Ptrdldo Conductiva Oononcla Conduct ivo
Cond*n$aclon
12 14
10
260 270 280 290
GEOTERMOMETRO DE S I L I C E ( ° C )
300
F I G U R A 8 .- CAMBIOS EN LA CONCENTRACIÓN DE CLORUROS Y T E M P E R A T U R A DE S Í L I C E CONRESPECTO AL TIEMPO PARA EL POZO H-l.
o>E
zUJ
cooCEZ)ce3o
100-
90-
80
70
60
50
40 _260
PERIODO 12-08-85 a 13-10-86E V A P O R A C I Ó N
Perdida Conducta<?. *
lando Conductiva
Condtniac lon
270 280 290
GCOTERMOMETRO DE S Í L I C E ( ° C )
300
100
90
80
UJ
V) 70Oceceo_iO 60
50
40260
PERIODO 13-10-86 o 19-07-88
3S-
270 280 290 300 3/0
GEOTERMOMETRO DE SÍLICE ( °C)
K) F I G U R A 9 . - C A M B I O S E N L A C O N C E N T R A C I Ó N DE CLORUROS Y T E M P E R A T U R A DE S Í L I C E CONRESPECTO AL T IEMPO PARA EL P O Z O H-l
150 -
140 -
130 -
LJ
oo:3oco_Jo
PERIODO 11-07-86 a 19-07-88
100 -
270 280 290 300
GEOTERMOMETRO DE S Í L I C E ( ° C )
310 3 2O
F I G U R A 1 0 . - C A M B I O S EN LA C O N C E N T R A C I Ó N DE C L O R U R O S YT E M P E R A T U R A DE S Í L I C E CON RESPECTO A L T I E M P OP A R A EL P O Z O H-6
(vapor y agua) en el yacimiento y su separación parcial en elacuífero antes de entrar al pozo. En la tabla 1 está referidala entalpia de mezcla de los pozos de Los Humeros. Si seobserva en la tabla 1 la entalpia de descarga del pozo Hlcorresponde a la de un liquido saturado a la temperatura dereservorio; por consecuencia el exceso de vapor a condicionesde yacimiento es pequeño. Esto sugiere que el yacimientopenetrado por el pozo Hl es de liquido dominante ( Tello,H.E.,1984a).
Para el resto de los pozos la entalpia de mezcla es muchomayor que la entalpia de un liquido saturado a la temperaturade yacimiento, cuando esto se presenta se debe a que a condi-ciones de yacimiento en forma natural existe una fracción devapor coexistiendo con la fase liquida.
324
Una de las causas del exceso de vapor o entalpia de lospozos es que las fases de vapor y agua no son transmitidas através de la formación en el reservorio con igual facilidadporque las dos fases tienen diferentes permeabilidadesrelativas. Es también concebible que el exceso de vapor sedeba a la evaporación de agua por conducción de calorproveniente de la roca del reservorio, ya que la caída depresión causada por la extracción del fluido por los pozosproductores origina el enfriamiento ( por ebullición )de estefluido y por esta razón hay flujo de calor conductivo de laroca a ese fluido. Sin embargo, hay dificultades involucradasen la evaluación química del agua del reservorio, basada sobredatos de exceso de vapor de los pozos. Si el reservoriocontiene solamente agua, entonces es fácil relacionar lacomposición de las muestras a superficie con las condicionesde reservorio. En el caso contrario, si el fluido en el pozoes un fluido de dos fases con exceso de vapor, no es posiblerelacionar la composición química del fluido en la superficiecon las condiciones de reservorio, sin hacer algunassuposiciones fundamentales. Por tanto, para este estudio lacomposición química del agua en Los Humeros fue evaluadasuponiendo dos posibles causas del exceso de vapor de lospozos: a) transferencia de calor conductivo de la roca b)efectos de permeabilidad relativa para el flujo de ambas fasesvapor y agua. Se consideró que ambos procesos ocurren en formaexcluyente. El primer caso supone que la transferencia decalor involucra evaporación de agua desgasificada (yahervida), de tal forma que se añade vapor libre de gas alvapor formado con anterioridad. Este modelo supone que ladescarga total representa una sola fase líquida en elyacimiento (Pozo Hl). En el segundo caso, se supone que elexceso de vapor es debido a los efectos de permeabilidadrelativa para las fases agua-vapor. Aquí, los cálculos de lasconcentraciones del agua en el yacimiento supone dos fases enel reservorio, lo cual ocurre en los pozos H6, H8 y H13. Esimportante mencionar que para los pozos H9, H10, Hll, H12,H16, H17, H18, H19, H20, H23, H27, H28, H29, H30, H31, H32,H33 el método de exceso de entalpia no es posible aplicarlopara hacer alguna corrección de los solutos en el yacimientodebido a que algunos pozos producen poca agua, y la poca queproducen es difícil muestrearla, o bien presentan entalpiasque corresponden a las de un vapor saturado. Por tanto, si seaplicara este método de corrección de la composición químicadel agua basados en los datos de entalpia se tendrían datoserróneos para estos pozos.
6. COMPOSICIÓN A DESCARGA TOTAL Y EN EL YACIMIENTODependiendo de la presión ejercida en la descarga, las
concentraciones de varios solutos varían en las fases indivi-duales agua-vapor; mientras que la concentración en ladescarga total permanece constante para condiciones deyacimiento constantes. Los componentes son distribuidos entrelas fases de acuerdo con la fracción de vapor, la cual es unafunción de la presión.
325
Para la concentración de solutos de los fluidos evaporadosa presión atmosférica se corrigieren a descarga total usandola siguiente ecuación:
Ao = Aa(l - Xv) ( 3 )donde Ao es la concentración a descarga total del componenteA, Aa es la concentración a presión atmosférica del componenteA, y Xv es la fracción de vapor, la cual se calcula de acuerdocon,
Xv = Ho - Ha / Hv - Ha ( 4 )donde Ho es la entalpia de mezcla y Ha y Hv son la entalpia delíquido y vapor a la temperatura de saturación respectiva-mente, que en el caso de los Humeros es de 91°C.
Es importante determinar las concentraciones a condicionesde yacimiento, ya que es posible detectar variaciones en lacomposición del fluido del reservorio. Estas concentracionespueden ser calculadas por medio de la siguiente expresión,
Ay = Ao / ( 1 - Xv ) ( 5 )donde Ay es la concentración a condiciones de yacimiento delcomponente A, Ao es la concentración del componente A en ladescarga total y Xv es la fracción de vapor, la cual se calcu-la de acuerdo con,
Xv = Ho - Ha / Ha - Hv ( 6 )donde Ho es la entalpia de mezcla y Ha y Hv son las entalpiasdel líquido y vapor a la temperatura del yacimiento.
En las figuras 4 y 5 están representados los cloruros deyacimiento y en la descarga total contra el tiempo para lospozos Hl, H6, H8, y H13. Se puede ver en estas figuras quesolamente las concentraciones del Hl concuerdan muy razonable-mente tanto a descarga total como a condiciones de yacimiento.Ahora bien, el hecho de que se agrupe el contenido de clorurosen la descarga total como a condiciones de yacimiento signifi-ca que el pozo está produciendo en una zona de líquido domi-nante (Tello,H. E. 1984 a). Por lo tanto, cuando esto ocurrela composición química a condiciones de yacimiento es igual ala descarga total. Para el pozo H6 la composición química enla descarga total no es la misma que a condiciones de yacimi-ento. Esto indica que está produciendo en una zona de dosfases ( Figura 4 ).
Para los pozos H8 y H13 nuevamente se presenta laseparación entre ambos valores, lo que indica que tambiénestos pozos producen en una zona de dos fases ( Figura 5 ).
Para el resto de los pozos como se mencionó en párrafosanteriores no es posible aplicar este método para corregir laconcentración de los solutos a descarga total y a condicionesde yacimiento. En la tabla 4 está referida la composiciónquímica a descarga total y a condiciones de yacimiento de lospozos Hl, H6, H8 y H13.
326
TABLA 3.- COMPOSICIÓN QUÍMICA DE LOS GASES DE LOS POZOS DE LOS HUMEROS .PUEBLA.EL CONTENIDO DE LOS GASES ESTA DADO EN % PESO. LA ENTALPIA ESTA DADAkJ/kg Y PRESIÓN DE CABEZAL ESTA DADA EN (MPA)
POZOH-1H-6H-7H-8H-9H-10H-11H-12H-1 3H-15H-16H-17H-18H-20H-23H-24H-27H-28H-29H-30H-31H-32H-33
FECHA121089
12108919108912108926108919078911108912108912108970689131089131089131089060989210189180589130489180789131089140389121089131089171089
ENTALPIA13852378258721682662266226362596167121162498266225302628206424912260173126622662248926372662
TEMPSEP126133137177128125133275144136241134105143160124143129
137112113188
xg5.693.512.275.611.391.432.787.393.432.891.943.0236.52.385.176.424.671.437.672.492.561.501.39
COZ98.2791.8793.6496.7384.9691.4394.8494.3693.6084.8689.2795.4995.6691.7790.3096.4891.2980.0688.4588.8289.9291.5985.40
H2S0.926.265.592.37
10.525.773.813.764.61
11.268.603.111.736.137.991.585.10
12.499.086.407.337.10
12.10
NH30.640.710.200.391.741.150.760.490.490.370.450.730.220.640.190.330.412.370.280.170.660.211.21
HELIO0.00010.00000.00000.00000.00080.00000.00000.00000.00000.00220.00000.00000.00000.00000.00050.00000.00000.00000.00000.00000.00000.00000.0003
H20.00060.00610.03090.02400. 16500.05740.02700.06200.05900.14400.08100.02700.09900.07090.11400.07540. 14500.20900.11100.26000.10400.05500.0946
Ar0.000.000.000.000.000.000.000.000.000.000.000.000.000.000.000.000.000.000.000.000.000.000.00
N20.1540.5460.5040.4661.4301.5700.4311.2401.0303.1800.7950.3330.1860.4861.0600.6503.0304.8401.9902.9900.6230.9940.827
CH40.0090.5460.0250.0131.1700.0010.1290.8000.2170.1800.8000.2492.1000.9160.3220.8790.0180.0330.0842.0501.3600.0360.820
GGDP155269277269301319244246264310300249239257287238286282305289268285280
UJK)-J
TEMPSEP.- TEMPERATURA DE SEPARACIÓN EN ( °C)
GGDP.-GEOTERMOMETRO DE GASES DE D'AMORE Y PANICHI EN ( °C )
LOS ANÁLISIS FUERON REALIZADOS POR CFE
TABLA 4.- COMPOSICIÓN QUÍMICA DEL AGUA SEPARADA A DESCARGATOTAL YA CONDICIONES DE YACIMIENTO. LA ENTALPIAESTA DADA ENkJ/kgYLA CONCENTRACIÓN EN mg /1.
POZOH-1H-6H-8H-1 3
FECHA121089121089121089121089
ENTALPIA1385237821681671
CONCENTRACIÓN EN LA DESCARGA TOTALNa
1582861152
K2651029
Ca1.00.10.52.0
Mg0.030.006
0.020.03
Si02510142214412
Cl563121
171
B13832109116
TNa/K263273263278
POZOH-1H-6H-8H-1 3
FECHA121089121089121089121089
ENTALPIA1385237821681671
CONCENTRACIÓN A CONDICIONES DE YACIMIENTONa
182110160213
K30202640
Ca1.00.41.02.0
Mg0.030.020.040.04
SÍO2596552562577
Cl6512255239
B161122286162
TNa/K263273263278
TNa/K.- GEOTERMOMETRO DE SODIO / POTASIO ( °C )
7. EQUILIBRIO QUÍMICO DEL YACIMIENTO Y GEOTERMOMETRIA DE FASELIQUIDA
Se ha demostrado que el estudio del equilibriosoluto/mineral es de gran ayuda para hacer una evaluacióncompleta de un sistema geotérmico. Esto está basado en elhecho de que la química de los fluidos está primeramentegobernada por la solubilidad de los minerales, lo cual esimportante para poder entender los factores termodinámicos queafectan el equilibrio. Por ejemplo el uso de geotermómetrosquímicos, y la predicción de la tendencia de la depositación eincrustación de los minerales depende grandemente del estadode saturación que se haya logrado entre el fluido y la roca.Debido a la gran importancia que esto representa para el mejorentendimiento del estado termodinámico del yacimiento. Se hanrealizado estudios con este enfoque desde 1986 los cualesfueron realizados por ( Tello, H. E. 1986 ) ,(Tello,H.E.,1987), ( Tello,H.E.,1989ab). Ahora bien, sabiendoel estado de equilibrio agua-roca sirve entre otras cosas parapredecir las tendencias de depositación de los minerales. Essabido que las especies químicas son controladas por la solu-bilidad de los minerales y que el estado de saturación esaltamente dependiente de los procesos físicos que puedanocurrir durante el transporte del fluido del reservorio a lasuperficie» Estudios sobre la cinética de interacción agua-roca, ebullición,desgasificación,enfriamiento y dilución confluidos más fríos han demostrado que estos son los principalesprocesos los cuales causan que algunos minerales se desvien desu estado de saturación original ( Arnorsson et al.,1982b).
Por ejemplo se ha visto que una ebullición con una máximadesgasificación produce un fluido supersaturado con respecto ala calcita, y de una progresiva ebullición y enfriamiento atemperatura más bajas se obtienen fluidos saturados en síliceamorfa. Es por tanto, deseable para la propia interpretaciónde los datos químicos, que cada factor sea tomado en cuenta.
328
8. ESTADO DE SATURACIÓN DEL FLUIDO PROFUNDO RESPECTO AMINERALES SELECTOS
En las figuras 11 se muestra el estado de saturación delagua a profundidad respecto a la calcita, anhidrita y cuarzo,a la temperatura medida en la zona principal de producción.Los valores de solubilidad para cada uno fueron calculadosa partir de los datos termodinámicos de Helgeson (1969), ylas actividades de las especies se calcularon a partir de losdatos analíticos, usando un programa de computadora desarrol-lado por Arnorrsson et al (1982 a). El estado de saturación decada mineral es expresado como:
LOG K anhidrita = a Ca * a S04 (7)LOG K calcita = a Ca * a CO3 (8)LOG K cuarzo = a H4SiO4 (9)
En la figura llb puede observarse que los pozos Hl, H6,H7, H8, Hll, H12, H20 y H23 están sobresaturados con respectoa la calcita. El pozo H9 está insaturado con respecto a lalínea de equilibrio. Cabe mencionar que las aguas geotérmicasa profundidad se equilibran con la calcita. Sin embargo, lasobresaturación con respecto a la calcita que presentan lacasi totalidad de los pozos que producen agua, se debe aprocesos de ebullición que ocurren alrededor de la zonaproductora de estos pozos. Por tanto, esas aguas profundasparecen supersaturarse y este mineral tiende a precipitar.
Con respecto a la anhidrita se puede decir que los pozosHl, H8, y H12 están en equilibrio ( Figura lia ). El H7 y H6tienden al equilibrio, mientras que el Hl, H20, y H23 estánsobresaturados con respecto a la anhidrita. El pozo H9 presen-ta una gran insaturación con respecto a la anhidrita.
Con respecto al cuarzo (Figura lia) los posos Hl, H6, H8,tienden al equilibrio aunque presentan una pequeñainsaturación con respecto a línea de equilibrio. El resto delos pozos están insaturados con respecto al cuarzo. Ahorabien, puede concluirse que la variación en la composiciónquímica de los pozos que producen agua se debe a procesos queocurren en forma normal en un sistema geotérmico y que lavariación en la producción se debe a la depositación de lossolutos debido a los cambios en la concentración y temperaturaque originan dichos procesos. La depositación e incrustaciónde algunos minerales depende de su estado de saturación. Cabemencionar que la depositación de algún mineral de alteración,como regla general ocurre en aguas sobresaturadas. Con base enlo anterior y después de observar el estado de saturación delos pozos con respecto a minerales selectos, puede decirse quelos pozos Hl, H6, H7, H8, Hll, H20 y H23 pueden presentarproblemas por depositación de carbonato de calcio, debido aque están sobresaturados con respecto a la calcita o El H9 esprobable que no presente depositación de este mineral porestar completamente insaturado. Los pozos H7, Hll y H23 pueden
320
-t
-IO
-II
-12
-1*
-14
-16
-l<
-17
-It
SOBRESATURADOS
H-lH tO
H-r
( a )
H-ll
CdS04 =
LOGK=6
SOO IZO S40 340
TEMPCRATUKA MIOIOA (C)
14
soo ato 340 saoTCMPCRATUMA WCMOA (c)
OMK<3O
Oo
-S
-Sí
-1.*
S08RESATURADOS
¿ í-6 ®
(c ;H-6
®H-20
H-23 H-7 H-12
H-ll
INSATURADOSH-9© LOO K Cuarzo= 0.41- -
»4O MO
MCOWA (C )••O
FIGURA. II SATURACIÓN DE SÍLICE, CALCIO, SULFATOS Y CARBONATOSEN AGUAS GEOTÉRMICAS CON RESPECTO A LA SOLUBILIDADDE ANHIDRITA, CALCITA Y CUARZO.
330
presentar depósitos de sulfato de calcio por estar sobresatu-rados con respecto a la anhidrita. Los pozos H6 y H9 es proba-ble que no presenten depositación de sulfato de calcio porestar insaturados con respecto a la anhidrita. Con respecto alcuarzo la totalidad de los pozos están insaturados, por tanto,es probable que no haya depositación de SiC>2 en la zona deproducción. Sin embargo es probable que deposite SiO2 dondehaya cambios de fase; o bien cuando haya presencia dehidróxidos de algún metal, los cuales actúan como puente paraque se inicie la depositación de sílice. Esto puede ocurrircomo en el caso del pozo H16 presentó insaturación con respec-to al cuarzo, sin embargo había depositación de sílice, peroesto ocurrió debido a la presencia de hidróxidos de fierroproductos d& la corrosión (Tello, E.H. 1989).
9. CARACTERÍSTICAS ISOTÓPICAS DE LA FASE LIQUIDA DELYACIMIENTO Y GEOTERMOMETRIA ISOTÓPICA
En la tabla 5 se presentan los resultados del análisis deconcentración de oxígeno-18 y deuterio respectivamente, enmuestras de vapor de pozos de Los Humeros. Cabe mencionar que,la base de datos incluye información sobre 16 pozos, loscuales fueron muestreados en mayo de 1987, diciembre de 1988 =Esto con el objeto de establecer la reproducibilidad de lametodología de muestreo, y de esta manera poder detectarvariaciones reales correlacionables con cambios en otrosparámetros físicos y químicos. En las mismas tablas se presentan las temperaturas de separación de cada fase, y la entalpiade mezcla solamente en los pozos Hl, H6 y H8, H13, H15 y H20la muestra del vapor separado se tomó a una temperatura distinta que la temperatura de la muestra líquida. En los casosde pozos equipados con separador de producción, se tomó lamuestra de vapor a la salida de este. En el caso de no contarcon separador se usó un separador portátil para la toma de lamuestra de vapor, mientras que la muestra de agua separada setomó en el vertedor. Con los resultados del análisis deconcentración de oxígeno-18 y deuterio en el líquido separadoy en el vapor separado, se calculó la concentración de ambosen la descarga total de los pozos, y los resultados se presentan en la tabla 5. Para estos cálculos se utilizaron 2 procedimientos los cuales se describen a continuación:a) Se utilizasolamente los datos de concentración de oxígeno-18 y deuterioen la fase líquida analizada. Esto es para cuando se toma lamuestra de agua en el vertedor, se hace uso de las siguientesecuaciones:
<S18Odt=£a180-eXv (10)5Ddt=íaD-eXv (11)
Donde Xv es la fracción de vapor a la presión atmosférica, queen el caso de Los Humeros es de (0.073Mpa) y se calcula deacuerdo con la ecuación (4). Para el cálculo del coeficientede partición (e) se hace de acuerdo con las siguientes ecua-ciones:
e180 = 9.2 - 0.04t°C (12)eD = 68 - 0.4t°C (13)
331
TABLA 5.- COMPOSICIÓN ISOTÓPICA DEL VAPOR SEPARADO DE LOS POZOS DE LOSHUMEROS. LA ENTALPIA ESTA DADA EN kj/kg.
POZO
H-1H-1H-6H-6H-7H-7H-8H-8H-9H-9
H-10H-10H-11H-11H-1 2H-1 2H-1 3H-1 5H-16H-16H-17H-17H-1 8H-1 8H-1 9H-20H-27
FECHA
140587290988140587290988140587280988140587280888140587270988140587280988140587280988140587290988290988280988140587270988140587280988140587290988140587280988290988
ENTALPIA
1286124818302382260024922034224926282757
2595266226362527173022652618243125952662
25282662
TEMPSEVAPOR
°C127160144150139136138139172170124216248248261139120128170163175144194178192152119
OXIGEN-18VAPOR-5.45-2.24-3.76-4.51-3.18-3.35-4.17-4.33-1.47-1.63-3.56-4.93-2.52-2.84-1.38-3.21-4.19-4.11-4.34-7.22-3.72-3.48-1.87-2.81-2.96-4.41-3.29
DEUTERIOVAPOR-80.4-65.9-70.6-71.5-66.2-70.8-73.8-72.7-62.1-68.0-66.0-68.0-68.0-64.3-60.1-67.3-73.9-69.3-70.6-82.2-64.9-68.2-46.0-53.4-60.4-71.7-66.2
CARBON-13EN EL GAS
-3.5
-4.1
-3.7
-3.3
-4.5
-4.0
-3.5
-5.4
-4.4
-4.0
-5.8
-3.2
DESCARGA TOTALOXIGEN-18
-2.61-2.35-2.09-3.88-3.18-2.91-2.89-3.42-1.47-1.63-3.56-4.93-2.52-2.84-1.38-3.21-2.2
-3.23-4.10-6.7
-3.72-3.48-1.87-2.81-2.96-4.0
-3.29
DEUTERIO-77.6-72.3-67.8-70.8-66.2-70.3-77.5-71.7-62.1-68.0-66.0-68.0-68.0-64.3-60.1-67.3-71.9-68.4-70.4-81.6-64.9-68.2-46.0-53.4-60.4-71.3-66.2
DEUTERIO Y OXIGENO-18 FUERON ANALIZADOS POR EL HE, MÉXICO
EL CARBONO-13 SE ANALIZO EN EL USGS LABORATORY. MENLO PARK, Ca.
donde (t) es la temperatura de saturación a la altura de 2800msnm que es la de Los Humeros.Estas ecuaciones 12 y 13 sonválidas para temperaturas entre 80 y 100°C.
b) Se utilizan los resultados de los análisis de la fasevapor separada, de acuerdo con las siguientes ecuaciones:
5180dt=5v180 +103lna(Xa)
£Ddt=<SvD+103lna (Xa)
(14)(15)
donde Xa es la fracción de agua a la temperatura de separacióny se determina de acuerdo con la siguiente expresión:
Xa = Hv - Ho / Hv - Ha (16)
332
donde Ho es la entalpia de mezcla, Hv y Ha son la entalpia delvapor y l iquido respect ivamente a la temperatura deseparación. El coeficiente de partición (103lna) se determinade datos experimentales desarrollados por Botinga (1969).
En la figura 12 se encuentran graficados el contenido dedeuterio contra el contenido de oxígeno-18 de los pozos deHumeros. Se puede observar en la figura que los pozos pre-sentan un corrimiento de oxígeno-18 característico de fluidosgeotérmicos. Este enriquecimiento de oxígeno-18 se debe a queel equilibrio agua-roca se llevó a cabo a altas temperaturas.Además como referencia se incluyó en la figura la composiciónisotópica del agua de manantiales y norias aledaños al campo.Estos datos de deuterio, oxígeno-18 y tritio están referidosen la tabla 2. Las muestras de Arteziano (47) y S.N. pizar-ro (M8) presentan un marcado corrimiento de oxígeno-18 respec-to a la linea meteórica, producto de la interacción agua-rocaa altas temperaturas. La muestra proveniente de la laguna deAlchichica ( M 3 7 ) presenta una composic ión isotópicacaracterística de aguas m o d i f i c a d a s por procesos deevaporación a temperatura ambiente. El resto de los manant-iales se ubican en la línea de agua meteórica (Tel lo ,H.E.1988).
-24 -
-48 -
ODiU
UlQ
-72 -
-96 -
-120
A
/LCHCHIC*
IMANANTIALESPOZOS (DESQUEJA. TOTAL)
-13 -11 -7 -5 -3OXIGENO 18
-1
FIGURA 12.- COMPOSICIÓN ISOTÓPICA DE MANANTIALES Y POZOSDEL CAMPO GEOTÉRMICO LOS HUMEROS, PUE.
333
10.GEOTERMOMETRIA ISOTÓPICA DEL CO2-CH4Una extensiva investigación de la composición isotópica
del dióxido de carbono y metano presente en fumarolas, manant-iales calientes y fluidos geotérmicos se han llevado a cabo enlos últimos 10 años. Estas investigaciones se han realizadocon el objeto de: 1) Determinar si el contenido de C en elCO2 presenta alguna variación significante en relación concambios del gradiente geotérmico. 2) Probar la validez delgeotermómetro isotópico de CO2-CH4.
Ahora bien, el C02 es el compuesto más abundante en losfluidos geotérmicos investigados los cuales varían de acuerdocon su ubicación geográfica. Las variaciones que presenta noson al azar, ya que valores más altos de 8 C están asociadoscon anomalías termales (Panichi y Tongiorgi, 1975). Por tanto,las variaciones de S C en el C02 natural puede considerarsecomo una herramienta auxiliar en la prospección de nuevasáreas geotérmicas.
Pequeñas cantidades de metano e hidrógeno siempreacompañan al CO2 en fluidos geotérmicos. Suponiendo que elequilibrio isotópico ocurre entre C02, CH4, H2, y H2O a travésde la reacción.
CO2 + 4H2 = CH4 + 2H2O (17)
El fraccionamiento isotópico observado entre los pares CO2- CH4, H2O - H2 y H2 - CH4 han sido usados para evaluar latemperatura a profundidad en varias áreas geotérmicas (Craig,1953; Hulston y McCabe,1962; Los datos isotópicos del conteni-do de SC en el CO2 y CH4 para dos de los pozos de Los Humer-os se refieren a continuación.
POZO FECHA Tsep 5l3ccH4 5l3cCO2 T°C
H17 14-V-87 175 -22.9 -4.O 400H18 14-V-87 195 -24.6 -5.8 400
Cabe mencionar, que tanto el contenido de S C, como lastemperaturas fueron proporcionados por A.Truesdell(comunicación personal, 1988) . La constante de equilibrio parael sistema C02 -CH4 ha sido calculada usando los gráficoselaborados por Botinga (1969) para temperaturas entre O a 7000 C. En el rango de temperaturas de interés geotérmico, porejemplo, de 100 a 400 ° C, los valores reportados pueden serdeterminados por medio de la ecuación:l'o3lna = -9.01 + 15.301 * 103T~1 + 2.631 * 106T~2 (18)donde a = (13C/12C) CO2/ ( C/ 12C) CR4 y T es la Temperaturaexpresada en °K.
Si se aplica la ecuación (18) para la evaluación de latemperatura, se debe suponer que existe equilibrio isotópicoentre el CO2 y CH4. Sin embargo, esta suposición ha sido muycriticada por varios investigadores entre ellos (Craig, 1963).Por lo tanto, es importante discutir la validez y limitacionesdel geotermómetro isotópico de CO2 - CH4:
334
I) En todos los campos investigados, incluyendo LosHumeros, el geotermómetro isotópico CG>2 - CH4 da temperaturasgeneralmente de 50 a 200°C más altas que la temperatura medidadirectamente en el cabezal.
II) El C02 y CH4 pueden estar en equilibrio isotópicosolamente cuando ambos se originen del mismo proceso químico.Esto es cierto para sistemas de baja temperatura como se haobservado en gases naturales producidos por procesos bacteria-nos y no hay razón porqué no sea cierto también para sistemasde alta temperatura (Panichi, C. , Ferrara, C., Gonfiantini,R., 1976).
III) La 513C del metano varía de -20 a -30 %. en sistemasgeotérmicos. Mientras que, el metano que se origina a másbajas temperaturas presenta valores más negativos de 13C. Parael caso de Los Humeros los valores de <S13C varían de -22.9(H17) a -24.6 (H-18).
Esto es una evidencia de que la composición isotópica delCH4 refleja la alta temperatura a la cual se formó o bien a unintercambio sucesivo con el CO2 dentro del sistema geotérmico.
IV) Generalmente se admite que la velocidad de reacción esmuy lenta en sistemas geotérmicos.
De acuerdo con lo anterior puede concluirse lo siguientesa) Debido a la falta de equilibrio isotópico, las tempera-
turas evaluadas a partir de la composición isotópica de CO, yCH4 pueden no tener sentido (400°C para el H17 y H18). b) Sinembargo, también puede ser que estas temperaturas sean respre-sentativas de fluidos más profundos.
11. GEOTERMOMETRIA ISOTÓPICA DEL DEUTERIO EN H2 y CH4También se aplicó la geotermometría isotópica del deute-
rio en H2 y CH4 a los pozos H17 y H18. Los resultados serefieren a continuación:POZO FECHA Tsep. S DR2 S DCH4 T°CH17 14-V-87 175 -435 -154 400 (D)H18 14-V-87 195 -392 -155.5 430 (D)
530 (C)donde (D) y (C) significa que el coeficiente de partición fuecalculado a partir de los datos experimentales desarrolladospor Botinga ( 1 9 6 9 ) . Cabe mencionar que estos valoresanalíticos fueron proporcionados por A. Truesdell (1988). Comose puede observar las temperaturas calculadas son muy altas.Esto sugiere o bien que no exista el equilibrio isotópico oque estas temperaturas sean representativas de fluidos deorigen más profundo.
335
12o CONTENIDO DE 513C EN EL C02 (gas)Se determinó el contenido de C en el CO^ en muestras de
gases de los pozos de Los Humeros.Como puede observarse en latabla 5 existe una diferencia bastante notoria en el contenidode <S13C entre pozos localizados en ambientes volcánicos (LosAzufres) y pozos localizados en ambientes sedimentarios (LosHumeros). La S 13C de rocas sedimentarias marinas (calizas) esdel orden de cero. Mientras que la 613C de CO2 de origenmagnético es del orden de -7 a -8 %. para los pozos de LosHumeros el contenido de 513C de CO2 en la muestra de gas varíade -3.2 a -5.8 de acuerdo con lo anterior puede decirse queexiste un aporte de carbón de origen sedimentario. Siendo lospozos Hl, H6, H8, Hll y H19 los que presentan un mayor aportede C, además de estar más cercanos al alto estructural delas calizas. Estos pozos están ubicados en la parte noroestedel campo. Los pozos H6, H12 y H18 los cuales están ubicadosen la zona sur del campo, presentan valores más negativos de6 C lo cual refleja que si hay aporte de C de origensedimentario, pero predomina el C de origen magmático0Esto es cierto ya que los pozos H12 y H18 los cuales estánubicados cerca del cráter de Maztaloya uresentan los valoresmás cercanos al valor que presenta el 13C de origen magmático-7 a -8 %.(Figura 13).
13. CAMBIOS SECUNDARIOS EN LA COMPOSICIÓN QUÍMICA E ISOTÓPICADE LOS FLUIDOS.
La composición química e isotópica de los fluidos produ-cidos por los pozos productores de Los Humeros han sido utili-zados en el modelo geoquímico del campo (Tello, H. E. 1989).
La existencia de procesos de mezcla, condensación dentrodel campo han sido evidenciados, por medio de los análisisquímicos e isotópicos, el problema ahora es definir y delimi-tar las áreas dentro del yacimiento geotérmico que son afecta-das por esos fenómenos secundarios.
El presente trabajo representa un intento por correlacionarun cierto número de parámetros geoquímicos. Por tanto, se haconsiderado además de parámetros isotópicos, algunas especiesquímicas encontradas en los fluidos geotérmicos.
I QLos análisis combinados de SO y 6D como se menciono enpárrafos anteriores nos han servido para definir lacaracterísticas isotópicas del vapor profundo y nos puedeayudar a conocer las propiedades del agua más somera quepudiera estar recargando el acuífero geotérmico. Además deindicarnos la dirección del flujo subterráneo.En la figura 14se presenta una configuración de isovalores de oxígeno-18 paralos pozos de Los Humeros. Se puede observar en esta figura quelos valores menos negativos se encuentran al sur de la zonaperforada y estos valores siguen una tendencia SE-NW lo cualcoincide con el alto estructural de las calizas. Esto sugiereque existe un enriquecimiento en isótopos pesados producto dela interacción del agua con calizas, a altas temperaturas.
336
6S9500 662500 665500
2178000 ¿
2175000
2172000 -
2169000
- 2178000
- 2175000
2172000
2169000659500 662500 665500
Figura 13.— Isovalores de carbono-13 (Los Humeros)
337
659500 662500 665500
2178000 ¿
2175000 <r
2172000 -
2169000
- 2178000
- 2175000
- 2172000
659500 662500 6655002169000
Figura 14.— Isovalores de oxigeno—18 (1988) Los Humeros
338
En la figura 15 se presentan los isovalores de temperatu-ras medidas en los pozos a condiciones de equilibrio se puedever en esta figura que las temperaturas más altas se encuen-tran en la parte norte de la zona perforada, concretamente enpozos localizados dentro del colapso central, los pozos H4,
659500 665500
2178000 -
2175000
2172000 -
2169000
- 2178000
- 2175000
2172000
659500 6625002169000
665500
Figura 15.— Isotemperaturas medidas en Los Humeros.
339
Hll, H20 son los que presentan estas temperaturas las cualesson mayores a 300°C. Los pozos localizados en la zona sur delcampo en el Xalapasco Mastaloya también se detectantemperaturas mayores a los 300°C. Es importante hacer notarque los pozos H13, Hl, H7 , H8 están ubicados cerca de laisolinea de 300°C, y están ubicados entre la falla de lasvíboras y la falla de Los Humeros.
Al configurar los isovalores de temperaturas calculadaspor medio del geotermómetro de gases de D'Amore y Panichi(1980) (Figura 16), se encontró que los valores más altoscorresponden a pozos localizados en la falla de Los Humeros(H7, H8, H10) y otros dentro del colapso central H17, H15, H16y H9 (Tabla 3) .
De acuerdo con lo anterior puede concluirse que las zonasmás importantes en cuanto a temperaturas se refiere puedenclasificarse como sigue: 1) la zona del colapso central contemperaturas medidas y/o calculadas de 307°C hasta 384°C. 2)la zona del Xalapasco Maztaloya con temperaturas que van de302 C a 356°C y 3) el corredor central o falla de Los Humeroscon temperaturas medidas y/o calculadas que van de 267 a 315°C
14. CONCLUSIONES
De acuerdo con su composición química el agua producidapor los pozos de Humeros se clasifica en bicarbonatada-sódica,sulfatada-sódica y clorurada-sódica. Al hacer la corrección dela composición química del agua separada de los pozos, seencontró que el pozo Hl, produce en una zona de líquido domi-nante. El resto de los pozos de Los Humeros se encuentranproduciendo en una zona de dos fases.
De acuerdo con el estado de equilibrio los pozos Hl, H6,H7, H8, Hll, H12, H20 y H23 pueden presentar depositaciones decarbonato de calcio, ya que están sobresaturados con respectoa la calcita.El pozo H9 lo más probable es que no presentedepositación de carbonato de calcio ya que está insaturado conrespecto a la calcita. Con respecto a la anhidrita se concluyeque los pozos que pueden presentar depositaciones de sulfatode calcio son los pozos Hll, H20 y H23 por estar sobresatura-dos con respecto a la anhidrita. El pozo H9 puede no presentardepositaciones de sulfatos de calcio ya que presenta una graninsaturación con respecto a la anhidrita. Los pozos Hl, H8,H12 están en equilibrio soluto/mineral y los pozos H6 y H7tienden al equilibrio por tanto, es probable que se depositesulfato de calcio. Con respecto al cuarzo la totalidad de lospozos están insaturados, por tanto, es probable que no hayadepositación de sílice en la zona de producción. Sin embargo,es probable que deposite SiO2 donde haya cambios de fase. Obien cuando haya presencia de hidróxidos de algún metal, loscuales actúan como puente para que inicie la depositación dela sílice.
340
659500 662500 665500
2178000
2175000
2172000 -
2169000
- 2178000
- 2175000
- 2172000
659500 662500 6655002169000
Figura 16.— laovalores del geotermo'metro de gases.
341
1 fiLos pozos de Humeros, presentan un corrimiento de SOcaracterístico de fluidos geotérmicos. Esto indica que lainteracción agua-roca se efectuó a altas temperaturas. Lacomposición isotópica del agua de los manantiales se agrupanen su mayoría en la linea de agua meteórica (Tello, H.E.1988)o
También se analizó el contenido de 513C en el CO2 en mues-tras de gases de los pozos de Humeros. Se encontró que existeuna gran diferencia en el contenido de <S13C entre pozos local-izados en ambientes volcánicos (Los Azufres) y pozos localiza-dos en ambientes sedimentarios (Los Humeros), ya que en primerlugar se sabe que la 8 C de rocas sedimentarias marinas(calizas) es del orden de cero. Mientras que la <S13C de CO2 deorigen magmático es del orden de -7 a -8 %. . Para los pozosde Humeros la £13C de C02 en la muestra de gas varía de -3.2 a-5.8, por lo que puede concluirse que existe un aporte decarbón de origen sedimentario. Siendo los pozos Hl, H6, H8,Hll y H19 los que presentan un mayor aporte de 513C de origensedimentario, lo cual se debe a que están más cercanos al altoestructural de las calizas. Ahora bien los pozos H6, H12 y H18los cuales están ubicados cerca del cráter de Maztaloya pre-sentan valores más negativos de 8 C (-4.1 a -5.8) lo cualrefleja que si hay aporte de 8 C de origen sedimentario, peropredomina el 513C de origen magmático.
De acuerdo con las temperaturas geotermométricas de agua ygases complementados con temperaturas medidas se encontró queexisten 3 zonas bien definidas, los cuales pueden definirse deacuerdo a sus temperaturas como sigue 1) la zona del colapsocentral con temperaturas medidas y/o calculadas de (307C a384C), 2) la zona del xalapasco Maxtaloya (302 a 356°C) y 3)corredor central o falla Los Humeros con temperaturas de 267 a315°C.
RECONOCIMIENTO
El autor agradece al Ing. Marco Antonio Torres R., por elapoyo recibido en la impresión final de este trabajo.
REFERENCIAS
Arnorsson, S., Sigurdsson, S. 1982a. The chemistry of geother-mal water in Iceland I. Calculation of aqueous speciation from0°C a 370°C. Sciencie Institute, University of Icelan.Arnorsson, S., Gunnlaugsson, E. 1982b. The chemistry of geo-thermal waters in Iceland II. Mineral equilibria and Independ-ent variables controllings water compositions.Science Insti-tute, University of Iceland.Arnorsson, S., Gunnlaugsson, E. 1982. The chemistry of geo-thermal waters in Iceland III. Chemical geothermometry in geo-thermal Investigations. Science Insitute, University of Ice-land.
342
Botinga, y., (1969). calculated fraccionation factors forcarbon and hydrogen isotope exchange in the system calcitecarbon-dioxide-graphite-methano-hydrogen-water vapor "Geochim.Cosmochim. Acta, V. 33, p.49.Craig, H., 1963. The isotopic geochemistry of water and carbonin Geothermal areas. (Edited by Tongiorgi, E) C.N.R., Spoleto.Craig, H., 1953. The geochemistry of stable carbon isotopesGeochim. Cosmochim. Acta 33. pp. 49-64.D'Amore, F. and Panichi, C. 1980. Evaluation of deep tempera-tures of hydrothermal systems by a new gas geothermometer.Geochim, cosmochim. Acta, 44, pp. 549-556.Ellis, A. J., and Mahon, W.A.J., 1977. Chemistry and Geo-thermal systems, Academic Press, New York. pp. 299 - 301.Fournier, R.O., 1977. Chemical geothermometer and mixingmodels for Geothermal systems Geothermics, vol. 5 pp.47 -49.Giggenbach, W., 1986. Graphical techniques for the evaluationof water/rock equilibration conditions by use of Na,K, Mg andCa-contents of discharge waters.Chemistry Division Departmentof scientific and Industrial Research.Petone, New Zealand.Giggenbach, W., 1988. Geothermal solute equilibria. Deriva-tion Na-K-Mg-Ca-geoindicators. Geochim. Cosmochim. Acta, 52pp.2749-2765.
Giggenbach, W., 1989. Techniques for the interpretation ofwater and analyses in geothermal exploration. ChemistryDivision. Department of Scientific and Industrial Re-search. Petone, New Zealand.Helgeson, C.H., Kirkham, D. 1969. Theore tical prediction ofthermodynamic Behavior of aqueous electrolytes at High pres-sures and temperatures: II Debye-Huckelparameters for activi-ty coefficients and relative partial molal properties. Depar-tament of Geology and Geophysics, University of California.Hulston, J.R. and McCabe, W.J.,1962. Mass spectrometer measurements in the thermal areas of New Zealand carbon isotopicratios. Geochim. Cosmochim. Acta 26, 399-410.Tello, H. E., 1984a. Qulmica de los fluidos de descarga delpozo Humeros 1.CFE. Gerencia de Proyectos GeotermoeléctricosInforme 16-84. Morelia, Michoacan, Mexico.Tello, H. E., 1984b. Metodologia para la estimacion de lacomposiciôn de la fase liquida a descarga total y a condi-ciones de yacimiento. CFE. Gerencia de ProyectosGeotermoeléctricos. Informe GQ - 4 - 84. Morelia, Mich.,Mexico.Tello, H. E., 1986. Quimica de los fluidos de los pozos delcampo geotérmico de Los Humeros,Puebla. CFE. Gerencia deProyectos Geotermoeléctricos. Informe 21-86.Morelia,Mich.Mexico.
343
Tello, H« E., 1987. Características Geoquímicas de la descargade los pozos de Los Humeros, Puebla. CFE. Gerencia de Proyec-tos Geotermoeléctricos. Informe 12-87. Morelia,Mich.,México.
Tello, H. E.,1988. Características químicas e isotópicas delagua de manantiales aledaños al campo geotérmico de LosHumeros. CFE. Gerencia de Proyectos Geotermoeléctricos. In-forme 22-88. Morelia, Michoacán, México.
Tello H . E . , 1989a. Química de los fluidos de descarga del pozoH-16. CFE. Gerencia de Proyectos Geotermoeléctricos. Informe68-89o Morelia, Michoacán, México.
Tello, H. E. 1989b. Modelo geoquímico del campo geotérmico deLos H u m e r o s , Pueb la . C F E . Gerencia de ProyectosGeotermoeléctricos. Informe 07/89. Morelia,Michoacán,México.
Truesdell , A., 1988. Resultados analí t icos de deuterio,oxígeno-18 y carbono-13 de los pozos de Los Humeros. Comu-nicación personal.
344
GEOCHEMICAL REPORT ON THE CHALLAPALCA ANDTUTUPACA GEOTHERMAL AREAS, PERU
G. SCANDIFFIOEnte Nazionale per 1'Energía Elettrica,Pisa, Italy
D. VERASTEGUI, F. PORTILLAUnidad Investigaciones Geotermales,ELECTROPERU SA,Lima, Peru
Resumen-Abstract
INFORME GEOQUÍMICO SOBRE LAS ZONAS GEOTERMALES DE CHALLAPALCA Y TUTUPACA,PERÚ.
Se muestran brevemente las características más sobresalientes de cincozonas termales que se encuentran en la parte sur del Perú a lo largo de lacordillera andina y cerca de la frontera con Chile y Bolivia. Se han organi-zado estas zonas en dos regiones:
Challapalca, que comprende Challapalca y Paucarani;
Tutupaca, que comprende Calacea, Calientes y Tutupaca.
Los datos geoquímicos se refieren principalmente a la composición químicae isotópica del agua en determinados puntos; si bien la composición del gasde estos caudales geotérmicos es casi desconocida, se obtuvieron indicacionesmuy favorables. En las zonas de Calientes y Challapalca existen amplios cir-cuitos termales; sus temperaturas profundas deberían sobrepasar los 200°C.
GEOCHEMICAL REPORT ON THE CHALLAPALCA AND TUTUPACA GEOTHERMAL AREAS, PERU.
The most outstanding characteristics of five thermal zones,
located in the southern part of Peru along the Andean Cordillera
and near the border with Chile and Bolivia, are briefly discussed.
These zones were organized into two areas:
- Challapalca, comprising Challapalca and Paucarani;
- Tutupaca comprising Calacoa, Calientes and Tutupaca.
The geochemical data mainly regard chemistry and isotopes of
water points; even if the gas composition of these geothermal
345
discharges is almost unknown, very favourable indications cameout. In the Calientes and Challapalca zones wide thermal circuitsexist; their deep temperatures should exceed 200 °C.
1. Previous studies
The Challapalca and Tutupaca areas have been the subject ofstudies by AQUATER-INGEMET [1] and Burgess [2],
The AQUATER survey regarded a region covering tens of thousandsof square kilometres, so that it can only be of interest forgeneral informations.
BURGESS, instead, carried out, with financial support of OLADE,an a detailed study of the areas in question sampling 70 waterpoints. Only chemical data were found in the records ofELECTROPERU, the peruvian Agency attending to the development ofthis Coordinated Research Programme. Isotopic data and thesampling map of the manifestations were missing.
2. Geologic outline and stratigraphy
This study regards an area of approximately 4500 sq km runningbetween the Western Cordillera and the Peruvian Plateau. Thegeological and structural setting is constitued by Mesozoic Units,made up of terrigenous sediments, intensely deformed and foldedover which lie in unconformity the products of several volcaniccycles spanning from Lower Tertiary to Quaternary, and clasticglacial or glacio-fluviatile quaternary deposits. Howevervolcanics are the most widespread formations in the area.
346
From top downward the stratigraphie sequence is made up of:
- Quaternary glacial or glacio-fluviatile erosion deposits;- Purpurini=îi- Tutupaca Upper Tertiary-Quaternary volcanic units;- Chila =!l- Capillune formation=i|- Seneca volcanics Limnic-Volcanic sequence;- Maure formation =="- Huaylillas Volcanic Complex;- Huilacollo formation=i| ow1"- Tarata formation =* Centre and SW Toguepala Group;- Toquepala formation NW =- Chachacumane format ion=ii- Atascapa formation = Yura GrouP-
This stratigraphie sequence can be recognized in the AA'section of Appendix 1C traced on the geological map of theTutupaca area in Appendix 1A.
2.1 Tectonics, volcanism and thermal features
During Tertiary the Andean tectonic in this area wascharacterized by brittle tensional features. The main phase datesback to Miocene. In places there was a reactivation of olderlineaments. As a result the region was divided into blocksunevenly displaced, which could be responsible for the arising ofgeothermal reservoirs.
Three main fault trends are distinguished, though some otherorientations occur: NW-SE (Andean), NE-SW (anti-Andean) and E-W;some of thes trends can be seen in Appendixes 1A, IB and 1C.
Over the older ignimbritic basal complex (Huaylillas) severalcentral volcanic apparatuses built up in various periods ofactivity and their products covered the whole area. Glacial orglacio-fluviatile erosion have partly modified the morphology dueto the different volcanic cycles, but the most recent volcanicsremained almost unaffected by weathering.
Three main volcanic phases are recognized: a) Chila;b) Tutupaca; c) Purpurini.
347
a) In a first period spanning between 5 and 8.4 M.a. theactivity was mainly effusive from central volcanoes (Nazaparco,Loma Pabellön, Novena, Jucure, Chila and San Francisco. The secondperiod reactivated the former apparatuses between 2 and 4 H.a.,and gave rise predominantly to effusions but also explosionsproducing andésites.
b) This second phase distributed its products over the wholearea, as isolated blocks, over the former volcanics. The glacialand glacio-fluviatile erosion has strongly altered the Chilaproducts while it has just touched the Tutupaca ones. The age ofTutupaca volcano is estimated around 0.7 M.a., to the same periodshould be ascribed also Yucamane, Yucamane Chico, Calientes, SanFrancisco, Casiri and a few domes; again effusive activity ispredominant with acid products (dacites), noteworthy are thepyroclastic deposits of Yucamane.
c) The last volcanic phase (Purpurini), displaced superficialdomes, probably the most differentiated products of regionalmagmatism. These domes of dacitic compositions could point to theexistence of magmatic chambers not at a great depth. In the localmorphology the domes appear as isolated shapes, untouched byerosion, at times related to collapse structures.
Radiometrie age of Purpurini volcanics is around 0.1 Ma, of thesame cycle are Kere, Casiri and Paucarani.
The volcanic apparatuses related to the study zones of Tutupacaand Challapalca are singled out in Appendixes 1A and IB.
Several thermal manifestations are known (thermal waters,fumaroles, solfataras, geysers), almost all occurring close to theyoungest volcanic apparatuses, along Andean and anti-Andean faultsystems.
348
Fumaroles and geysers are mainly sited toward Tutupaca andCalientes, while solfataras occur mostly around Paucarani sector.
Thermal waters spring almost always where there are fractures,even having sometimes their final seepage through glacio-fluvialcovers.
Three circuits can be distinguished on the basis of springfeatures :
1) subsurface freshwaters of meteoric origin infiltrating inthe volcanics and outcoming at the contact between lavas andpyroclastics as cold waters;
2) waters that through deeper circuit become contaminated bygeothermal fluids hosted by the permeable portion of the Capilluneformation, and rising as thermal waters of low salinity;
3) waters with higher temperatures and salinities, probablyfrom deep aquifers developed through regional fractures well belowthe Huaylillas ignimbrites.
It is however difficult to distinguish "reservoir" and "cover"geologic units because of the nature of the outcrops.
Mesozoic terrains have various lithologies and permeabilitieswith a noteworthy silt-clay component; the limnic-volcanic uppertertiary sequence can make up for a partial cover and is wellspread; volcanics could act as recharge areas or covers as afunction of fissuring or/and hydrothermal alteration achieved. Infact such alteration processes are known in the whole area, mostlyrelated to the occurrences at the highest temperatures, mainly asdeposits of kaolinite or silica/carbonate incrustations.
3. Sample collection and field determinations
The sampling carried out over a total of 11 days was mainlyperformed in three zones: Challapalca, Paucarani and Tutupaca.
349
Thirtyone manifestations were visited, 28 samples of water weretaken and 3 of gas; samples location is shown in Appendixes 1-1B.Some of the water points recorded in the previous studies were notfound owing to imprecision as to their location or to theimpossibility of reaching them.
Water samples of others zones not visited (Calientes andCalacoa) were later sent to Italy for further analyses.Unfortunately, for these samples, that proved to be of maximumimportance, some field determinations were not carried out and nosample of gas is avalaible. Water samples were collected generallyin 6 separate aliquots for analyses of major and traceconstituents, monomeric aluminum silica, stable isotopes (18O andD) and tritium.
Temperature, pH, conductivity and alcalinity were determined inthe field. Major and trace chemical costituents were analyzed inENEL laboratory in Italy; isotopes were analyzed in IAEA and DSIRlaboratories.
4. Water Chemistry
4.1. Classification of water samples (Tutupaca)
The analytical results of the fluids sampled in this area areshown in tables 1 (waters) and 3 (gases) . The code CA and CSidentify manifestations in the Calacoa and Calientes zonesrespectively.
The sample classification was mainly done by simple correlationdiagrams involving both major and trace chemical components.
In the Piper diagram of fig.l, all the samples of the Tutupacaarea (with the exception of TP12) are distributed in the NE and SE
350
Table 1 - Tutupaca water samples analysess = spring
Samplt
CA1CA2CA3CS1CS2CS3CS4TP10TP11TP12TP1BTP2TP3TP4TP6TP7TP8TP9
; Date Cod. S.T
30/05/86 1755 S30/05/86 1756 S30/05/86 1757 S30/05/86 1758 S30/05/86 1759 S30/05/86 1760 S30/05/86 1761 S21/05/86 1669 S21/05/86 1670 S21/05/86 1671 S19/05/86 1660 S20/05/86 1661 S20/05/86 1662 S20/05/86 1663 S20/05/86 166521/05/86 1666 S21/05/86 1667 S21/05/86 1668 S
. T.f.•C
60.091.014.086.086.080.014.050.046.014.031.036.056.056.086.03.0
12.060.0
pH.f. (
6.307.96
7.657.046.942.702.907.566.416.406.446.563.636.756.793.00
:on.f. /
MS
17104130
5480398051602100205062
1790247028403080577230144
2100
Uc.f. 1meq/l
3.83.2
2.02.92.20.00.0
10.910.811.20.00.0
0.0
r.i.°c
202020202020202020202020202020202020
pH.l. (
6.278.307.127.548.236.957.602.722.856.507.507.977.768.003.106.456.202.88
:on.l. /
MS
174029203315600567055803532390226067
226027602810303072302431492420
Uc.l. (meq/l
3.63.61.12.13.22.40.40.00.00.47.9
10.310.110.80.00.70.50.0
¡Ic.l.meq/l
3.63.61.22.13.22.40.40.00.00.47.9
10.310.110.80.00.70.50.0
Ca
58.867.432.742.445.048.331.3
152.0151.03.344.830.155.523.219.821.311.4
152.0
Mg
5.709.707.300.493.504.30
10.9039.0041.001.805.809.406.306.15
13.706.502.6040.30
Na
319.0602.019.2
1270.0777.01000.0
16.3166.0163.05.5
472.0589.0619.0641.029.912.810.1
179.0
K
30.153.04.6
99.154.942.56.7
19.918.72.536.645.943.649.918.13.44.823.8
Cl S04
404.0 126.0702.0 368.09.5 77.1
1960.0 93.41140.0 100.01550.0 112.0
1.2 124.0162.0 1040.0150.0 1020.01.2 8.6
322.0 314.0428.0 372.0444.0 390.0452.0 412.0
1.7 248.01.5 66.42.0 32.2
168.0 1000.0
HC03
225.8195.373.2
122.0176.9134.224.40.00.024.40.0
659.0659.0683.40.00.00.00.0
Si 02
530.0766.00.0
969.0836.0850.050.0
185.0178.046.0109.0172.0120.0126.0190.042.038.0196.0
H3B03
67.5121.0
1.1323.0194.0255.00.2
30.528.90.256.072.775.779.10.70.30.2
32.5
Br I
0.98 0.001.76 0.600.00 0.003.44 0.502.10 0.302.70 0.300.00 0.000.35 0.000.42 0.000.00 0.000.97 0.701.24 0.501.30 0.801.32 0.000.00 0.000.00 0.000.00 0.000.37 0.00
Table 1 - ( cent.)
Sample
CA1CA2CA3CS1CS2CS3CS4TP10TP11TP12TP1BTP2TP3TP4TP61H7TP8TP9
Code
175517561757175817591760176116691670167116601661166216631665166616671668
F
2.003.200.002.701.501.800.004.304.600.000.961.001.040.000.280.160.004.80
Li
1.703.003.009.805.807.600.000.500.500.001.401.801.902.1010.0010.000.000.59
Rb
0.270.490.491.300.540.500.000.120.130.000.190.250.240.240.060.000.000.15
Cs
0.500.000.003.401.902.800.000.340.000.000.461.500.700.700.000.000.000.28
NH4
0.2000.3000.0000.6000.5000.5000.0000.1000.2000.0001.100
1.4001.1000.2000.0000.0000.200
Sr Ba Zn
1.10 0.080 0.0371.50 0.050 0.0161.50 0.050 0.0161.10 0.020 0.0000.99 0.040 0.0000.86 0.060 0.0070.11 0.018 0.0000.14 0.018 0.4000.09 0.018 0.4000.03 0.000 0.0001.20 0.035 0.0001.00 0.001 0.0001.50 0.039 0.0001.40 0.028 0.0009.00 0.011 0.0810.17 0.010 0.0000.13 0.024 0.0000.04 8.000 0.420
Sb
0.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.000
As Fe ALT Al.M
1.60 0.420 0.270 0.0002.80 0.220 0.000 0.0000.01 0.000 0.000 0.0009.30 0.030 0.200 0.0005.60 0.080 9.000 0.0008.00 0.440 0.120 0.0000.00 0.020 0.000 0.0000.33 29.00 35.50 0.0000.29 27.20 34.90 0.0000.00 0.000 0.000 0.0000.49 0.760 9.000 0.1700.67 0.000 0.000 0.0000.51 0.470 0.050 0.0000.61 0.000 0.000 0.0000.00 1.800 6.500 0.2100.02 0.000 0.000 0.0000.02 0.000 0.000 0.0000.35 32.20 23.80 0.000
TDS
155027031574789317938922411866182169
18581727176417975501651021864
TAN
meq/l
17.730.73.159.237.148.33.026.225.50.623.530.631.432.55.21.40.725.6
TCAT
meq/l
18.332.23.661.438.648.53.318.618.60.624.429.331.731.25.33.71.319.4
5180
o/oo
- 1 1 . 78-11.28-11.36-12.67-13.36-14.04-14.13-13.44-14.11-13.16-13.36-13.39-13.46-12.86-11.84-12.73-13.34-14.20
5D
o/oo
-91.0-89.3-82.6
-111.4-112.3-118.4-111.3-103.4-104.9-96.6-102.8-105.8-103.3-100.4-102.6-98.2-96.2
-106.1
Tnt.
U.T.
1.20.42.00.40.4
2.70.72.20.60.7
1.70.30.4
5180/S04 534S/S04 Alt.
o/oo o/oo m.
360029003000
-6.00 9.20 4340-3.00 8.20 4315
0.00 42800.00 43200.00 43900.00 43300.00 42600.00 4120
2.10 12.80 41402.50 12.60 4050
0.00 40400.00 44800.00 41400.00 41400.00 4290
Table 2 - Challapalca water samples analysess = spring
SampU
PA1PA2PA3PA4PC1PC10PC2PC3PC4PC5PC6PC7PCSPC9
; Date Cod. S.T
17/05/86 1656 S17/05/86 1657 S17/05/86 1658 S17/05/86 1659 S14/05/86 1673 S16/05/86 1682 S14/05/86 1674 S14/05/86 1675 S15/05/86 1676 S15/05/86 1677 S15/05/86 1678 S15/05/86 1679 S16/05/86 1680 S16/05/86 1681 S
. T.f.
°C
45.09.062.038.043.018.087.08.012.072.022.058.024.068.0
pH.f. (
6.595.056.505.986.816.727.929.028.206.126.286.106.726.10
:on.f. /MS
10102941880890
1350473546033929220747545705504630
Uc.f. •meq/l
11.20.04.54.53.81.91.40.00.00.70.04.41.83.1
r.i.°c
2020202020202020202020202020
pH . I . (
7.533.847.287.127.176.847.356.756.676.756.757.057.067.10
:on.l. /
MS
1120274193010401470611619031429122050640505023860
Uc.l. P
meq/l
10.60.04.44.33.71.91.21.01.20.61.53.71.72.9
ilc.l.
meq/l
10.60.04.44.33.71.90.61.01.20.61.53.71.72.9
Ca
75.615.865.393.835.223.267.618.128.98.715.643.116.538.7
Hg
35.405.8038.3029.1016.3017.000.443.607.300.7212.2017.6010.9010.10
Na
127.012.8241.093.1249.067.2
1420.037.216.426.659.1848.065.9807.0
K
42.94.290.331.029.416.491.98.95.18.214.898.110.184.0
Cl
3.70.6
498.06.1
354.095.0
2320.05.46.51.0
102.01330.084.2
1240.0
S04
116.0125.070.8344.041.458.775.579.066.241.716.4107.029.698.7
HC03
677.30.0
274.6268.5231.9115.985.461.073.242.790.3268.5109.8189.2
Si 02
192.053.0244.0132.0137.085.4273.037.443.4120.093.9210.099.9165.0
H3B03
4.00.4
147.04.591.88.4
591.01.71.50.610.2233.017.7220.0
Br !
0.00 0.000.00 0.001.65 0.500.00 0.001.18 0.300.12 0.007.42 1.300.00 0.000.00 0.000.00 0.000.21 0.005.28 0.600.36 0.004.96 0.70
Table 2 - ( cont.)
Sample
PA1PA2PA3PA4PC1PC10PC2PC3PC4PC5PC6PC7PCSPC9
Code
16561657165816591673168216741675167616771678167916801681
00000010000
F
.12
.27•22..21.22.20.84.22.30.15.20
0.8601.24.04
Li
0.1610.001.000.151.750.15
11.500.030.030.000.174.900.304.70
Rb
0.190.000.380.170.190.001.100.000.000.000.000.760.040.76
Cs
0.000.000.280.500.760.005.700.000.000.000.002.500.002.60
NH4
0.4000.0.0.0.0.0.0.0.2.0.1.0.1.
000600900100000700000000400000500000500
Sr 8a Zn Sb As Fe Al.T Al.M
0.71 0.120 0.000 0.000 0.35 5.500 0.050 0.0000.11 0.010 0.073 0.000 0.00 0.250 6.400 0.1000.60 0.210 9.000 0.000 4.50 0.140 0.050 0.1701.10 0.023 0.000 0.000 0.36 5.400 0.070 0.0000.53 0.016 8.000 0.000 3.30 0.000 0.110 0.0900.24 0.020 0.000 0.000 0.11 0.000 0.000 0<t)001.50 0.065 0.026 0.000 18.60 0.000 0.070 0.0500.15 0.010 0.000 0.000 0.05 0.000 0.000 0.0000.19 9.000 0.000 0.000 0.05 0.000 0.000 0.0000.13 0.043 0.007 0.000 0.02 0.390 1.100 0.2100.17 0.000 0.014 0.000 0.32 0.020 0.080 0.0001.50 0.170 0.000 0.000 12.30 0.070 0.050 0.0500.15 0.000 0.000 0.000 1.30 0.000 0.000 0.0001.500.1900.000 0.000 11.400.2000.0000.000
TDS
6052351416742971372489319218521241629173372698
TAN
meq/l
13.62.620.011.714.65.8
68.42.82.81.64.744.14.8
40.1
TCAT
meq/l
13.31.9
19.211.914.75.9
67.53.02.91.94.7
43.04.8
40.0
¿180
0/00
-15.84-14.37-14.74-15.96-15.25-16.76-10.95-11.68-12.96-3.76-16.64-14.57-16.58-14.58
¿D
o/oo
-117.5-111.5-118.6-121.7-119.0-125.6-104.9-99.2-103.0-89.2
-124.6-116.9-126.2-116.3
Trit.
U.T.
0.7
0.50.10.30.00.23.06.40.52.80.00.50.0
5180/S04 ¿34S/S04 Alt.
o/oo o/oo m.
460046004750
1.20 -1.50 45404370442043954360437047004200435043804370
Tab. 3 - Chemical analyses of gases
Sample
TP1BTP4TP6PC6
Date
20/05/8620/05/8620/05/8615/05/86
C02%
65.6593.5498.7522.90
N2%
34.006.171.24
77.10
H2Sppm
2000
CH4ppoi
36327702
H2ppm
4.22.9
450.04.2
COppm
2.015.448.66.5
Cl- + S04—
ra2
100
+ TP12
+ TP2+ TP4
HC03- + C03—
CAÍ
100
..TP7
::TFn
++
Fig. 1 - Piper diagram for Tutupaca area water samples.
quadrants. Points TP7, TP8, TP12, CA3, and CS4 are cold meteoricwaters with a maximum temperature of 14 "C and a TDS rangingbetween 70 and 240 mg/1.
TP6, TP9, TP10 and TP11 are hot acid waters, the first of wichis a boiling pool (86 °C) with a TDS of 550 mg/1, whereas theothers have a temperature varying between 46 and 60.4°C and a TDSaround 1850 mg/1. TP6 and TP9 are located in the quebrada AzufreGrande, the other in the quebrada Azufre Chico. Along the shoresof these torrents, located in two large faults beneath the slopesof Tutupaca volcano, many warm and hot springs are present.
Their chemical characteristics are similar displaying a highearthy-alkaline sulphate component (approximately 60%) and aalkaline sulphate component of some 25%; the rest is alkalinechloride.
355
All the other samples of the Tutupaca area are located in theSE quadrant, where the alkaline chloride waters are found.
For better discrimination the Piper diagram was plotted withchlorides as separated anión (fig.2). In this figure the richchloride samples can be clearly distinguished into threesubgroups.
The first includes the samples TP1B, TP2, TP3 and TP4, whichare hot samples with temperature ranging between 31 and 56°C and aTDS around 1750-1850 mg/1; their composition is alkalinebicarbonate chloride with a high sulphates percentage. In the zonewhere these samples are located there are many emergences of warmand hot springs and numerous streams with considerable flowratethat may interact.
T
T
+ TP6
ra-2.
+ TP8TP9
TP12
T
Cl-
.00
C A Í
+ CA2
^TPS+ TP4
HC03- S04--
100
nQ)
Fig. 2 - Piper diagram with Cl as separated a n i ó n .
356
The second subgroup is composed of the samples CAÍ and CA2,these were taken in the zone of Calientes, several kilometres awayfrom the water points with the code TP. Their temperature valuesare 60.5 and 91'C respectively (this latter, decidely higher thanthe water boiling point at the emergence altitude, is probablyjustified by the formation of superheated steam); the sodiumchloride component exceed 65%, the TDS ranges between 1550 and2700 mg/1 and the SiC>2 content, very high, is respectively 530 and766 mg/1.
The third and last subgroup includes the samples CS1, CS2 andCS3, located about 15 km to the west of the Tutupaca volcano.Their temperatures and TDS are very high ranging 80°-86°C (boilingpoint) and 3200-4800 mg/1 respectively; the sodium chloridecomponent varies between 86 and 93% and the SiOo content between836 and 969 mg/1.
In fig. 3 is shown the relative Cl, S04 and HCO3 content of allthe previous described water points. On the basis of this diagram,excluding the cold waters from the discussion, the samples TP6,TP9, TP10 and TP11 (hot acid waters), located in the left corner,could be considered as steam heated waters enriched mainly ingeothermal H2S. This statement is actual for the water point TP6,the upper one sampled in a creek where some mud pools and steamingpools are present; as regard the others three, the total absenceof gas, along the lower part of both the streams Azufre Grande andAzufre Chico, leads one to believe that the conspicous presence ofsulphates, rather than to an H2S oxidation, is linked to a sulphuroxidation. This is a product of the past activity of the Tutupacavolcano in such large amounts that not far from the summit asulphur mine exists.
357
o.
STEAM HEATEa WATERS
100 80 60 40
S04 (mg/l)
0
Fig. 3 - Relative Cl, S04, HC03 content.
The samples TP1B, TP2, TP3 and TP4 (alkaline bicarbonatechloride with a high sulphates percentage) are found in the"peripheral" waters sector of the diagram. These mixed samples canbe considered the boundary part of a geothermal system, leavingthe central core, the deeper chlorinated inflow uprising at thesurface reduces, but the CO2 and H2S, reaching surface nappes richin oxygen, give aggressive solutions that can considerably alterthe host formations and produce alkaline bicarbonate-sulphatewaters. In the Tutupaca area the peripheral waters retain a slightdeep liquid phase contribution and this explains its chloridecontent.
358
All the samples in the SE quadrant of fig. 2 are also in theupper sector of fig. 3, in particular CS1, CS2 and CS3 are locatedin the "mature waters" zone and can be used for geothermometricpurposes.
The linking between these alkaline chloride waters and theprevious "mixed waters" is clearly appears in fig. 4, where therelative content of Li, Rb and Cs is shown: the representativepoints cluster in the same area of the diagram proving that asingle deep liquid feeds the manifestations.
Another diagram that provide general informations about anysteam and gas inflow contribution is the fig. 5 one, where therelative content of Cl, H3BO3 and NH4 is shown. The chloride
0,
¿o
i i i i i i i i i i i~i i i i i i i ] i i i i100 80 60 0
RB (mg/L) * 4
Fig. 4 - Relative content of Li, Rb, Cs.
359
60 40 0Cl (mg/i)
Fig. 5 - Relative content of NH4, Cl, H3BO3.
concentration is a direct function of the water-rockequilibration process, H3BO3 behave in a similar way, but prove tobe enriched where phenomena of steam inflow occur; the NH4concentration, finally, in addition to phase separation phenomena,may depend on gas inflow.
Excluding the cold waters, whose NH4 content is zero and lie onthe diagram base, in the lower part are located the mostrepresentative alkaline chloride samples, where the NH. content isvery reduced. Subsequently the group of acid waters TP10, TP9 andTP11 is encountered, this confirms their low steam inflow; athigher values of NH4 content the samples TP4, TP3 and TP1B is met,for this "mixed water" group the supply of deep gases has alreadybeen underscored.
360
At the top of the diagram is located TP6, where the presence ofcondensed steam were evident even if the presence of gas is notparticularly abundant. The gas sample associated with this waterpoint (see table 3) contains H2, relatively high quantity of CO,but H2S and CH4 are below the detection limit, making any gasgeothermometry unreliable; the tritium content of the associatedliquid phase (1.7 T.U.) is, finally, quite comparable with thatfound in the cold waters. This situation agrees with a mixingbetween a local groundwater, that circulates in hot subsurfacerocks and boils without reaching high temperature and pressure,and a deeper gas which encounter this fluid and considerablyenriches it in NH¿ and SO..
4.2. Classification of water samples (Challapalca)
The analytical results of the fluids sampled during the surveyin this area are reported in tables 2 (waters) and 3 (gases) . Thesamples with PA code identify manifestations of the Paucaranizone.
The classification of samples was carried out in the same wayas for the preceding area.
In the NE quadrant of Piper diagram ( fig. 6) the earth-alkaline sulphate bicarbonate samples PA4, PC4 and the earth-alkaline sulphate sample PA2 are found; the first has atemperature of 38 °C and a TDS of 740 mg/1, the others are cold andcharacterized by a TDS lower than 250 mg/1. Sample PA1 (45°C and605 mg/1) located along the dividing line between the NW and SWsectors is a alkaline earth-alcaline bicarbonate sulphate water.In this latter sector the PC3 and PC5 samples are found; theirTDS, aroud 200 mg/1, is similar, but the temperature quitedifferent: the first is a cold water, the second a steaming pool(72°C).
361
At the boundary between SW and SE sectors are located PCS andPC10, alkaline earth-alkaline bicarbonate chloride tepid sampleswith a TDS around 350 mg/1.
Other 6 samples, PA3, PCI, PC2, PC6, PC/ and PC9 are foundwell inside the SE quadrant. These mainly alkaline chloride watershave temperatures ranging between 42°C and 86.9 °C and TDS between1 and 5 g/1. Excluding PA3 belonging to Paucarani thermal zone,all these last samples either line up in the diagram or show agood correlation between chloride content and temperature.
The same conclusions can be drawn from the triangular Cl, SO4HCO3 diagram (fig. 7) , where, the all the water points found inthe SE quadrant of fig. 6, fall into a line in the "mature waters"area.
Cl- 100
+
+
raz .
+ PC4
fPA2
>PA4
PAi
+ PC- +PC3
i
+ PC5
............ .1...... .. i..... .,..__,.. i _..._.. — i... — ........
10
PCS +
+ PA3
+ PC1
+ CS +PC
n- 0)
. +
ia
100 HC03- + S04—
362
Fig. 6 - Piper diagram with Cl as separated anión forChallapalca water samples.
TI I I I I I I I T I I I I I I I I I T I I I I I I I I M I I I I I I I I I II I I I I I I I I
S04 (mg/l)
Fig. 7 - Relative content of Cl, SO¿, HCO-,.
This trend is evidently due to a dilution process of a deepwater whose most representative uprising end-member is the PC2sample.
4.3. Geothermometry
Fig. 8 and 9 show the Na, K, Mg diagram [3] for the two areassamples of Tutupaca and Challapalca. The deep temperatures for themost representative springs CS1 and PC2, evaluable from the Na/Kratios, are respectively 215°C and 20CTC that is 20°- 30°C higherthan those computable with the K/Mg ratios. This shift could beexplained in terms of the different re-equilibrations rates
363
N a 1 0 0 0
K/100100
Mg Í0.5
Fig. 8 - Na, K, /~Mg triangular diagram for Challapalca area water samples.
involving the considered cations during the deep fluid uprising atthe surface. Both the geothermal areas present dilution phenomenaare as confirmed, in the corresponding diagrams, by the samplespoints lining between the most important springs (CS1, PC2) andlocal groundwaters (right corner in the diagrams).
Also in fig. 10 (where the values of 10*Mg/ (10*Mg+Ca) areplotted vs. lO*K/(10*K+Na)[3]) springs CS1 and PC2 fall very closeto the equilibrium curve confirming the reliability of thegeothermometric estimation. These temperatures, however, should beconsidered as the minima to be found in the reservoirs.
As regard the SiO2 geothermometer values, these must becritically evaluated: PC2 spring (Challapalca area) and CS1, CS2,
364
Na/1000
100MGÍ0.5
Fig. 9 - Na, K, 7~"Mg triangular diagram for Tutupaca area water samples.
CS3 springs (Tutupaca area) are steaming ones; on the hypothesisof the "quartz adiabatic cooling" the resulting deep temperaturesare over 186°C [4] for the the former area and 265°C for thelatter.
The former value is consistent with the Na/K and K/Mg resultsfor the same water point, the second is decidedly higher. As above200 °C the re-equilibration speed of SiO2 is very high and not evenmassive phenomena of boiling can explain this incongruence, theonly explanation to justify the concentration found is that thethermal fluids meet formations with SiO2 which can easily beleached. These latter could be the ignimbrites schematized in the
365
10 Mg
10 K
o
Fig. 10 - 10 K/(10 K + Na) vs. 10 Mg/(10 Mg + Ca) diagram for both the areas.
geological section of the Tutupaca area (see appendix 1C). In thiscase, the SiO2 content would be controlled by the amorphous phaseand the resulting equilibrium temperature ranging between 185 °Cand 200°C [4] perfectly merging the solute chemicalgeothermometers values.
At last in fig. 11 are shown the log (AP/K) values for PC2spring composition vs. temperatures ranging between 120°C and240 °C (with the only constraint of the calcite equilibration atthe different temperatures)[5]. The curves for a series ofvolcanic rock components, and the phases derived from theirhydrothermal alteration converge at a value around 210°C, veryclosed to the Na/K temperature.
366
Sample PC2
Added Carbon to solut.13.17 mmol/Kg
120 140 160 180 200Temperature °C
220 240 260
Fig. 11 - Log AP/K values vs. temperature.
5. Remarks on the isotopic data
The relative results of water points sampled for isotopicanalyses are given in tabs. 1 and 2. The SD vs. <518O diagrams forthe Tutupaca and Challapalca areas are shown in figs. 12 and 13.
The first shows a substantial shift (at least two <S18O units)for the samples CS1, CS2 and CS3 confirming that these are fluidswith high deep temperatures.
The second (fig. 13) the cold samples PC3, PC4 and PC11, whichconsiderably swerve from this one, are brook waters subjected todaily cicles of freezing and thaw and could, in the warmer hours,partially evaporate. This last process certainly is at the base of
367
-70
O<NX
-90
1 30~ I I I I I I 1 I I I I I 1 I I I 1 I I I I I I I I I I I I I I I 1 ! I I 1 I I I I I I I I II I I i 1 I I ! I I I I I I I I I I I I I I I 1 I I I I I I
-110
•15 -13180 IN H20
12 -11
Fig. 12 - 6 D vs. <518O diagram for Tutupaca water samples.
-60-1
-80-
oIEzjQ
•100-
-120
-140-18 -16 -14 -12
Fig. 13 - S D vs. £^°0 diagram for Challapalca water samples,
368
the isotopic composition of PCS (72°C, 212 mg/1) which,characterized by a small flow rate, results isotopically veryincreased: <S18O = -3.76, SD = - 89.2.
These evaporation phenomena are confirmed by the lining of allthe points till now mentioned on a straight line with the equation
<SD = 1.53*<S18O - 81.6 (R2 = 0.96) which can exatly taken as the"evaporation line". A further support is constituted by theposition of sample TP6, which also evaporates, that even locatedin the Tutupaca area, falls on the same line.
As regard the remaining points, sample PC2, the mostrepresentative of the reservoir deep fluid, shows an appreciableoxygen shift of about three <518O units; PC7, PC9, PCI, PC6, PC8and PC10 can be considered as mixings in different percentages ofthis deep fluid with a surface groundwater whose isotopiccomposition should be: <518O = -16.85, SD = -126.5.
The SD vs. Cl content diagram of fig. 14, furtherly confirmsthis hypothes is.
100-f.3
O
OCMX
-120*O
)ñ i.C)
o o o
500 1000 1500Cl (mg/l)
2000 2500
Fig. 14 - 5 D vs. Cl diagram for Challapalca water samples.
369
To end up, the tritium content of the samples decreas,generally, as a function of TDS and temperature with the exceptionof TP6; here, notwithstanding a temperature of 12°C, the tritiumvalue is 1.7 T.U. confirming the large supply of groundwater.
The 180°C and 240°C temperatures computable for the exchangereaction of 18O between dissolved sulphate and water [6] for theCS2 and CS1 samples also agree with the chemical geothermometersresults.
6. Gas chemistry
The number of free gases found in the two areas, not takinginto account the Calacoa and Calientes zones not visited, israther small. The manifestations are characterized by a veryreduced gas flow. Moreover, some samples were damaged during theair freight.
Shortly, for the Tutupaca area only three gases are available,while for Challapalca the only gas resulted so polluted to looseany meaning. The few analytical results are in tab. 3.
The composition of TP1B and TP4 agree with the assumption of amixture between a thermal fluid and surface waters initially inequilibrium with the atmosphere. Instead TP6 contains an higherquantity of H2 confirming a stronger interaction of surface waterwith a deeper gas of vulcanic origin.
7. Conclusion
In the Tutupaca and Challapalca areas exist geothermal fluids,their temperatures should be higher than 200°C. As regard theformer, samples coming from the Calacoa and Calientes zones, evenif some tens of kilometers far each from the other, seem to be theemergecy of the same big thermal system or, at least, twodifferent hydrologic circuits with similar characteristics.
370
References
[1] AQUATER - INGEMET (1980) - Proyecto de InvestigaciónGeotérmica de la República del Perú - Región V. Lima -PERÚ.
[2] BURGESS W.G. (1985) - Proyecto Integrado del Sur,PERU: contribution to the geothermal assessment.WD/OS/85/19.
[3] GIGGENBACH, W.F. (1988) - Geothermal solute equilibria.Derivation of Na-K-Mg-Ca geoindicators. GEOCHIMICA ETCOSMOCHIMICA ACTA. 52, 2749-2765.
[4] FOURNIER, R.O. & POTTER, R.W. (1982) - A revised andexpanded silica quartz geothermometer. GEOTHERMALRESEARCH COUNCIL BULLETIN. 11, 3-9.
[5] REED, M. & SPYCHER, N. (1984) - Calculation of pH andmineral equilibria in hydrothermal waters with appli-cation to geothermometry and studies of boiling anddilution. GEOCHIMICA ET COSMOCHIMICA ACTA. 48, 1479 -1492.
[6] MIZUTANI, Y. & RAFTER, A.T. - Oxygen isotopiccomposition of sulfates. NZ J SCIENCE, 12, 54-59.
371
U)~Jto
Appendix 1
LOCATION OF THE INVESTIGATED THERMAL ZONES IN SOUTH PERU
Appendix 1A
GEOLOGICAL MAP OF THE TUTUPACA AREA
Appendix 1B
MAP OF THE CHALLAPALCA AREA
-j-t- > -- ' Av-s-Mt^ífti.„£/( %!ifjf|'
te¿^y-' J-X J^, ~»C"k l w¿^n, \ /' - . v ,J
^s^^S£^^l-M;-- •* j^^t /?r x^S;~ "H0,"-)" T'""*. "'' S)S4 !*>* ''-. — 1" A I l\\u\ ll ^ * /^• » j- - t.'"?"'; !-*\ ''* /^-^ '" v T V . , , / " /N^'x ^x N; ^1 f7*-i,c»i .1, vi.'Giv/ S\ ,-,<l L '••<• ^-p-rv-v '-*%¿;"" / ^ „ x / ^\^ \-) X\ 4 - >¿__•eiítoj""- ' X^/O^^ ^ , , x A;-r °-l - " ' \ V 'J-- '-^^^^S^^-^^'^rV/v^^í^tv V<, v- > I>N / - - -^A -V— *v
Appendix 1C
SCHEMATIC GEOLOGICAL SECTION OF THE TUTUPACA AREA(see Appendix 1A)
VOLCAN TUTUPACA
_
avaspi roc last i cos (tufos)" " " (redeposition)ignimbrite
VOL CAN LOP E7 EX TRA NA
RIO CALIENTES
5500
5000
«500
4000-igmmbriteJ50U
-areniscas3000
L E Y E N D ADEPÓSITOS ALUVIALES Y FLUVIO.GLACIARES
\ Qp-VlcZ VOLCÁNICO TUIL'PACA CALIENTES
lUv-XVl TtO-Vicl VOLCÁNICO Pf?E-TUTUPACA CALIENTES
L"l'\-':';] T«-Co Fm. CAPILLUNE
KTI-to f'PUPO TOOUEPALA
JSKI-/U GRUPO YURA
MANANTÍA CALIENTE
GEOTHERMAL EXPLORATION BY GEOCHEMICAL METHODSOF THE THERMAL AREA EL PILAR-MUNDO NUEVO,STATE OF SUCRE, VENEZUELA
F. D'AMORE, G. GIANELLIIstituto Internazionale per le Ricerche Geotermiche,Consiglio Nazionale delle Ricerche,Pisa, Italy
E. CORAZZAIstituto di Geocronologia e Geochimica Isotópica,Consiglio Nazionale delle Ricerche,Pisa, Italy
J. JAUREGUI, P. VÁRELADirección General Sectorial de Energía,Ministerio de Energía y Minas,Caracas, Venezuela
Resumen-Abstract
EXPLORACIÓN GEOTÉRMICA MEDIANTE MÉTODOS GEOQUÍMICAS EN EL ÁREA TERMAL DE ELPILAR-MUNDO NUEVO, ESTADO DE SUCRE, VENEZUELA.
En 1985-1986 se efectuó un estudio geoquímico en la zona de El Pilar -Hundo Nuevo, península de Paria, Estado de Sucre, Venezuela. El objetivo erahacer comprobaciones para una posible explotación geotermoeléctrica de la zo-na, que se caracteriza por muchas manifestaciones naturales con temperaturasdel orden de 80-100°C.
Mediante una serie de clasificaciones diferentes de las muestras de aguay gas, fue posible hacer una selección de las composiciones químicas más im-portantes que permitieron obtener indicaciones geotermométricas útiles. Paraevaluar algunos de los parámetros del depósito, se han aplicado al mismo tiem-po varias técnicas geotermométricas basadas en la interacción fluido-roca.
La zona en cuestión parece prometedora respecto a la recuperación de re-cursos geotérmicos de entalpia elevada; parece que hay un depósito profundocon salinidad relativamente baja (< 5 000 ppm), con salmuera neutra a tempera-turas que se calculan entre 250 y 300°C y con presión parcial de CO eleva-da. Se supone que existe un segundo depósito a menos profundidad, con tempe-raturas del orden de 200-220°C. La acción de ambos depósitos está dominadapor el agua, y parece que se produce la ebullición a 160°C, a un nivel muy po-co profundo. El nivel piezométrico (elevación de 150 m) regula la distribu-ción en la zona de fumarolas y manantiales de agua. La autoobturación hace
377
posible la acumulación de fluidos, al menos en el depósito profundo, el cual
puede realimentarse con agua meteórica del lugar. Se deduce que a cierta pro-
fundidad hay un batolito que proporciona calor a los fluidos profundos.
GEOTHERMAL EXPLORATION BY GEOCHEMICAL METHODS OF THE THERMAL AREA ELPILAR-MUNDO NUEVO, STATE OF SUCRE, VENEZUELA.
A geochemical survey was carried out in 1985-86 in the El Pilar - Mundo Nuevo area, Pariapeninsula, state of Sucre, Venezuela. The aim was to ascertain a possible geothermoelectricexploitation of the area, characterized by many natural manifestations with temperatures in the range80- 100°C.
Through a series of different classifications of water and gas samples it was possible to operatea selection of the most significant chemical compositions that allowed useful geothermometricindications. Several geothermometric techniques based on fluid-rock interaction have been applied atthe same time to evaluate some reservoir parameters.
The area of concern seems to be promising for the recovery of high enthalpy geothermalresources; a deep reservoir appears to be constituted by a relatively low salinity (< 5000 ppm), neutralbrine with computed temperatures between 250 and 300°C and high CÜ2 partial pressure. A second
shallower reservoir is supposed to exist, with a temperature of the order of 200 - 220°C. Bothreservoirs result to be water-dominated, and boiling appears to take place at 160°C, at a very shallowlevel. The piezometric level (elevation 150 m) regulates the areal distribution of water springs andfumaroles. Self-sealing allows accumulation of fluids at least in the deep reservoir which can receiveits recharge from local meteoric water. A batholith is inferred at some depth, supplying the heat to thedeep fluids.
INTRODUCTION
From the Preliminary Report on the Evaluation of the Geothermal Potential of north-easternVenezuela (Gonzales et al., 1981), and also from the results obtained in the Geothermal ResourcesInventory in eastern and central Venezuela (Hevia and Di Gianni, 1983; Urbani, 1984), the El Pilar -Mundo Nuevo area, located in the isthmus of the Araya-Paria peninsula, state of Sucre, Venezuela(Fig. 1), was selected as the one with the highest priority. The geothermal assessment of the Pariapeninsula allowed this area to be selected as the most promising for a geothermoelectric exploitment.The reason for this choice is due to some characteristics suggesting a local accumulation ofgeothermal fluids, like the highest density of surface manifestations in Venezuela as well as of hightemperature springs (80 - 100°C) and of the peculiar chemical composition of the discharged fluids.Urbani (1989) gave a preliminary assessment of the area and proposed a model implying ageothermal reservoir with a temperature of 200 -250°C.
378
.SEA**£ ATI ANTIC
Stau of SMCftÑ OCEAN
CPAB - Cariaco pull-apartbasin
CG - Cariaco graben
GG - Guaraünos graben
SOG - San Juan graben
EPFS - El PHar faultsystem
SFF - San Francisco fault
UF - Úrica fault
BF - Bohordal fault
LBF - Los Bajos fault
ESF - El Soldado fault
r Area of interest (El Pilar-Casanaygeotherraal district)
r Geolonical mapsc."1:25.000
r Geological mapsc. 1:10.000
P Zone of highest temperature springs
22 39© Towns• Springsx Tree hydrocarbons
A Cerro La Pica""-• Fault traces o 20km
Fig. 1. Location of thermal springs in the Araya-Paria isthmus, stale of Sucre (from Urbani,1984,modified), and geostructural sketch map (from Várela and Hevia, 1990, modified). Thecenter-most rectangle is represented in detail in Fig. 2.
In the years 1985 and 1986 several thermal manifestations were sampled and analyzed with thepurpose of a regional survey from the Cariaco gulf to the Paria gulf, and inland to the San Juanvalley, thus characterizing the boundaries of the interest area outlined in Fig. 2 (Campos, 1981;Gonzales, 1981; Jáuregui etal., 1985; Jáuregui et al., 1986; D'Amore and Gianelli, 1986; Hevia andJáuregui, 1988). Many more analyses of diluted waters, both from cold and hot springs, are reportedin Hevia and Jáuregui (1988), namely for the central area. The location of springs in Fig. 2 wasderived from the map in the same article by Hevia and Jáuregui (1988).
The marginal springs shown in Fig. 1 have been included in the present work with the purposeof a better evaluation of the thermal anomaly in the central area. In spite of their thermality (up to55-60°C) and of their salinity, since the beginning of the survey these springs demonstrated by theircompositions to be related to marginal circuits. Their locations are roughly aligned E - W on the activeEl Pilar fault trace or on secondary ones related to it.
The geothermal manifestations in the area of concern (Fig. 2) form a hydrothermal system,constituted by several boiling springs of sodium chloride type, associated with large alteration zoneswith fumaroles and mineral deposits of sulfur, gypsum, anhydrite, calcite, and silica. Being only ofshallow origin, fresh diluted waters are found sparsely in the whole area, including also thefumarolic zones.
In this study several geothermometric techniques based on fluid-rock interaction have beenapplied at the same time, with the aim of a better definition of the thermal potential of the field. Thesurvey allowed also a thorough test and intercomparison on the reliability of different methods inevaluating the main reservoir parameters. The interpretation of data is based also on a selection ofavailable samples, depending on their modifications that might occur during their ascent to thesurface; this selection was applied both to water and to gas samples. Notwithstanding the highdensity of the tropical vegetation covering the studied area, enough geological and structuralinformation has been obtained, as briefly exposed in the geological chapter.
Though the above limitations are objective, the study encouraged by the involvement of IAEAcertainly allowed some conclusions to be reached about the high geothermal potential of the El Pilar -Mundo Nuevo system, positively ascertaining high thermal conditions at depth and possibly a sealingthat produces the condition necessary for a true geothermal field: the fluid accumulation.
The interpretation given in this work may give useful indications for further steps of thesurvey, like deep explorative drillings.
GEOLOGICAL BACKGROUND
The northeastern region of Venezuela is located in a transition zone at the contact between theSE end of the Caribbean plate and the NE edge of the South American plate. Such a contact is poorlydefined, and it is marked by a wide strip affected by deformation and block faulting. This character isthe consequence of the stress generated by the right transcurren! movement and the oblique collisionof the Caribbean plate toward ESE, against the counterclockwise turning South American plate. Theresult at depth is a transcurrent strain (at the base of the thinned continental crust which is near the
380
- '\\ m
/ \ M rW*\ V.™J\/
O NH4-S04 springs (fumaroles) • N a - C l springs A Diluted waters • Houses
UJ00
Fig. 2. Location of thermal springs in the area of concern, El Pilar - Mundo Nuevo. Elevations are inm a.s.l. (from Hevia and Jáuregui, 1988, modified). Dashed lines represent thehydrogeological pattern.
ductile-fragüe limit, at a depth of some 15 km) and pressure deformation along the dominatingdirections NNW-SSE, E-W, and NW-SE; in turn this is complicated by the pre-existing directions(e.g. N 70 E) as well as by some more directions complementary to the previous ones (SW-NE,N-S). The block faulting takes place either through faulting with variable directions around NW-SE,or through reactivation of old faults (Úrica, San Francisco, Santa Rosa, and others more,delimitating the Caribbean plate), or else through recent faults like Bohordal, Los Bajos, El Soldado(Fig. 1).
An important feature is the clear steady decrease from W to E of the right transcurrent E-Wmovement, between the Cariaco graben (almost 100 km), the Cariaco gulf (25-40 km) and thewestern part of the Araya-Paria isthmus (20-25 km), reaching a full stop in the Cerro Las Minas"push-up" block (Vierbuchen, 1984). This is in agreement with the interpretation given by Speed(1985) about the present transcurrent activity of the El Pilar fault system, as the consequence of asuture resulting from the angular collision in this area between the two plates.
After the Paleocene-Eocene great displacements that gave the region its basic architecture, themain deformations took place in middle Miocene (Macsotay et al., 1986), in Pliocene-Pleistocene,and in Quaternary. The most recent evidence are the marine terraces in the Araya peninsula, uplifted12 and 8 m, 125000 and 25000 years ago respectively; another consequence is the subsidence provedby microgravimetric methods in the area around and ESE of El Pilar (Bravo et al., 1986), as well asin other areas of the SE quadrant. According to all authors working in the area the above inference issupported by the interpretation of the present seismicity.
In the central area Mundo Nuevo - Las Minas - El Pilar the compressive deformation toward Sand SE produced many scales, faulted folds, wedges either raised or lowered, structures likesemigraben or klippen, all of them with an average vergency to SE.
Since in the region almost all contacts and the same stratigraphic column have been alteredtectonically, the following list represents the formations and the lithologies of the area:
- Barranquin formation (KB): average or very coarse quartz sandstones, shales, calcarenitesand limestones. Age: Barremian. Thickness: 600-1200 m.
- El Cantil formation (KEC): clear massive limestones and some calcarenites. Age: Albian.Thickness: 300-700 m.
- Querecual formation (KQ): limestones with alternated dark shales. Age: Turonian-Coniacian.Thickness: 150-300 m.
- San Antonio formation (KSA): limestones, hard siliceous shales and some shales andsandstones. Age: Maestrichtian. Thickness: 200-400 m.
- Tunapuy formation (KMT): phyllites and calcareous schists, either quartz-graphitic ormicaceous, with intercalations of lenses of metamorphic limestone, of some quartzites and ofquartz-mica-chlorite schists. Age: lower Cretaceous. Thickness: 800-1500m.
- Gliinimita formation (KMG): sericitic-graphitic phyllites, marls, meta-arenaceous limestones.Age: lower Cretaceous. Thickness: 200-300 m.
- Lechozal complex (KTpL): informal name for a mixture of deformed fragments ofmetamorphic rocks (KMT, KMG, KM Macuro, etc.) and of sedimentary rocks (BB, KSA, etc.).Age: undetermined, later than Paleocene-Eocene. Thickness: 100-150 m.
382
- Chaguaramas complex (Ts-QCH): informal name for a polymictic conglomerate or mixture ofreworked clasts, ranging from a coarse conglomerate to a fine sandstone and to a pelite, of greenishcolor. It results from the demolition of the southern edge of laminations and scales, and from othererosive products of metamorphic rocks (KMT, KMG, etc.) as well as of sedimentary rocks like KBand KSA. Thickness: 500-800 m.
- Los Arroyos formation (TmLA): informal name for deep trench turbiditic sediments with acomposition generally pelitic. Age: middle Pliocene. Thickness: 600-1000 m.
- Guatamarito formation (TpsG): informal name. Clear calcareous shales and greenish-grayclays from lagoons and marshes of upper Pliocene. Thickness: 150-400 m.
- Mamporal formation (QM): clear polymictic conglomerate, poorly cmented, with layers offine sandstone. Age: upper Quaternary. Thickness: 300-800 m.
- Recent sediments from alluvium and flooded plains. Thickness: from a few meters to morethan 100 m.
The recharge of water takes place through the intensive faulting of the rocks all over the area.The sealing could be provided by some low permeability lithologies of the above mentionedformations, or by a self-sealing, or else by the compression of certain sections of the fault planes.
Approximately 8 km to the north of the area with thermal manifestations several apophyses ofa granitic porphyrite outcrop, with an age of 5 Ma (Moticska, 1987). This area corresponds to agravimetric anomaly suggesting the presence of a great sized intrusive body, either granitic orgranodioritic (Vierbuchen, 1984); possibly this intrusion is constituted by rocks derived from therecent igneous phase related with the volcanic activity in the southernmost section of the LesserAntilles arc (Schubert and Sifontes, 1983). According to the gravity model by Vierbuchen (1984) thetop of the intrusion can be located at a depth of 6-9 km. The batholith has a maximum extension ofabout 10 km, and a maximum thickness of 5 km (undefined width). A hypothesis about the heatsource can be forwarded, attributing it to the pressure stresses producing the fracturing and thedeformation of the plate edges, and capable of generating heat anomalies in the deep crust, up to thelocal melting. The heat anomaly would produce a high heat flow capable of generating deepreservoirs of hot fluids. The hot fluid uplift can take place through fracture systems associated tofaults, the most known directions of which are NNW-SSE and N-S, as resulting from thedistensional discharge of the compression of blocks limited by minor faults.
The geological conditions of the region are highly favourable to the presence of reservoirs atdepth, with a medium-high enthalpy.
CLASSIFICATION OF WATER SAMPLES
The results of water analyses are given in Table 1 for the main components; trace elements forselected samples are given in Table 2. Table 3 reports d^O, QD and tritium values for samplescollected in May 1985. Fig. 3 is a ¿r °O vs. dD diagram for the rainwater values of the Maracay area(Venezuela, Lat. 10.25°N, Long. 67.65°W) and Barranquilla area (Colombia, Lat. 10.88°N, Long.
383
$ TABLE 1. Chemical analyses for main components of the springs in the El Pilar area. All analysesperformed at 25°C, expressed in mg/\; temperature and pH are measured in the field;conductivity is in nS/cm; blanks = not determined.
sample
712131415
106114
4
1718454647495051
109110111112
2224397788
M2193136425253596566717883
115116
date
5/852/865/855/855/852/866/86
5/85
5/855/855/855/855/855/855/854/863/863/863/863/86
5/855/855/855/855/85
5/855/855/855/855/855/854/865/854/865/855/854/865/852/862/86
t'C
98386 580
10099 597 5480
497
87954904824285536435394493275599
476244284392565
240305324373253465295340333243 537355332
pH
6775966879469577825
595
6226505752852804352363642852402781 83
6 16691605667588
7 06976846616363632677386356570158644654
conductivity
4930
478040305710
5060
1070133126602750130069305640
24303600349030201130
658990725713888573
461
852927
609
Na
763675689634939650400
779
6 73 61 30 83 50 73 13 23 73 26 42 5
477908
1030816176
8794 83128 163 8
6 35 2503 8
69 563 24 86 24 03 9
K
290287273188364263130
147
1 21 61 40 62 62 9
19 75 33 73 58 57 5
21 218 215 742 4195
1 512 3
1 51 78 21 51 40 50 38 85 70 94 00 70 7
Ca
136229181122110371179
466
10628 32 56 6
74 81 08 8
30 116278 3
38116 1
176191232141106
1431469 0 4
12414417916910895 5
14714278 3
304143136
Mg
11 725213 521 7082
42 116 1
373
15390400923403555579
21 5743276251
529820336 8295
2913279167136 669129353
11 411 343589
13785
Cl
154019001415122018701400705
1190
684 72 22 24 70 44 41 00 92 52 51 8
34185 3
5371496 5 7
15214756 536 198 58 7
1211 56 4
111969 8
10 58 180
SO4
16617716863 5827
154144
877
287474
11601140482
31701870545
1130123116157
7 410318725 839
62 220 296 3
13924 824 416 09 60
14 819 905 3
2210 3
HCO3
12935022226249
43076
465
257117
0000000000
1090259017201420637
348329262182399338540281290336420241409314345
SiO2
15014414378 1
15210976 5
59 5
28 526 825 186 429 336 8
20145 277 353 870 968 8
33 821 522 926 235 4
8 928 713 612 318 742 425 513 213 124 018 215 258 91515
NH 4
27
261333 5
5 4
61 5i7546240345
1080304
1 9233 45 43 8
I 40 20 20 30 20 8
0 3
0 20 2
0 4
B
36 530933 724 545 242 317 7
16 6
0 050 050 6 50 040 03
< 0 0 1008
2 60 246 24 52 9
0 053 10 060 51 70 04
<0 01
2 21 7
008
TABLE 2. Chemical analyses for trace elements of selected springs, El Pilar area. All analyses performed at 25°C, expressed in ppb; n.d. - not determined
sample
71314154171845464749502224397788525983M2193136426671
u11800114009900154009000<10<IO<10<10<io<10307401109801400540<10<1050
1200<10<10610830580
Rb
52005100400063001800<40<40<40<40<40<40<40<40<40<40<40<40<40<40<40<40<40<40<40<40<40<40
Cs
27002400250032001500<400<400<400<400<400<400<400<400<400<400<400<400<400<400<400<400<400<400<400<400<400<400
As
1000110015001100110<10<108020404010010308020
<10<20<1020501010205020
Br
81007100500093006500120
<100160100
<100740200570
<1001000870190
<IOO<100<100<100590260130330380120
NO3
2500910<200<200<200<200<200<200<200<200<200<2003700
11500079002800<200240<200<2003300<2003300<2001020040006900
Ba
370270390620504040<3010070<3040
36002030
8093044080<30130201604090180140180
Sr
18001700110021003600180300
<100<100140
<100<1004600170091039003900210130400
<100370410500360300320
F
18001900160017001800<500<500<500<500<500<500<50011001200<50034001100<500<500<500<500600<500<500<500<500<500
Al
40<309060607208001000500
10600430
54900<3040<30<30<30100<30<3030<30<30<30<30<30<30
AlOnXO
20<20<2040<2016032060
n.d.n.d.130n.d.<20<20<20<20<20<20<20<20<20<20n.d.n.d.<20<20<20
Fe
«101503020
1900110090
1900210022500
5606100
8010130<10602501403080<10<1020<10<10<10
Sb
23018013087019030
<30<30120<30<30301702201901509020<3040<304030
<3030
<3020
Pb
500460390530470<20<20<20<2030<206026040046060015040306020703030509080
Zn
<5<5<5<59<5<51050603013020<5180<5<577<57<5<5<5<5<5<5
H2S
160012002901100<10044001380028009207704000160*
1700100670380790790
<100730590
<100<100660
<100730380
O Al is expressed as total Al and monomeric Al (separated in the field)
oo
TABLE 3. Isotopic composition of H^O. 5 is in °/°° SMOW. *518O for 804.
sample 618O 6D T.U. (±1.0)
7 -2.45 -17.813 -2.59 -20.0 1.214 -3.14 -17.315 -1.88 -16.0 0.115 +6.6*
4 -3.63 -19.2
17184545464749502224397788M2193136425259667183
-2.77-0.33-0.18-1.88-2.67-3.76-0.39-3.81-3.29-3.61-3.91-3.90-3.45-3.91-4.06-3.70-3.88-3.73-3.71-3.86-4.00-4.24
- 7.4+ 0.7+ 7.3- 0.4- 6.9- 4.8+ 0.2-21.2- 9.2-18.4-23.6-20.5-16.8-19.5-19.5-18.5-19.2-18.6-18.1-20.0-20.2-20.2
2.23.11.4
1.51.13.4
0.5
0.65.10.79.44.20.90.3
74.78°W), as well as some average rainwater values of Venezuela and Colombia (from IAEA,Vienna; R. Gonfiantini, personal communication). A slope of 8 seems fairly reasonable, although theaverage rain values might not be fully representative of the average ones in the El Pilar area.
The chemical classification of waters was accomplished taking into consideration: i) absoluteconcentrations of the main components characterizing each type of water; ii) correlations amongselected components.
Five types of waters can be recognized on the basis of the former method. In Fig. 4 the fivewater types are exemplified by particular samples.
386
10-
-10-
Q-30H
-50-
-70-
O • a1 2 3 4
-10 -8 -6 -4Ó180
-2
Fig. 3. Rainwater 5^80 versus 5D (°/°° SMOW) in northern Venezuela and Colombia. 1: average in
Venezuela; 2: average in Colombia; 3: Barranquilla area (Colombia); 4: Maracay area,Venezuela.
Na K Ca Mg Cl HC03 S04 NH4 B Li Rb Cs SiQ2 Na K Ca Mg ClxlO «10 KlO «10 xlO3 «!03 «10 «ID »10
4 B Li Rb Cs SiO?x10»10 <103 «103 xlO
80-
o
80
NACÍ water
n
Mixed water
n n n
O
80
» 4 0 -
NH4 SÜ4 water Na HC03 water
XL ñu -Q. H n
diluted water diluted water
nn n___n PI 1 1n n n n n
Fig. 4. Examples of the Five types of waters, as described in the text.
387
Group 1). The first group consists of seven samples (7, 12, 13, 14, 15, 106, 114). Completechemical analyses are available for springs 7, 13, 14, and 15 only, sampled in 1985. From Piper's(1944) modified diagram (Fig. 5) it is evident that this is a homogeneous group of sodium-chloridetype waters. They are all located along an east-west strip of about 3 km in length, from El Palmar toLos Chirriaderos (see map of Fig. 2). The most representative water sample (15) is reported in Fig. 4for almost all chemical components (in meq/1). Despite its homogeneity some interesting differencescan be observed in Fig. 6 where sample 39 (located 15 km to the south) is also reported forcomparison (only samples collected in May 1985 are represented). In this figure several componentsare plotted versus chloride (ppm). It is evident that, as the Cl concentration decreases, the Mg andalkalinity increase, whereas Na, K, Li, Rb, Cs, Br, Nlfy, and B decrease. The SO4/H2^ ratio alsoincreases, due mainly to a decrease in H2S (oxidation). From stable isotope data (Table 3) the waters
show almost the same ÓD values (=18 i 1.5°/°°), while the d^°O value is -1.9%° for sample 15,
decreasing progressively towards -3.14%° of sample 14 and to -3.6%° for sample 39, in proportion to
the Cl content. The high arsenic content (over 1 ppm) in these waters is an indication of itsaccumulation in the geothermal water from a deepe and hot source. Antimony also is quite high inthese waters: in sample 15 it is just less than 1 ppm.
Despite the fact that the tritium content of fresh water is about 13 T.U., little tritium isapparent in these waters, at least in samples 13 and 15 which were the only ones analyzed in thisgroup.
Group 2). It includes spring 4 (Ño Carlos) only, located about 15 km east of the first group.The water chemistry shows an unusual composition: it contains about the same amount of Cl asspring 14, but more Ca, Mg, Na, as well as very high values of 804 and of HCC>3. Figures 4 and 5
show the complex chemistry of this water.
Group 3). One typical group of waters is that of the NH4-SÜ4 springs located in the fumarolic
fields of the Mundo Nuevo - Los Mereyes area (springs 45, 46, 47, 49, 50, 51, 112) and of ElSalvaje - Buena Esperanza area (springs 17, 18, 109, 110, 111). Their peculiar composition is putinto evidence in Figures 4 and 5.
Group 4). The fourth group of waters is of the Na-HCC>3 type (springs 22, 24, 39, 77, 88),
located far west and south of the area of concern El Pilar - Mundo Nuevo. They are all accompaniedby emissions of CO'2. Their chemistry is evidenced in Fig. 4 for sample 77, and in Fig. 5; they
usually contain large amounts of Mg. In Fig. 6 spring 39 is reported because of its Cl content inexcess of 500 ppm, despite its relative distance from the springs of Group 1. Compared to thewaters containing Cl reported in Fig. 6, its Na content increases with a decrease in Cl, whereas K,Li, Rb, Br, B and NH3 decrease; this is the consequence of its high HCC>3 content.
Group 5). This is made of low salinity waters (< 1000 ppm), with pH between 6 and 7. Adistribution can be made on the basis of their geographic location. Springs 52, 59, 65, 78, 83 and
388
*í»
Fig. 5. Piper's (1944) modified diagram plotting compositions of waters from Groups 1, 2, 3, and 4.
M2 are located in the Mundo Nuevo area. Springs 115 and 116 are located west of El Salvaje andnorth of Los Chirriaderos ( 1 - 2 km); these are typical local groundwaters of the Ca-HCC>3 type, as
shown in Fig. 4 (for sample 83). Springs 19, 31, 36, 42, 66 and 71 are all located west of the LosMereyes - Mundo Nuevo area (see example in Fig. 4). Springs 31 and 36 in the Casanay - LaFlorida area are characterized by relatively higher Mg and 804 contents with respect to the others of
this group.
Fig. 7 shows the 3D versus d^O diagram for the waters of Groups 1, 2, 4, and 5. Allsamples but Group 1 are positioned on the meteoric line (Fig. 3) at dD = -19°/°° and d^°O = -4°/°°
approximately. Diluted waters can be considered as representative of the local meteoric ones. Allsodium bicarbonate waters except spring 24 fall on the meteoric line, at about the same point asdiluted waters. The lack of an isotopic shift of bicarbonate waters can be an indication oflow-enthalpy systems in spite of the emerging temperatures. The position of spring 24 can beexplained taking into consideration its recharge from Cerro El Pato, with an elevation in excess of600 m; a negative shift for d^O can also be explained by an exchange with CÜ2: the manifestation
emits a huge amount of free gas with respect to the water flow. Springs of Group 1 show an evidentpositive shift in d^O, up to two units (spring 15), suggesting a high temperature water-rockinteraction.
389
Fig. 6. Plot of some components against chloride for some selected springs, from the central ones(center and right) to a peripheral one (left), showing the steady changes in major and minorcomponents. Chemical components are in ppmv.; 3'^O in %<> SMOW.
390
20-
10-
0-
Q-10-
20-
-30-
-40 H
4,36.39.52.59
31,71.19,42,66.8883
-7 -6 -5 -4 -2 -1
Fig. 7. 5D versus 5O diagram for the waters of Groups 1, 2, 4, and 5. The meteoric line isreported also (from Fig. 3). The oxygen isotopic shift for Na-Cl waters is marked by a brokenline.
Fig. 8 shows the isotopic composition for the NH4-SÜ4 type waters (Group 3). The values
are scattered over the diagram from about 0 to about -4°/°° of 3^O, across the meteoric line; the
exchange with CC>2 of the associated gas can displace the points toward more negative values. The
points fall along a straight line with a slope 3 which can be interpreted as due to condensation andevaporation processes at about 100°C; this line intercepts the meteoric line at more positive 3D valueswith respect to the springs of other groups. The positive shift in 5D can be explained by an exchangewith H2S of the associated gas phase, depending on the relative amounts of K^S and water.
"Most geochemical techniques may with confidence be applied only to specified typesof fluids with limited ranges of compositions. For instance, most ionic solutegeothermometers "work" only if used with close to neutral waters containing chloride asthe major anión. Any such interpretation of geothermal water samples, therefore, is bestcarried out on the basis of an initial classification e.g. in terms of their major anions Cl,SO4 and HC03" (Giggenbach and Goguel, 1989).
In the C1-HCO3-SO4 diagram of Fig. 9 the five groups of waters show the importance of
three main geochemical processes: a) steam heating from the absorption of high temperature,H2S-containing, "geothermal" steam into groundwater; b) uprise of neutral, low sulfate, high
chloride, "geothermal" waters along the C1-HCÜ3 axis, close to the Cl corner; c) dilution by neutral,
but high-bicarbonate waters. Considerable caution is required in the application of most"geo-indicators" for sulfate or bicarbonate waters, because the observed chemical composition ismainly due to attack of acidic gases on the local matrix rock. It is evident that all waters of Group 1follow almost a linear trend toward the shallow bicarbonate waters, starting from spring 15 which is
391
20-
Fig. 8. dD versus d^^O diagram for the NH4-SÜ4 springs of Group 3 (see text).
0,50
HCO,/Cl
STEAM - HEATED WATERS
112-51-109-110-111 115-53-65-116
Fig. 9. Cl - HCO3 - SO4 diagram (after Giggenbach and Goguel, 1989) for all five groups of waters.
The hatched area represents "mature" waters; symbols represent: solid circles waters of Group1, open circles waters of Groups 2 and 4, crosses waters of Group 3, open squares waters ofGroup 5.
392
regarded as the most "mature" water and which is likely to be the most representative of the deepreservoir. With respect to spring 15, the other Na-Cl waters become more and more enriched in Mgand bicarbonate upon dilution; this has been evidenced also in Fig. 6. All low salinity waters (Group5) fall in the vicinity of the bicarbonate corner; some waters far from the interest area show a certainscatter, like the ones from Pantoño and Casanay areas (Fig. 1). Springs 22 and 39 (out of the interestarea) show a relatively high content of Cl, likely to be related to a mixing with salty waters in recentsediments, from which they can strip also oxidized organic matter, like nitrates. Spring 4 also showsa high Cl content associated with a high sulfate, consistent with a saturation temperature for anhydriteclose to 50°C. The relatively high Br (6 ppm), sulfate, Ca and Mg values, and its location subjected tosea flooding, might indicate that a fraction of this water is likely to be related to a marine source. Itsisotopic composition, shown in Table 3, is consistent with a meteoric origin and a circulation withinmarine sedimentary formations, at low temperature.
Spring 24, located few kilometers south of Las Minas, is a typical sodium bicarbonate water; itshows anomalous contents of ammonium and mainly nitrate. The associated gas has a very high flowrate and contains large amounts of methane (close to 20 % in volume). All these evidences are infavor of a circulation in sediments rich in organic matter.
In the interest area the waters falling near the left corner of the diagram are generally acidic.Fig. 9 shows that springs 17 and 18 also contain a significant amount of bicarbonates. These twosprings (not acidic) are associated with the most powerful gas emissions east of the Mundo Nuevoarea. All these waters are interpreted as cold groundwaters that have been heated and contaminated bygeothermal steam. Indeed, all these springs are characterized by a strong bubbling of gas containinglarge amounts of H^S. One interesting feature of these waters is their large seasonal variation of pH.
During the rainy season oxidation of FTjS is stronger and the pH lower (less than 2).
It is interesting to point out that bicarbonate waters, such as 4, 22, 24, 31, 36, 39, aresurrounding the interest area of geothermal upwelling. Besides, the high sulfate and the steam-heatedwaters in the interest area are encountered at the higher elevations (see Fig. 2). In particular in theeastern area, while the Na-Cl waters (El Palmar - Los Chirriaderos) are found below 150 m, thesteam-heated waters and the main gas flows (El Salvaje - Buena Esperanza) are found at elevationsbetween 200 and 400 m. The piezometric level can be defined by the elevation of springs 14 and 15,about 130 - 150 m. In the Mundo Nuevo - Los Mereyes western area only acidic springs (as well asdiluted bicarbonate waters) are found; indeed the whole area has an elevation between 350 and 400m. Then (if locally present) the deep and chloride-rich waters are not allowed to reach the surfacebecause of their lower piezometric level. The circulation system is complicated by many shallowwaters which can also influence the composition of gases, e. g. oxidizing FÍ2S and H2-
The Cl-Li-B diagram shown in Fig. 10 is here proposed only for water samples wherechloride is above 10 % in weight among anions. According to Giggenbach and Goguel (1989):
"The alkali metal probably least affected by secondary processes is Li. It may, therefore, beused as a "tracer" for the initial deep rock dissolution process and as a reference toevaluate the possible origin of the two important "conservative" constituents of thermalwaters, Cl and B."
393
Cl/100
001
0.20
B/4
Fig. 10. Li - Rb - Cs diagram (Giggenbach and Goguel, 1989) for selected waters (Cl > 10% inweight among anions) (see Table 2); SW = sea water.
All waters of Group 1 fall in a single cluster, showing a leaching of rocks, possibly reaching a"maturation" stage. Sample 4, and to a further extent 22 and 39, show a chloride enrichment: this isconsistent with an addition of sodium chloride-rich waters (tectonic lows filled with marinesediments and/or sea water at high tide).
In the Li-Rb-Cs diagram (Fig. 11) the Na-Cl waters cluster again around a composition which isconsistent with the dissolution of an acidic rock, like rhyolite. Rhyodacitic subintrusive rocks (asdiscussed in the geological chapter) outcrop just north of the area, within a distance of 6 - 10 km.These three rare alkali elements, being little reactive, are conservative of the rock composition and canbe leached only at depth; their ratios are not affected by shallow processes (Giggenbacb and Goguel,1989). Only in waters of Groups 1 and 2 it was possible to find measurable concentrations of Rband Cs. These neutral waters (at least for their deep components) possibly have lost Rb due to uptakein K-silicates, but not Cs.
As an introduction to the next chapter dealing with water geothermometry, the Na-K-Mgdiagram proposed by Giggenbach and Goguel (1989) has been used only for qualitative classification(Fig. 12). All points representing waters of Group 1 fall aligned in a straight pattern from the Mgcorner of shallow waters to the farthest sample 15. This is an indication that they all form a singlefamily, confirming the previous observations. The relative position of the alignment is due mainly tothe Na-K and K-Mg geothermometric equations chosen by Giggenbach and Goguel. It is evident thatin this case the use of the two geothermometers yields an inconsistency. In spite of the generalreliability of Giggenbach and GoguePs (1989) geothermometer, the Ca concentration should betaken into account in sedimentary rocks, like in this case. Different temperature calibrations of the
394
100
IOCS
Fig. 11. Triangular plot for Li, Rb, and Cs (after Giggenbach and Goguel, 1989, modified) forselected waters (see Table 2).
Na/1000
K/100 A A A A A A A
"VMg
Fig. 12. Triangular plot for Na, K, and Mg (after Giggenbach and Goguel, 1989, modified).Temperatures are in °C.
395
empirical K-Mg curve result in changes of the "full equilibration" and temperature lines. In any case,the diagram suggests that all these waters are likely to be partially equilibrated or mixed waters, theleast immature being again spring 15.
CLASSICAL WATER GEOTHERMOMETRY
Empirical cation geothermometry has been used as a first attempt to calculate the sub-surfacetemperatures of the springs. A particular emphasis will be given to the sodium chloride waters, withthe aim of evaluating the thermal conditions of the deep reservoir, even though all these waters appearto be affected by some dilution processes, on the basis of the classification outlined above.
It is well known that many different geothermometers (Table 4) rarely give the same computedtemperatures. Table 5 summarizes the results obtained using some among the given geothermometersfor the high salinity groups of waters (Groups 1, 2, 4). Geothermometers should be used carefully,and some comments are necessary for their application.
Silica geothermometer: Quartz (conductive cooling) can be used with a certain confidence for allthe sampled springs except for the Na-Cl waters. For this group in fact the relatively low computedtemperatures (of the order of 160°C in the El Palmar - Aguas Calientes zone) have been interpreted asthe ones at which the gas phase separates from the liquid phase owing to boiling and cannot beassumed to be representative of the deep reservoir temperatures. Fig. 13 is a dissolved silica -enthalpy plot (Truesdell and Fournier, 1977) for determining the temperature of a hot watercomponent mixed with a cold water, yielding a warm spring water. Group 1 only is reported in thediagram; considering among the sodium chloride waters the ones between the boiling point and thepoint where all local diluted waters merge, the extrapolation to the quartz solubility curve gives atemperature of 250°C approximately. Of course, this temperature is only indicative of a high enthalpysystem, namely because of the few data available. For Na-bicarbonate waters (Group 4) temperaturescomputed from the chalcedony equation are also reported in Table 4, considering low enthalpysystems (t< 150°C).
Na - K geothermomcten Several Na-K geothermometers can be applied; all of them lie in abroad band between the albite-microcline and the albite-adularia curves (Fig. 14; from Fournier,1990); empirical curves tend to converge above 300°C. In Table 5 Arnorsson's (1983)geothermometer only was used as an instance for Na-K calculated temperatures. For the Na-Clwaters the Na/K ratio appears to give too high temperatures, as if Na were deficient with respect topure albite. For springs of Groups 2 and 4 a Na-K geothermometer based on montmorillonites hasbeen applied (as in Fig. 14) In fact, it must be taken into account that in sedimentary basins exchangereactions involving clay minerals can substantially change the Na/K ratio (see also Henley et al.,1984). The computed temperatures are much lower than the ones obtained by application of the Na-Kgeothermometer based on feldspars.
Na - K - Ca geothermometer: It takes into account reactions involving exchanges of the threeelements with a mineral solid solution (Fournier and Truesdell, 1973). Fournier (1990) makes severalcomments on the application of this geothermometer for waters with reservoir temperatures below200°C, of sodium bicarbonate type, and/or rich in Mg (see also Giggenbach, 1989). The composition
396
TABLE 4 - Equations expressing the temperature dependence of selected chemicalgeothermometers. For each geothermometer used in Table 5 there is an abbreviation;concentration units are in mg/Kg, except for Fouillac and Michard (1981) (molar units).
Geothermometer Equation Source
Amorphous silica t°C = 7314.52 - log SiO- - 273 (<250°C) Fournier (1981)
Chalcedony (TCH) t°C = 1(324 69 S;Q -273 (<250°C) Fournier (1981)
Quartz (maximum steam loss) t°C = 15225.75 - log SiO2
-273 (<250°C) Fournier (1981)
Quartz (no steam toss) (TQC) t°C = 5 ^ f c " 2?3 (<25(f C) Fournier (1981)
Na-K t°C = 8560.857 +log (Na/K) -273 Truesdell(1976)
Na-K t°C = 8330.780 + log (Na/K) -273 Tonani(1980)
Na-K(TNK) 933____0.993+
273ll5 Amorsson(1983)
Na-K(TNK) AmOISson(1983)
Na-K f0/"*._ _____1L1 /_____ JTl1.483 + log (Na/K) Fournier (1981)
Na-K t°r = ____*178____ . 2731.470 + log (Na/K) Nieva and Nieva (1987)
Na-K toc= 1390 .2?31.750 + log (Na/K) Giggenbachetal. (1983)
K-Mg(TKM) t«0————M1Í————-27313.95-log (K2/Mg)
Giggenbachetal. (1983)
397
TABLE 4. (continued)
Li-Mg CTM L) ft> 22005.470 -log(Li/VMg)
==r-273 Kharaka and Manner (198 9)
Na-Li t°C = 15900.779 + log (Na/li)-273 Kharaka et al. (1982)
Na-Li (TNL) <ci<o 3 M) fC = 1000>»T rr *\ ~ ¿* i J0.389 + log (Na/Li) Fouillac and Michard (1981)
Na-Li (ci>o 3 M) 11950.130 + log (Na/Li).x - 273 Fouillac and Michard (1981)
Na-Ca t°C = 1096.73.080 -log(Na/VCa)
-273 Tonani(1980)
K-Ca t°C = 19303.861 -log(Na/VCa)
-273 Tonani(1980)
Na-K-Ca(TNKC) fC = 1647bg(Na/K)+p[log(VCa/Na)+2.06]+2.47
P = 4/3 for t<100°C, = 1/3 for t>lOO°C
- 273 Foumier andTruesddl (1973)
Na-Ca-Mg(TNKCM) t°OTNKC-DEL (see below) Fournier and Potter (1979)
1 Compute t by TNKCIf t<70°C, no correction to TNKC
2 Compute R = (Mg/(Mg + Ca + K)) 100 (in equivalents)If R>50, no correction to TNKC (cold environment)
3 Compute DEL - correction to TNKCIf 5<R<50 and T is computed by TNKC
DEL - 10 66-4 7415R+325 87(log R)2-l 032 105(log R)2/T-1 968 107(log R)2/T2+l 605 107(log R)Vr2
If 0 5 <R<5 and T is computed by TNKCDEL = -1 03+59 971 log R+145 05(log R)2-3671 l(log R)2/T-1 61 107log R/T2
If DEL <0 5, no correction4 To compute R- Ca - Ca(ppm)/(1000 20 04)
K = K(ppm)/(1000 39 102)Mg = Mg(ppm)/(1000 12 156)
185 O 1 000 In a - 2.88 (lO6!'2)- 4.1
1000 + 5
1000
MacKenzie and Truesdell (1977)
398
TABLE 5. Temperatures (in °C) computed by water geothermometers
sample TQC TCH TNK TNKA TNKC TNKCM TNL TML
TQC: quartz, no steam loss (Fournier, 1981)TCH: chalcedony (Fournier, 1981)TNK: Na-K (Arnorsson, 1983)TNKA: Na-K from clay equilibrium (Fournier, 1990)TNKC: Na-K-Ca (Fournier &Truesdell, 1973)TNKCM: Na-K-Ca-Mg (Fournier & Potter, 1979)TNL: Na-Li (Fouillac & Michard, 1981)TML: Mg-Li (Kharaka & Mariner, 1989)TKM: K-Mg (Giggenbach et al., 1983)
TKM
47121314152224397788106114
1101611591581241628466697486142123
81—————5334374255——
2713493643553193521257459136206357330
140—————5429-256197——
15328128027825829193948513391268249
1252311902281452916334856362168167
281322—333324331100—73106147——
142173—1691552466723868767...—
1211641501591382316655708770138129
of CO2-rich waters tends to be controlled by the attack of carbonic acid on feldspars (and on other
mineral phases); this will cause water composition to plot off the empirical composition trends thatresult from NaCl geothermal fluids in equilibrium with feldspars. The NKC temperatures for watersof Group 1 appear to be significant of the deep reservoir conditions, ranging between 250 and 290°C.
Na-K-Ca-Mg geothcrmometcn The presence of relatively high contents of Mg tends to lower thetemperatures given by the previous geothermometer; Fournier and Potter (1979) introduced the Mgcorrection. About this geothermometer, Fournier (1990) made the following comments:
"Mg concentrations in geothermal fluids decrease rapidly as temperature increases and allMg-rich fluids found in nature have undergone water-rock reaction at a relatively lowtemperature. Furthermore, as geothermal fluid flows from a high-temperature environment
399
to lower temperatures, it appears to pick up significant amounts of Mg from thesurrounding rock relatively easily and quickly. This leads to major ambiguity in theapplication of the Fournier and Potter (1979) Mg correction; that is, a trulylow-temperature water may require a Mg correction to give a correct temperature, but a Mgcorrection applied to a high-temperature water that has picked up Mg during upflow willyield too low a reservoir temperature."
The Mg correction for the springs here considered appears to give reasonable results only forbicarbonate waters. For NaCl springs the decrease of the calculated temperature is in correlation withthe decreasing salinity; it seems to confirm that the whole Group 1 constitutes a family of mixedorigin waters, among which spring 15 is the purest term.
K - Mg and Mg - Li geothermometers: Giggenbach et al. (1983) and Kharaka and Mariner(1989) proposed these two empirical geothermometers respectively, using equations based on watercompositions of well fluids. Application of both geothermometers to the springs of Group 1 yieldslower temperatures than the ones inferred in the deep reservoir because of the possible uptake of Mgduring ascent, along with dilution. According to Fournier (1990)
700-
600-
500-
400-
O>^¿O5
JECMo
300 H
200-
100-
0-100 200
Enthalpy (cal/g)300
Fig. 13. Dissolved silica vs. enthalpy diagram (from Truesdell and Fournier, 1977) for selectedsamples. Solid circles represent Na-Cl waters; all groundwaters fall within the solid square.
400
3.00
2.50-
_ 2-°°~i¿\055 1.50H
1.00-
0.50-
60 100i
200 250 300 Ci . i i
3.50 3.00 2.50 2.00 1.50
1000/T(°K)
Fig. 14. Variation of log (Na/K) as a function of the reciprocal of absolute temperature. Theoreticalcurves for low albite - microcline, low albite - adulada and Na-montmorillonite -K-montmorillonite calculated using equilibrium constants in Arnorsson (1982). Various Na/Kgeothermometers are labelled VG (Giggenbach et al., 1983), RF (Fournier, 1979), DN (Nievaand Nieva, 1987), SA (Arnorsson, 1983), AT (Truesdell, 1976), FT (Tonani, 1980). (AfterFournier, 1990).
"Water-rock exchange reactions involving Mg proceed relatively fast at lowtemperatures and K //Mg and Li//Mg ratios appear to be good indicators of the lasttemperature of water-rock equilibration in an ascending water. A contributing factor in thisapparent fast reequilibration is that both Mg and Li tend to be minor constituents in mostthermal waters, so a relatively small amount of reaction involving one or both of theseconstituents may have a large effect on the ratios of cations remaining in solution. Whenthere is good agreement in the temperatures estimated using K//Mg, Li//Mg, and othergeothermometers (particularly Na-K-Ca and silica), one can be fairly certain thatwater-rock equilibration has occurred at the estimated temperature and that there has beenlittle water-rock reaction during subsequent flow to the surface. Agreement betweenK//Mg and Li//Mg geothermometer temperatures, but higher temperatures estimated usingother geothermometers, may indicate a relatively short time in a reservoir at intermediatedepth and intermediate temperature where water-rock reequilibration occurs only in respectto the most reactive phases."
In this study the Na bicarbonate waters follow the former proposition (equilibrium), while theNa-Cl waters (as always, possibly except spring 15) contain significantly more Mg (and less Li) thaninitially was present in the deep reservoir water (dilution).
401
Na - Li gcothermometen Two geothermometers of this type have been proposed by Fouillac andMichard (1981) and by Kharaka et al. (1982) (Table 4). In Table 5 only the temperature computedaccording to Fouillac and Michard's (1981) equation is given. Since Na is a major constituent and Lia minor one, slight changes in the latter (e.g. exchanges in sedimentary formations) can heavilymodify the Na/Li ratio. It is used here only to differentiate the high temperature sodium chloridesystem (Group 1) with respect to the low enthalpy sodium bicarbonate waters (Group 4). The hightemperature calculated for spring 4 by means of this geothermometer is worth of attention, because ofits geographic location: the temperature might be interpreted as the deep reservoir originaltemperature, before any possible reequilibration in a shallow and low enthalpy reservoir. Moreover,application of the TNL geothermometer often results in overestimated temperatures, possibly becauseit has been calibrated mostly in granitic systems.
K - Ca geothermometer A chemical reaction governing CC>2 pressure through the formation of
calcite in a full equilibrium system corresponds according to Giggenbach (1988) to:Ca-Al-silicate + K-feldspar + CÜ2 = K-mica + calcite (1)
The temperature dependance of this univariant reaction is given by:
log PC02 = o-° 168 t-3.78 (2)
where ?CO2 *s 'm ^>ar an& *m °C-As an example, this equation gives 2.6 bar at 250°C and 18 bar at 300°C. A possible relation
linking CC>2, Kand Ca in a full equilibrium system is (Giggenbach, 1989):
3 K-feldspar + CC>2 + Ca2+ = K-mica + calcite + 2 K+ (3).
Its PcC>2 dependence corresponds approximately to:
log (CK2/CCa) = log PC02 + 3-0 (4)
where Cs are concentrations in mg/1.
For spring 15 this geobarometric equation gives a PcO2 °f 1-2 bar, consistent with a reservoir
temperature close to 230°C if calcite is considered a stable phase. This is the same temperature as thatcomputed by the K-Mg geothermometer.
As a general conclusion on the application of the aforementioned geothermometers, the generalremarks by Fournier (1990) are to be regarded consistent with the results obtained in this study:
"In evaluating cation geothermometer temperatures, attention must be given to possibleeffects of continuing water-rock interactions and mixing of different waters during upflow.In addition, variations in estimated cation geothermometer temperatures may result fromvariations in the mineralogy of the reservoir rocks that are reacting with the fluid. Thecomplete conversion of all feldspar in contact with reservoir water to smectite or illite willgenerally lead to erroneous cation geothermometer temperatures. In particular, this is alikely source of error when dealing with CU2-rich waters that contain mostly NaHCO3-
Additional errors arise from not taking into account activity coefficients of both the solidand ionic reactants. Under the best conditions cation geothermometers have an uncertaintyof at least ± 5 - 10°C, and commonly much greater than 20°C.
402
Even with their inherent uncertainties, cation geothermometers can be very useful forestimating approximate temperatures in geothermal systems and for investigating effects ofwater-rock partial reequilibration during upflow. Reactions involving Na and K appear toproceed very slowly while those involving Mg appear to proceed relatively quickly. In thisregard, the salinity of a solution is an important factor because a given amount of reactionof highly saline fluid with wall rock during upflow will have little effect on the ratios of themajor cations in solution. On the other hand, the same amount of reaction of a dilutesolution with wall rock may drastically change cation ratios. Therefore, cationgeochemistry applied to hot-spring waters is likely to be most reliable for saline waters andleast reliable for dilute water. Increasing Mg , in general, and decreasing K/V~Mg andLi//Mg, in particular, are very sensitive indicators of water-rock reequilibration withdecreasing temperature and of mixing of high- and low-temperature waters."
Some more remarks can be made about the results reported in Table 5. The general trend oftemperatures follows the classification groups made on the basis of salinities and chemicalcompositions. Sodium chloride waters of Group 1 show in general a strong thermal anomalycentered in the area of the order of 1 - 2 km^ between El Palmar and Aguas Calientes. The thermalanomaly is likely to be extended 3 - 4 km westward, as far as spring 14 (Los Chirriaderos). All thesewaters appear to be more or less affected by dilution and mixing with a shallow diluted water, asalready outlined. In spite of this, the composition of spring 15 (El Palmar) allows a first evaluation ofthe deep thermal conditions to be made through the geothermometers presented in Table 5. Asmentioned before, the temperature calculated from quartz for this spring might well represent thetemperature of the flashing point. One can suppose that a liquid phase (equilibrated at hightemperature with quartz) rising from the deep reservoir is cooled by conduction and becomesoversaturated in silica; at a certain shallow depth, depending on local structural and physicalconditions, the liquid phase starts boiling, segregating a gas phase. The sudden increase inconcentration of silica and the violent expansion of the gas phase produce a stirring of the residualliquid, and as a consequence precipitation of silica. The solid phase thus produced can be partiallydeposited locally in the fractures and/or carried to the surface by the upflow; it is then eliminated at thesampling point by filtration of the sample. The explanation for the whole process was proposed by R.Celati (personal communication, 1986).
The average temperature calculated for spring 15 by means of the other classicalgeothermometers is of the order of 290 ± 50°C, with all the already outlined limitations. It is worthnoting that the use of the isotopic SÜ4-H2O geothermometer reported in Table 4 (Mackenzie and
Truesdell, 1977) gives a temperature of 206°C for the same spring. This represents the isotopicequilibrium temperature between water and sulfate ion. Of course, water collected from this springdoes not necessarily have the same isotopic composition as the deep hot water because of possiblesteam losses or dilution and mixing phenomena which here are not accounted for. The temperatureobtained from the isotopic geothermometer can represent the equilibrium between water andanhydrite, below which solid sulfates dissolve yielding sulfate ions. That is, above this temperatureSÜ4 and HSO4 species are not present in the solution; this is supported by the calculated
403
oversaturation of anhydrite found for this water for temperatures above 200 - 230°C. In the deepreservoir instead sulfur is supposed to be controlled by the E^S - pyrite equilibrium.
Temperatures computed for spring 4 (Group 2) by TQC, TNKCM, TNKC, TNKA, TKM,TML are of the order of 132 ± 16°C, indicating a situation of medium enthalpy as can be inferred by aleak from a deep reservoir and a later reequilibration.
Temperatures computed for bicarbonate waters of Group 4 (ruling out the TNK and TNLgeothermometers) are all below 100°C. As an example, spring 22 can be considered representative ofthe group; it is located south of the anomaly, with an emerging temperature of 48°C and an averagecalculated temperature of 69 ± 15°C.
From the geothermal point of view Group 5, including all diluted waters, was not taken intoaccount, having been considered too immature for having any significance. The NKC computedtemperatures are in any case lower than 50°C.
GEOTHERMOMETRY AND SATURATION INDICES
Despite the fact that all the classical geothermometers indicate a strong thermal anomaly localizedin the El Palmar - Aguas Calientes area, the range of values is pretty wide for a precise temperatureevaluation. Application of fluid-mineral equilibria governing the chemical composition of water candecrease the uncertainty with regard to the temperature estimates for the deep aquifers (e.g.Amorssonetal., 1982; Michard and Rockens, 1983; D'Amore et al., 1987; Celati etal., 1991). This
can be achieved by the analytical determination of monomeric aluminum (AP+) and (AKxJOw)2' w¡th
x = OH", F-, SO^-, etc. and z = 3 - wy. Monomeric aluminum must be separated in the field.
The method by Arnorsson et al. (1982) was used through the computer program WATCH inorder to calculate the water speciation and the saturation indices for several hydrothermal minerals.
The Saturation Index (SI) is the logarithm of the ratio (at a given temperature) between theactivity product for hydrolysis reaction of a given mineral (AP) and its thermodynamic equilibriumconstant (K). Assigning a decompression temperature (that is when a two-phase system is produced
in the ascending fluid) equal to the quartz geothermometer temperature and a SiC>2 concentration
corresponding to quartz saturation, it is possible to calculate at selected temperatures the pH, thespeciation of the fluid (as activities of aqueous species), and then the saturation indices. Thesesaturation indices are plotted versus temperature for a number of selected minerals. For each mineralthe intersection of the SI vs. temperature curve with the zero line gives its equilibrium temperature.Considering the uncertainty of measured Al concentration and of the thermodynamic data, the field ofstability for a given mineral is assumed to be between temperatures corresponding to a saturationindex of 0 ± 0.25. Table 6 reports the temperature range for selected minerals andsprings (4, 15, and22). For sodium chloride waters spring 15 was chosen as the most representative. The values of thesaturation indices vs. temperature for spring 15 is reported in Fig. 15. Taking into consideration themaximum temperatures of the ± 0.25 range, the following high temperature mineral assemblage canbe found for spring 15: albite (293°C), K-feldspar (288°C), Mg-chlorite (305°C), calcite (275°C),epidote (268°C). The computed mean temperature is 286 ± 15°C; this is very close to the value of291°C given by the TNKCM geothermometer. On the other hand, considering the minimum
404
temperatures of the ± 0.25 range, a different mineral assemblage can be also found in the diagram:albite (226°C), K-feldspar (210°C), wairakite (225°C), prehnite (235°C), calcite (225°C), anhydrite(225°C). This gives a mean temperature of 224 ± 8CC. This temperature is comparable with that of theTKM geothermometer (231°C).
This bimodal distribution of temperatures can be interpreted as two limiting thermal conditionsexisting in different and independent parts of the reservoir: a deep zone characterized by temperaturesclose to 290°C and a relatively shallow zone with a temperature of about 225°C. If spring 15 ispartially mixed, the bimodal distribution is the result of the mixing. Of course, the choice of mineralspecies is somewhat arbitrary, at least in the early survey phase, until proved by drill cores. Thismethod cannot be applied with confidence to the other sodium chloride waters because their higherdilution and mixing produce large variations of the computed saturation indices of minerals.
Waters of spring 4 (Group 2) and of sodium bicarbonate springs (Group 4) appear to be inequilibrium with the following minerals: albite, K-feldspar, laumontite, montmorillonites (Table 6and Fig. 16). Undersaturation for alumosilicates such as epidotes, prehnite, Mg-chlorite, can beascribed to the incompatibility of the reservoir temperature with the stability ranges of these high
-6200 250 300
T(°C)350
Fig. 15. Saturation indices vs. temperature (see text) for selected minerals, for spring 15 (at quartzsaturation).
405
temperature mineral phases. This type of waters, always accompanied by free gas, may be the resultof a release of CC>2 at depth from hydrolysis of limestones and interaction of the produced carbonic
acid with silicate minerals at relatively low temperatures. All waters of Group 4 yield temperatureslower than 100°C with the "classical" geothermometers; saturation indices give consistent indicationswithin a temperature range of 60 to 90°C.
For spring 4 (see Fig. 17) the computed temperature is 104 ± 8°C, in spite of its high chloridecontent; it confirms a contribution of a deep component, with reequilibration at low temperature. Thepresence of the deep component is witnessed by the relative abundances of conservative species likeLi, Rb, Cs, Cl. Waters of Groups 2 and 4 are noteworthy for the agreement between the temperaturescomputed by the method of saturation indices and by the "classical" geothermometers.
TABLE 6. Computed temperature range (°C) for equilibrium conditions of selected mineral species.Equilibrium occurs when SI = 0.00±0.25. Spring numbers as column headings.
mineral system 15 22
AlbiteAdularíaMicroclineK-FeldsparO)PrehniteEpidoteZoisiteWairakiteMg-ChloriteCalciteAnhydriteNa-Montm.K-Montm.Ca-Montm.Mg-Montm.
MuscoviteLaumontite
Na-Al-Si-OK-Al-Si-0K-Al-Si-OK-Al-Si-OCa-Al-Si-O-HCa-Fe-Al-Si-O-HCa-Al-Si-O-HCa-Si-O-HMg-Al-Si-O-HCa-C-OCa-Si-ONa-Al-Si-0-HK-Al-Si-0-HCa-Al-Si-O-HMg-Al-Si-O-HK-Al-Si-0-HCa-Al-Si-O-H
260±33 (x)289±29oversat. t>232249139 (x)241 ±6 (x)264±4 (x)232±5 (x)233±7 (x)302±2 (x)250125 (x)oversat. t>200
oversat.<2>oversat. t>205oversat/2)
oversat/2)
oversat/2)oversat/2)
70±5undersat. t>60undersat.
85±5undersat.undersat.undersat.undersat.undersat.oversat. t>48
undersat.
88±182±193+194±1
oversat. t<H579±4
90±6 (x)undersat. t>90undersat.112112 (x)undersat.undersat.undersatundersat.undersat.oversat i>59oversat. t>82
10311 (x)9911 (x)
110+1 (X)
110+1 (X)
oversat. t<!4010215 (x)
(x) This symbol indicates the use of the mineral for equilibrium temperature evaluation (see text)(') Assumed with a structural state intermediate between microckne and adularía(2) Oversaturated in the temperature range 200-325°C
406
- 6 -
80 100T(°C)
Fig. 16. Saturation indices vs. temperature (see text) for selected minerals, for spring 22 (at quartzsaturation).
- 6-
~4Ó ' §T 80 100 "iío ' iSó ' Í8Ó~TÍ°C)
Fig. 17. Saturation indices vs. temperature (see text) for selected minerals, for spring 4 (at quartzsaturation).
407
GAS GEOCHEMISTRY
Gas classificationIn Table 7 the gas compositions as vol. % are given; analyses were carried out on total dry
gases. Data are reported after correction for C>2, considered as an index of air contamination. In order
to use the gas composition as a tool to evaluate deep temperatures, a classification of gases was madeby means of three suitable diagrams.
Fig. 18 is a diagram comparing the main component CÜ2 against two reduced and reactive
parameters: CH4 and the sum H2 + H^S. Methane is an ubiquitous component of all kinds of gases ;
it is one of the slowest species to equilibrate (Giggenbach, 1982) with respect to "faster"components. The samples in the diagram follow a certain geographic distribution from E to W and tomarginal springs: the compositions of the eastern samples, occurring from El Palmar as far as ElSalvaje, show important concentrations of Ü2 and/or H^S. Moving farther to W (Mundo Nuevo -Los Mereyes) the gases show lesser and lesser of these components, up to nil (or close) in the gasesassociated with the marginal bicarbonate waters. An exception is the gas of spring 88, with highconcentrations of H2S (and to a lesser extent of H2) in spite of its marginal position, its low salinity
and its low (< 100°C) temperature computed by water geothermometry.Another classification is given by the reduced and hydrogenated minor components H2, H2S,
CHÍ4, in the plot of Fig. 19. A geographic trend similar to what obtained from Fig. 18 can be seen in
this diagram, the marginal gases being richer in methane. The shift to the right, that is toward theÜ2S - CFÍ4 edge, of gases associated with NaCl springs like 7, 14, and 15 can be explained as a
hydrogen loss. The most likely reason for this can be a steam loss from the original fluid, of whichthe high elevation gas emissions like 17 and 18 represent the gas fraction; the NaCl springs insteadrepresent the liquid fraction. It should be pointed out that the former are within 1.5 km of the springs,at an elevation about 200 m greater. The gas composition of 17 and namely of 18 samples is thushere considered as the most representative one of the gas at equilibrium in the deep reservoir, thecorresponding water being represented by spring 15. A second hypothesis for the H2 loss can be
reequilibration in water during ascent. The gases from the Mundo Nuevo - Los Mereyes area lie in anintermediate position between 17 and 18 gases and the methane corner. The explanation for not beingthese gases in the same position in the diagram as samples 17 and 18 can be attributed to a certaincontamination of the deep gases in the latest stages of their ascent, that is after separating from theliquid phase at the piezometric level (as already mentioned in the water classification): in the area infact the villages tap local water tables at very shallow depths. The least affected by this oxidation isthe gas from spring 45, the strongest gas manifestation in the area. The position of sample 52(air-contaminated and associated with a diluted neutral water, the richest in tritium) is odd: it is clearlythe result of oxidation in the surface water. Because of the large amount of C>2 found in the gas, its
position in the diagram suggests that if no oxidation did occur (complete loss of Ü2 and partial of
Ü2S) its point would shift toward the position of gas 18. The same can be valid for sample 88.
408
TABLE 7. Composition of free gases in springs, El Pilar area. All analyses expressed in vol %; n.d. = not determined; blank = not detected.
sample
714151718
5
454546474950
222424397788
5283
C02
98.597.389.093.292.4
94.6
95.996.496.095.697.295.4
95.871.379.498.996.267.2
72.091.2
H2S
0.512.012.900.971.90
0.330.270.0810.090.170.21
0.45
1.15
CH4
0.0740.0925.403.491.54
1.09
2.251.842.262.421.502.39
2.4625.718.45.5-10-4
1.383.50
3.112.77
H2
2.9-10-4
0.00840.0410.430.52
3. HO-4
0.0630.0760.0330.0580.0620.028
2.6-10-4
2.7-10'4
n.d.4.0-10-4
2.0-10-4
6.5-10'4
1.2-10-4
N2
0.850.577.401.893.65
4.27
1.711.381.611.841.011.92
1.733.02.731.042.37
28.3
23.45.93
AT
0.0180.0120.114
2.5-10-34.3-10-3
2.7-10-2
5.7-10-33.6-10-3
5.1-10-38.2-10-33.7-10-37.4-10-3
4.6-10-25.5-10-2
3.9-10-22.5-10-21.6-10-20.49
0.346.6-10-2
He
tracestracestraces
2.4-10-3
2.5-10-3
4.2-10-3
1.5-10-31.1-10-31.7-10-32.2-10-37.8-10-4
1.6-10-3
traces2.9. 10-4
traces2.9-10-4
1.1-10-31.6.10-2
5.9-10-32.6.10-3
CO-104
4.45.06.33.13.3
3.3
4.8n.d.0.90.241.40.96
9.20.2n.d.0.21.01.8
n.d.0.37
COz/100
A————A————A————A————A————A————A————A————A————'(Hj.Hj.SJxK)
Fig. 18. Triangular diagram for CC>2, Cffy and (H2 + H^S). Crosses refer to the easternmost
springs in Fig. 2, from El Palmar to Los Chiorriaderos and from Buena Esperanza to ElSalvaje; open circles represent the Mundo Nuevo - Los Mereyes area; triangles representperipheral springs, as in Fig. 1.
/\ A A A A A A
Fig. 19. H2S-H2-CFI4 triangular diagram. Symbols as in Fig. 18, except solid circles identifying
true fumaroles (Buena Esperanza - El Salvaje).
410
The diagram built on the N2-Ar-He inert components (Fig. 20) (Giggenbach et al., 1983;
Giggenbach and Goguel, 1989) yields more information about the origin of gases as well as on theeffect of air contamination. The He corner, named "crustal", represents radiogenic helium, as can beobtained from a long residence in old rocks; the N2 comer is close to the "magmatic" gases, nitrogen
being an ubiquitous component, also in its non-atmospheric fraction. Along the N2 - Ar edge the two
end points of air and water-dissolved air are lying; the air-saturated water generally represents the endmember on this side. Most samples cluster into two distinct compositions: one groups all fumaroles(with associated condénsales as the acidic waters of Group 3) in a position intermediate between the"crustal" and "magmatic" end members; the second group includes all gases associated with neutral"real" thermal springs. Once more, samples 17 and 18 appear to be the most representative of thedeep gas, lying right on the "crustal"-"magmatic" line; the gases from the Mundo Nuevo - LosMereyes area appear to be more or less contaminated by an atmospheric component supplied byair-saturated water. In particular, gases from springs 52 and 83 (diluted neutral waters) are moreheavily contaminated by dissolved air. For what the central study area is concerned, these resultsconfirm that any geothermometry based on gas equilibria becomes significant only when the aircontamination is small: reactive species (namely H2) in fact can reequilibrate at low temperature.
N,/100
HexlO
Fig. 20. Inert gas triangular plot (N2, He, Ar) (after Giggenbach et al., 1983).Solid circles represent
true fumaroles (Buena Esperanza - El Salvaje); open circles represent gases associated withNa-Cl waters; X's refer to the Mundo Nuevo - Los Mereyes area; squares and triangle refer toperipheral waters.
411
Among gases associated with bicarbonate waters (all falling near the air-saturated water endmember) exceptions are shown by gases 5 (Ño Carlos) and the two ones from the Cariaco gulf (77and 88). All of these are right on the El Pilar fault trace or on secondary ones, and in spite of theirapparently odd location there can well be a contribution from deep gases. This anomaly was pointedout also in the section dealing with waters, namely for conservative species, e.g. Li.
Gas Geothermometrv.On the basis of the above information the geothermometric methods based on gas equilibria
will be applied only to some gases considered as the most representative.The gases from manifestations 18 (El Salvaje, to the east) and 45 (Mundo Nuevo, to the west)
were tested by means of the method by Saracco and D'Amore (1989). This method takes intoconsideration the three contemporary equilibria:
CH4 + 2H2O = CO2 + 4H2 (5)
H2 + 3/2FeS2 + 2H2O = 3H2S + l/2Fe3O4 (6)
CH4 + 3CO2 = 4CO + 2H2O (7)
A numerical approach is used to solve a set of three non-linear equations derived from theabove equilibria:
4 log (H2/CO2) - log (CH4/CO2) = 4.635 log T - 12144.08/T + 6.69
+ 4 lo A - 3 log A - l°g A - 4
3 log (H2S/CO2) - log (H2/CO2) = - 0.412 log T - 10318. 15/T + 17.25
+ 3 log AH2S - 2 log ACOZ - lo« AH2 ' 2 '°g PCO2 (9)
4 log (C0/C02) - log (CH4/CO2) = 0.719 log T - 12913.84/T + 4.73
+ 4 log ACO - 3 i°g Aco2 - i°g ACEU (1°)
where T is in °K, PcO2 IS ^e partial pressure of CO2 at equilibrium, and Aj is a term
containing the reservoir vapor mass fraction y and the distribution coefficient B¡ of the gas i between
steam and liquid (Bj is the ratio between the concentrations in the steam and in the liquid):
Ai = y + ( l -y) /B i (11)
if y > 0 (steam present in a two-phase fluid), or:
A¡= l/Bjd + y-yBj ) (12)if y < 0 (liquid, vapor-depleted system).
The unknowns are three: T, PcO2> an^ v-
412
For gas 18 the following results are obtained:T = 250°C; y = 0.001; PCO2=2.8atm;
the computed redox conditions are the following:
log P02 = -37.7; log PS2=-13.4,
consistent with the values computed in many geothermal fields (D'Amore and Gianelli, 1984).For gas 45 the results are:
T = 230°C; y = 0; Pcc>2=1.0atm.As pointed out by Giggenbach (1987) this method, used to evaluate the reservoir parameters,
involves several species at the same time, with the limiting assumption that all of them are still foundin the sample in the representative proportions as originally present in the reservoir. Hence, thecomputed temperatures above shown are to be considered minimum values for the reservoir ifreequilibration (and oxidation at shallow depth) are actually occurring during the ascent to thesurface, with different reaction rates for each species and with different solubilities.
Calculation of deep temperatures for "good quality" samples can be achieved through use ofcarbon monoxide in equilibrium with the other main carbon components CC>2 and CIfy and with
water, according to equation (7) (Bertram! et al., 1985; D'Amore et al., 1987; Giggenbach, 1987;Chiodini and Cioni, 1989). Fig. 21 was taken from Giggenbach (1987), plotting the CH4/CC>2
versus CO/CÜ2 molar ratios. In the figure two major redox buffers are considered: one supposing
Ü2 - SC>2 coexistence for a magmatic gas phase, and another (more interesting for geothermal
systems) involving a general reaction likely to control the redox state:4(FeOL5) = 4(FeO) + 02 (13)
where the bracketed entities represent Fe2+ and Fe^+ incorporated in an unspecified ironoxide or an iron-aluminum mineral assemblage. For this buffer two curves are shown, according todifferent equilibrations in the liquid phase or in the vapor phase. In the upper right part isothermsrefer to the equilibration temperatures in the vapor phase, while in the lower part they refer toreequilibration in the liquid phase. In Fig. 21 the four most representative gases for the study area arereported: 17, 18,45, and 46. At the first glance the points fall on the curve of the iron buffer referringto the liquid phase; the same result was found according to the previous method. The calculatedtemperatures range from 250 - 260°C for the El Salvaje - Buena Esperanza area (eastern zone) to 220- 270°C for the Los Mereyes - Mundo Nuevo zone. The other gases (not reported) fall in a scatteredway, outside the liquid-vapor area, for temperatures between 200 and 300°C. This does confirm thelocal high temperature anomaly, but it confirms also what stated above about the non-equilibriumstate of these gases, due to vapor loss and/or oxidation phenomena during the late stages of ascent.
The carbon monoxide content can also be used to estimate the CC>2 partial pressure using(Chiodini and Cioni, 1989) the following chemical equilibrium:
C02 + H2 = CO + H20. (14)
Arranging the expression of the equilibrium constant, the following relation is obtained(D'Amore, 1990):
413
5-
0-
CvjOo
C)O)
- -5-
-10-
1-8 -7 -6 -5 -4
log (CO/CO2)
Fig. 21. CH4/CO vs. CH4/CÜ2 molar ratios for the application of the CO geothermometer (after
Giggenbach, 1987) applied to selected samples (see text).
= °-491 + 192.4/T + 0.979 log T + log (CO/H2) -log ACÓ + log AH2 (15)
where T is in °K and A as defined in equations (11) and (12).This equation is almost independent of the temperature and the steam fraction . Between 130
and 320°C, and neglecting the effect of "y":
log PC02 = 3.52(±0.1) + log (CO/H2) (16)
The effect of considering a pure liquid phase will increase the computed log ?CO2 by ab°ut
0.2 units, considering the difference in solubilities between CO and H2- Application of equation (16)
to the four samples appearing in Fig. 21 gives a range of CC>2 partial pressures close to 2 bar in the El
Salvaje - Buena Esperanza and of the order of 10 bar for the Mundo Nuevo - Los Mereyes areasrespectively.
414
The empirical gas geothermometer proposed by D'Amore and Panichi (1980) givestemperatures of 264 and 289°C for gases of springs 17 and 18 respectively, considering a
PCQ2-dependent correction term of 10 bar because of the composition constraints:
t(°C) = 24775/(2 log(CH4/C02) - 6 log (H2/CO2)- 3 log (H2S/CO2)
-71ogPc02 + 36.05)-273 (17)
Considering a liquid-dominated geothermal system, an attempt was made to apply the H2/Ar
geothermometer proposed by Giggenbach and Goguel (1989). According to these authors in the caseof boiling springs the H2/Ar ratio (when no oxidation from O2 nor Ar pollution from atmospheric
sources occur) appears to provide valid information on the thermal conditions within the deepequilibrated liquid phase. This method has been proposed and up to date tested only for high enthalpysystems; in the case of low enthalpies it needs further refinement. The equation used is:
t(°C) = 70 (2.5 + log (H2/Ar)) (18)
The calculated temperatures are 321 and 331°C for gases 18 and 17 respectively, and 258 and232°C for gases 45 and 46 respectively. These temperatures are in general agreement with the onesalready found by gas and water geothermometry, thus confirming a liquid-dominated system at highenthalpy.
D'Amore et al. (1989) proposed a geothermometric method devised for low enthalpy systemswhich can be applied to thermal springs where the gas is associated to water. The method is based onthe calculation of the CO2 pressure either from water chemistry and from gas composition. There is a
temperature at which the two PcO2>s are equal. This can be considered the temperature at which thegases are in equilibrium with the water sample at some point in the reservoir. This method of
comparison of the two PcO2>s can be use(^ as a geothermometer if the water sample is truly
representative of the reservoir water, where it is possible to assume steam fraction to be totally absent
in the reservoir. Using the chemical equation (5) and expression (8) with y = 0, the computed ?CO2
changes very rapidly with temperature. Table 8 lists the temperature-dependent term (KBT) of thefollowing equation:
l°gPcO2 = KBT - log(H2/CO2) + 1/4 log(CH4/CO2) (19)Using the water composition and starting from the field measured temperature, pH and
alkalinity it is possible to calculate the new values for pH and ?CO2 f°r eacn temperature, e.g. by
means of the program WATCH by Arnorsson et al. (1982). Change of P<X)2 w'tn increasing
temperature is moderate and smooth.The limitations of this method are mainly due to attainment of full equilibrium of the gases
involved in the chemical reactions considered (different reactions can be used), and to the waterwhich must be representative of the liquid phase in the reservoir. As an example, this method wasapplied to the Ño Carlos spring (water sample 4 and gas sample 5, a few meters away) and for spring
15 with the gas of samples 17 and 18. To calculate PcO2 in me water of spring 15 a decompressiontemperature of 160°C was chosen (as resulting from the silica geothermometer), taking into accountthe loss of carbonic acid as CO2 during boiling. This method gives a convergency of computed
415
TABLE 8. Temperature-dependent term KBT in eq. 19(t in °C)
KBT t KBT
40506070758090100110120125
-6.352-5.945-5.582-5.233-5.071-4.905-4.589-4.307-4.021-3.758-3.648
130140150'175200225250275300325
-3.516-3.290-3.100-2.607-2.169-1.796- 1 .443-1.110-0.808-0.470
2's of 2.5 bars at about 90°C for Ño Carlos and between 13 and 15 bars at a temperature of 306
± 8°C for the high temperature example (15, 17, 18), as shown in Fig. 22. The obtained temperatures
are consistent with the ones calculated by the previous methods. Besides, the higher PcO2 obtained
at 300°C with this method is consistent with 2.3 atm calculated with the method by Saracco andD'Amore(1989)at250°C.
CONCLUSIONS
The results of the geochemical survey indicate that the studied geothermal area can bepromising for the recovery of high enthalpy geothermal resources in the El Pilar - Mundo Nuevoarea.
About the springs outside the area of concern, chemical evidences indicate that some leaksfrom the deep reservoirs must exist, namely for spring 4 (Ño Carlos), with computed reservoirtemperatures generally in excess of 100°C, but lower than 150°C. This temperature is still of interestfor the exploitation of a low enthalpy system. Its position right on a fault plane of the El Pilar faultsystem is in favor of a deep contribution.
The springs with NaCl waters, located in the El Palmar - Aguas Calientes - Los Chirriaderos' area and the fumaroles located both in the Buena Esperanza - El Salvaje and in the Mundo Nuevo -
Los Mereyes areas are the result of a high local convective heat flow. A local heat source can bepostulated in a young magmatic body at an indefinite depth of some kilometers. Its residualtemperature at the top can still be high enough to justify the heat transmission to the fluids. The
416
5-
ra¿íoo 3-
O
equilibrium
40 60 80 100temperature (°C)
o
equilibrium
100 200 300temperature(°C)
Fig. 22. Computed ?CO2 vs- temperature for water and gas compositions, for selected samples, in
order to obtain the equilibration temperature between gas and water (see text).
inference of this magmatic body is justified by the occurrence of outcrops of rhyodacitic subintrusiverocks (5 Ma; Sifontes and Santamaría, 1972), located in the metamorphic unit about 6 km north of themain zone of thermal manifestations. These rhyodacitic subintrusives are likely to be apophyses ofthe deep batholith, possibly granitic or granodioritic; other younger intrusions not outcropping arealso possible to exist in the region just below the thermal area.
The local permeability is a consequence of the long-lasting and intensive deformations of thezone; the recent and present episodes are related to the active El Pilar right lateral strike-slip faultsystem (and to other systems complementary to the main structural feature) connected with thedisplacement of the southern end of the Lesser Antilles arc with respect to the continent. Possiblerecharge areas are located on both sides of the El Pilar fault: north of it the metamorphic formationsare highly fractured, and south of it limestones are fractured too by secondary tectonics. Both sidesform elevations in excess of 200 - 400 m of the fluid manifestations.
The zones with higher temperature manifestations are affected by carbonate, sulfate, and/orsilica mineralizations. Self-sealing phenomena can exist at depth, thus allowing the formation of animpermeable cover confining the fluid into a possible reservoir. Shallow oxidation is witnessed bythe remains of some old sulfur mining works in the study area; the fresh water wells in the villagesindicate the presence of shallow water tables that can mix with the thermal fluids of deep origin.
The processing of geochemical data led to some hypotheses on the chemical and physicalcharacteristics of the reservoir fluids, as well as on their modifications during ascent. All thefollowing considerations are summarized in Fig. 23.
417
GAS+STEAM |_J«LIQUID
BOILING + GAS SEPARATION (t =160 °C
LLIOce<oLUce
--300
--150
--0
---150
1000
Fig. 23. Sketch summarizing the possible model inferred for the geothermal field centered in LasMinas (Fig. 2), in a section approximately N - S, from Aguas Calientes to Buena Esperanzaand farther uphill. Some selected springs and fumaroles are labelled along the topographicsurface. Symbols represent: 1, the gas phase; 2, the liquid + gas and steam (2 phases); 3, theliquid phase; 4, the dominant phase(s). Stippled areas represent possible aquifers. The truereservoir is the deeper one; the shallow resrvoir is not certain to exist: it can be merely themixing and partial reequilibration level (see conclusions in the text).
A deep hot reservoir is inferred at a temperature of about 300°C; it is constituted by a sodiumchloride neutral water, without any free steam (y = 0), with a CC>2 partial pressure of about 14 bar.
Considering all classifications, a mixing results to be active for all springs, the "purest" being water15; various geothermometric methods give the highest calculated temperatures for this spring. Beingthis the least diluted (TDS = 3650 ppm), the total salinity in the deep reservoir cannot be muchhigher, of the order of less than 5000 ppm, taking into account a dilution of 10 % approximately(from Fig. 9) and a silica loss.
The lower temperatures obtained by the SO4-FÍ2O geothermometer (206°C) and the bimodal
distribution of temperatures obtained by water geothermometry indicate that a second reservoir islikely to exist, separated from the hotter one and with temperatures in the range of 200-230°C. Themixing with water of this second reservoir yields a dilution of the NaCl water and increases therelative concentrations of Mg, 804 and HCÜ3.
418
Considering the highest silica content (spring 15) a boiling temperature in the range160-170°C is found; the result is the production of a two-phase system, in which the steam-gas phasemigrates toward springs 17 and 18, while the liquid phase outcrops as spring 15. The piezometriclevel at an elevation of 150 m approximately controls this phase separation of the manifestations: allfumaroles are found at higher elevations than the low-gas Na-Cl springs. For all fumaroles the mostreliable temperatures computed by gas geothermometry are found around 250°C. This difference fromthe maximum temperatures of the deep reservoir obtained by water geothermometry can be due eitherto «equilibration in the upper reservoir or to interaction with shallow fresh waters. Using the boilingpoint curve for liquid H2O at low salinity and assuming a certain connection between the deep
reservoir and the surface, the high temperature reservoir can be estimated to lie at a depth greater orequal to 1100 m. The shallow reservoir, with a temperature of the order of 220°C, may also bepresent at a depth of a minimum of 300 m.
The relative position in the inert gas diagram (N2-He-Ar) of the most important fumaroles
(namely 17 and 18) indicates for certain a deep origin of these gases (no meteoric contribution). Thehigh He content can be explained only by a long residence of the fluid in the reservoir rocks (this wasfound e.g. at Larderello). This supports a possible accumulation of the fluid in the deep reservoir,which is inferred to be generally sealed by the rock alteration.
The geothermal system is supplied by groundwater derived from meteoric water which canoriginate from the two high elevations on both sides of the El Pilar fault: Cerro La Pica to the northand Cerro El Pato to the south, both higher than 600 m (recharge areas).
Heat and gas (mainly CÜ2) are assumed to be supplied by a buried batholith which heats a
convective cell of neutral pH, chloride type, water, and with a two-phase condition in the upper zone.This steam separation process gives rise to the fumaroles; steam can be absorbed by localgroundwater, with oxidation of H2S and production of steam-heated sulfate and bicarbonate waters
rich in boron and ammonia. Hybrid waters can be the result of mixings between the deep waters andshallow ones.
The outflow of chloride waters depends on local structural and topographic conditions:because of lateral flow the spring can be found kilometers away from the hot upflow point of thegeothermal system. The high relative permeability for the gas phase generally yields a direct verticalupflow from the reservoir, thus defining clear geographic distributions. In this area the main steamupflows are localized at Las Minas and between Mundo Nuevo and Los Mereyes (Fig. 2), whereeventually exploratory wells can be localized.
ACKNOWLEDGEMENTS
This work has been performed within the framework of the IAEA coordinated researchprogram on the "Application of Isotope and Geochemical Techniques in Geothermal Exploration inLatin America (Research Contract No. 3994/R2/IG), which is executed with the financial support ofthe Government of Italy. The Italian National Research Council (CNR) also gave a financialcontribution to this program.
419
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Várela, P. and Hevia, A. Report on the detailed geological exploration of the Mundo Nuevo - LasMinas - El Pilar geothermal area, (unpublished report).
Vierbuchen, R. (1984) The geology of the El Pilar fault zone and adjacent areas in northeasternVenezuela. Geol. Soc. Am. Mem. 162, 189-212.
422
ORIGINS OF ACID FLUIDS IN GEOTHERMAL RESERVOIRS
A.H. TRUESDELLUS Geological Survey,Menlo Park, California,United States of America
Resumen-Abstract
ORÍGENES DE LOS FLUIDOS ÁCIDOS DE DEPÓSITOS GEOTÉRMICOS.
Los fluidos ácidos en depósitos geotérmicos son raros. Su presencia ensistemas geotérmicos relacionados con volcanismo reciente (Tatún, Sumikawa,Miravalles) indica probablemente que el fluido en el depósito geotérmico pro-viene de fluido volcánico incompletamente neutralizado por la reacción confeldespatos y micas. El vapor recalentado que contiene HC1 (Larderello, LosGeiseres) forma ácido cuando se condensa o se mezcla con líquido a temperatu-ras moderadas (<300°C). El origen del vapor con HC1 es la reacción del NaClsólido con las fases minerales de la roca a temperaturas elevadas (>325°C).En Los Humeros se produce criptoacidez cuando se forma y neutraliza la acidezdel HC1 sin alcanzar la superficie.
ORIGINS OF ACID FLUIDS IN GEOTHERMAL RESERVOIRS.
Acid fluids in geothermal reservoirs are rare. Their occurrence in geothermal systemsassociated with recent volcanism (Tatun, Sumikawa, Miravalles) probably indicates that the geothermal reservoirfluid was derived from volcanic fluid incompletely neutralized by reaction with feldspars and micas. Superheatedsteam containing HCl (Larderello, The Geysers) forms acid where it condenses or mixes with liquid at moderatetemperatures (<300°C). The origin of steam with HCl is reaction of NaCl solid with rock minerals at hightemperatures (>325°C). Cryptoacidity occurs at Los Humeros where HCl acidity is formed and neutralizedwithout reaching the surface.
Introduction
The occurrence of acid fluid in well discharges from geothermal wells is of great theoretical andpractical interest. Chemical substances in geothermal fluids have been proposed to originatefrom leaching of rocks by hot water (e.g. Ellis and Mahon, 1964) or from neutralization ofvolcanic fluids (e.g. Giggenbach, 1981). Because volcanic gases and condénsales where theydischarge at the surface are acid, the occurrence of acid reservoir waters in some geothermalfields associated with volcanic activity supports the hypothesis that neutral geothermal fluidsalso originate from volcanic fluid. Deep, hot, acid water has not been successfully exploitedand some wells (and fields) have been abandoned as a result. Acid carried in superheated steamthat condenses only at the surface can be neutralized but at additional cost. Distinguishing thecause of acid fluid production can assist in management decisions involving treatment orisolation of acid reservoir fluids and possible abandonment of wells or fields.
423
In some fields, acid waters that come to the surface through springs or drill holes originatefrom acid reservoir fluids. In others, boiling of high-temperature neutral or acidic brines orreactions in vapor-halite-silicate assemblages generates HC1 gas that is carried in superheatedsteam and becomes corrosive when the steam condenses, usually at the wellhead. Ascendingsuperheated steam containing HC1 may also mix with overlying neutral waters to produce acidsolutions that rapidly corrode and scale well casings, possibly without acid fluid appearing atthe surface. Analyses (in milliequivalents per liter) of related acid and neutral geothermal watersand steam condénsales are given in table 1.
TABLE 1. SELECTED ANALYSES OF ACID GEOTHERMAL RESERVOIR WATERS AND RELATEDWATERS (in milliequivalents per litre unless otherwise specified)
No Well orspring
Temp<C
pH Na Ca Mg Fe a SO,) HCOjmmol
Rmmol
Cation Aruon Clsum sum excess
Tatun
1 Hsinpeitou2 E1013 E102
Sumikawa4 Tamagawa5 S-26 S-2A7 S-48 Yakeyama
Yellowstone
9101112
Green DragEchinus GEarUnnamed
Miravalles
13 PGM 214 PGM2A15 PGM Av
70175250
9824024528088
235235238
1 34
1 27 2267 604
85 2993 394 5 9 2188 22
2 28 1
424297396
49631 119910867
126696152013
962108977
103123854
1 66242531 33972
12 6213 1 864 35 19
1051 52280005101
661 585401842 8
4 826548 8
376020617000734 8
138 024 0041 00221 28 0095 0041 00790532 003 00012 -00128 001 00065 0011
673849683
34 001119 1 26325 0012
000471 3900036
104109158
91 416915389564
12932211 60076
112113109
66504172
277128373227175
341
286
2 8377407
1198715 6107
342 - 6536 02 -- 4 270521 308 27227 07
1 96 0 57415 61 17 0 689
1011 510 6
4 6
13 15 17123361
93 6114136
45367503123293
0741 1530204 8580361 16500093 0171
5 83 107657 1225 37 109
170 104114 21 5175 70 8
!2533 1526140739
16595165228
115129111
46 3
274
16 G 217 G-418 G-6
Krafla19 KG 1220 KG 12v
Los Humeros
21 H-422 H-1623 H-l
250>250
340
330330
300'316270
593 12 6
667348
0730957 00818
29 7397 7 2 1 585 128
0065 -17 1 66135 119
0826 012300074
066507161 25
144887
017 -1851600 0 667
010500008 000540592 02080021 00032 0932 3 16 -
48 -036 00033001 -
143742272
024 -398 2362 56 2 84
00683 4821 48 48 10 777 78 6
1 1
0 1910911 3
214154222
0 065 0 169 0 1196 185 1511410 1600 1100
1 060964
137227145
0808 -3 17 22
145138812
Sources of analyses 1-3.CH Chen (unpublished), 4. Ozawa et a] (1973) 5-7, Sakai et al (1986), 8, White et al (1963): 9-12, Roweet al(1973) and Thompson and Yadev (1979). 13-15. Majruen et al (1990). 16-20. Truesdell et al (1989) and Orkustofhun unpublished data,21-23, CFE (1990)
Other significant consDtuents (in meq/1) for analyses (#) Al #1. 17 3. #Z 24, #3. 15 6, «4, 17 6, and #8. 37 8 NRi #17.41 6, #18,486. and#21.05 F#4. 63. and #11. 14 Mn #1 0 6. #17. 6 8, and #18. 26 6 Li #9, 0.5. #11. 0 75. #13. 097. #14. 1 1, and #15. 1 0
424
Acid C1-SO4 Waters with Excess Chloride
Some reservoir water analyses show "excess" chloride defined as chloride equivalentconcentrations significantly greater than the sum of equivalent concentrations of cations otherthan H+. (High Fe++ and Mn++ in well discharges originate from reaction of H+ with casings,and these ions are also not included in the sum of cation equivalents.) These waters must havehad at least part of their acidity introduced as HC1, most probably from a volcanic source.Chloride excess waters from geothermal reservoirs are apparently rare, although manyexamples of similar waters are found in crater lakes or associated with active volcanoes (Whiteet al., 1963, table 19). Only the Tatun system in northernmost Taiwan has been sufficientlyexplored to show excess chloride waters present in a distinctly geothermal reservoir. Otherareas may have similar reservoirs but have not been drilled.The geology of the Tatun field has been described by Chen (1970) and the chemistry ofreservoir waters briefly reviewed by Truesdell et al. (1989). The Tatun geothermal field coversan irregular area about 5 x 10 km within the circular Plio-Pleistocene Tatun volcanic group.This volcanic center has a long history of activity and some morphologically young volcanoesbut there are no historic eruptions or published rock dates. The geothermal reservoir isdeveloped partially within andesitic flows and tuffs but mostly within the underlying 900-mthick Miocene Wuchishan Sandstone consisting chiefly of thick beds of orthoquartzitecontaining only quartz, kaolinite and minor alunite, and elemental sulfur. Within the reservoirandesites are highly altered.
Reservoir waters range in salinity but have chemical compositions similar to each other and tothe Hsinpeitou spring at the western margin (table 1, analyses 1-3, and figure 1). The deepreservoir water at Tatun probably has a pH of 1.5 to 2, chloride from 3500 to 13000 ppm, anda temperature of 250 to 300°C (references in table 1). Steam boiled from this water containsHC1 gas. Condénsales from a shallow well producing dry steam contained 3500 ppm HC1 atpH 1 (Ellis and Manon, 1977) and condénsate with 400 ppm Cl was collected from asuperheated fumarole (Truesdell et al., 1989). The chloride excess at Tatun has not beenneutralized by reaction with rock because the reservoir contains no minerals (e.g. feldspar ormica) capable of neutralizing acid. The Tatun field was finally abandoned because no meanswas found to prevent rapid corrosion of casings.
34->JO+-I
03 0o 2coOO) ,O 1
O
a E-101O E-102+ Hsinpeitou
D
pH Na K Ca Mg Fe Cl S04 B Al
Figure 1. Schoeller diagram of analyses (in meq/1) of well waters and a spring water from theTatun, Taiwan field. In a Schoeller diagram the slopes of lines connecting logarithmicconcentrations indicate ratios and are not changed by boiling or mixture with dilute waters orsteam condénsate. pH values are shown directly. Data sources are given in table 1.
425
Other waters with chloride excess are closely associated with active volcanism. An example isthe famous Tamagawa hot springs on the western slope of Mt. Yake, north-central Honshu,Japan. These springs (table 1, analysis 4, and figure 2) discharge 9300 1/m at 98°C containingabout 3 g Cl/1, comparable on a yearly basis to an erupting volcano in the amount of heat andmaterial transported (Ozawa et al., 1973). The Sumikawa geothermal field on the north slopeof Mt. Yake produces mostly low-Cl (100-400 mg/1) neutral waters with one well producingacid SO4-C1 water (discussed later). The relation between the neutral waters, high-SU4 acidwater, and high-Cl acid waters occurring within 5 km has never been examined.
Acid discharge of well 4 at Los Humeros, Mexico, well 12 at Krafla, Iceland, and acid steamcondénsate at The Geysers, California, and Larderello, Italy, are also excess chloride watersbut were formed by a different mechanism involving transport of HC1 in superheated steam.These areas will be described later.
Acid SO4-C1 Reservoir Waters
Acid waters with SO4-C1 acidity are found at the margins of several geothermal systemsassociated with recently active andesite volcanoes. In these fields the acid zones werepenetrated by a small number of wells that produced acid fluid from the outset or afterdeepening. These wells were soon abandoned or used as injectors so geochemical data arelimited. The best documented examples are well S-2 at Sumikawa, Japan, and well PGM-2 atMiravalles, Costa Rica. Each of these wells became acid after deepening. Analyses of flashedwaters from these wells before and after deepening and of a typical neutral water of each fieldare given in table 1 (references in the table).
Well S-2 at Sumikawa was originally drilled to 900 m and deepened 5 months later to 1065 mwith the intention of increasing the flow. The fluid produced before deepening had high excesssteam (2428 J/g total enthalpy) and elevated Cl, SC<4, Ca, Mg and Fe relative to otherSumikawa well waters but was at near neutral pH (table 1, analyses 5-7, and figure 2). Duringthe first few months of production, Cl and SÜ4 increased but fluid pH remained neutral. After
10
crQJE 6
c•B 4cu
cuocoU OD3O-1 -2
-4
o S-2O S-2AA S-4+ Tamagawax Yakeyama
pH Na K Ca Mg Fe Cl S04 Al
Figure 2. Schoeller diagram of analyses (in meq/1) of waters from Sumikawa, Japan wells andassociated acid springs, on the flanks of ML Yake volcano. Data sources are given in table 1.
426
deepening (S-2A), the water no longer had excess steam (~1090 J/g enthalpy), but had muchhigher $64, Fe, Ca and Mg, and lower Na, B and HCOs, with pH values below 3. Oxygenisotope compositions were initially similar to those of other Sumikawa waters and increased0.8 permil after deepening, but deuterium compositions were initially 5 permil higher thanother waters and became 12 permil higher after deepening (MMC, 1990). The Cl in the acidwater was a little lower than the initial Cl but still higher than Cl in other well waters althoughthere was no chloride excess.
The chemical evidence for the source of the acidity is equivocal. Higher Cl and SC»4 comparedto other Sumikawa waters might suggest less dilution with meteoric water and less reactionwith rock to neutralize acidity and precipitate SÜ4 as anhydrite. Alternatively, the S-2 acidwater could be a mixture of normal Sumikawa water with acid SO4-C1 spring water similar tothe Yakejama water (table 1, analysis 8) but higher in SÜ4 relative to Cl. Higher 6D and 5O areconsistent with evaporation at surface boiling temperatures, suggesting a component ofsurface-evaporated, acid-sulfate water. The mixing of shallow high-SC»4 water with deep high-Cl water has been suggested to be the origin of acid SÜ4-C1 waters in the Palimpanon andBacon-Manito fields of the Philippines (Robinson et al., 1987) and of acid SC»4-C1 hot-springwaters at Yellowstone National Park, USA (table 1, analyses 9, 10, and figure 3). Thedeuterium contents of the Yellowstone waters were intermediate between those of neutral high-Cl waters and of evaporated, deuterium-rich acid-SU4 waters formed by oxidation of H2S(Truesdell et al., 1977).
10
cro>
co
§ü
8O)O
o-2
Green DragonEchinus GeyserEar SpringUnnamed
pH Na K Ca Mg Fe Cl S04 B
Figure 3. Schoeller diagram of analyses (in meq/I) of acid and neutral spring waters fromYellowstone National Park, USA. Data sources are given in table 1.
At Miravalles, Costa Rica, acid high-SC»4 water was discharged from well PGM-2 after it wasdeepened from 1200 m to 2000 m in 1984 (table 1, analyses 13, 14, and figure 4). Beforedeepening the well had produced water similar to other wells with neutral pH but slightlyelevated 804. After deepening (PGM-2A),pH decreased to 2.2 and SC<4, Mg and Fe increasedgreatly with smaller increases in Na, K and B, and decrease in Ca. The increased Fe probablyresulted from acid corrosion of the casing suggesting that the reservoir pH was probably lowerthan 2.2. This water showed almost no change in Cl and no excess Cl indicating that the aciditywas associated with higher SO4 not Cl.
427
10
cr03E 6
«,
I 4
0)ucoUO)o
O
-2
-4
o PGM-2O PGM-2AA PGM-Ave
pH Na K Ca Mg Fe Cl S04 B HC03
Figure 4. Schoeller diagram of analyses (in meq/1) of well waters from the Miravalles field,Costa Rica. Data from Mainieri and Vaca (1990).
As at Sumikawa the chemical evidence does not indicate a definite origin for the acidity. Acid,high-SO4 waters could have had a volcanic origin or could have been formed at the surface byoxidation of H2S. These acid-SC>4 hot-spring waters are well known and occur in almost everygeothermal system where steam escapes at the surface. Most of these spring waters containcations leached from adjacent rocks but some are nearly pure, dilute sulfuric acid (table 1,analysis 12). However, in order to increase SO4 in PGM-2 from 2 meq/1 to 16 meq/1 with only1% change in Cl requires a concentration of SÜ4 in the mixing water exceeding 1400 meq/1 orabout 65000 ppm SÜ4. This is about 20 times the most concentrated acid-SU4 water reportedby White et al. (1963). An acid-SO4 water mixing to produce the acid S-2A discharge atSumikawa would have to be twice as concentrated. Although not impossible, the existence ofthese waters seems very unlikely.
A Volcanic Origin for Acid C1-SÜ4 and SO4-C1 Waters
Acid SÜ4-C1 waters at Sumikawa and Miravalles seem most likely to be immature volcanic -geothermal waters not fully equilibrated with feldspar, mica, and Fe minerals in the reservoir.Giggenbach (1981, 1988) suggests that volcanic fluids containing acid, oxidizing gases"mature" to form neutral, reducing geothermal waters by reaction with rock minerals includingfeldspars, mica, and reduced iron oxides and silicates. HC1 is neutralized to NaCl, and SO2disproportionates to H2S and H2SÜ4, which may then precipitate as alunite and anhydrite. Thechemically distinct acid waters at Tatun and at Sumikawa and Miravalles appear to haveresulted from different reaction paths during maturation. At Tatun, SÜ4 is present in variableamounts but is not as important as HC1 in producing acidity. At Miravalles, acid and neutralwaters differ mainly in SÓ4 with nearly the same Cl. Sumikawa seems intermediate. At Tatun,conversion of SO2 to SO4 and precipitation as anhydrite and alunite appears to have precededneutralization of HC1. At Miravalles, HC1 neutralization apparently preceded SC»2 dispropor-tionation which produced additional acid. Sumikawa may be similar to Tatun but with moreneutralization of acid and less precipitation of sulfate.
Sulfur isotopes in SC»4 from Miravalles (Giggenbach and Corrales, this volume) and fromPalimpanon and Bacon-Manito fields in the Philippines (Robinson et al., 1987) have been
428
analyzed to determine whether SC«4 was produced by surface oxidation of H2S or by deepaddition of SO2- These isotope analyses were not entirely definitive and the interpretationswere quite different. In the Philippines fields most S^S values were near zero permil, whichwas interpreted to indicate that all sulfur was originally present as H2S and that H2S hadoxidized near the surface to acid-SO4 water, which infiltrated the geothermal reservoirs.Analyses of Miravalles SO4 were interpreted to show that mixing with surface-produced acid-SC»4 water had not occurred and that SÜ4 was of deep origin. It is unfortunate that detailedanalyses of Philippine acid waters are not available for geochemical interpretation.
HC1 in Superheated Steam
The appearance of superheated steam containing HC1 at the major vapor-dominated geothermalsystems of Larderello and The Geysers has led to several studies of its occurrence and originalong with programs to mitigate its corrosive effects. At Larderello, HC1 was first observed in1960 (Allegrini and Benvenuti, 1970; D'Amore et al., 1977) and at The Geysers, in the mid-1980s (Haizlip and Truesdell, 1988). The almost simultaneous appearance of HC1 in manywells at Larderello was probably caused by widespread drying of the reservoir and to a largeextent HC1 disappeared when liquid injection was begun (Truesdell et al., 1989). The solubilityof HC1 in solutions differing in temperature, pH, and Cl concentrations was calculated byHaizlip and Truesdell (op. cit.) who concluded that at The Geysers and Larderello (240-260°C), HC1 is transported only in superheated steam because below 300°C it is very soluble inneutral liquid and would have been removed from the vapor if condénsate were present.Saturated steam from the normal Geysers reservoir (table 1, analysis 16, and figure 5) has littleCl, but superheated steam at normal and high temperatures may have high HC1 concentrationsindicated by acid condénsate with high Cl and high Fe and Ca (analyses 17, 18). The origin ofthe HC1 in these fields is related to high-temperature reactions but whether it forms mainly fromboiling of near-neutral NaCl brines or from reaction of halite with silicates (D'Amore et al.,1990) is still controversial.
At The Geysers and Larderello, HC1 in steam seems closely related to the existence of high-temperature zones below the main vapor-dominated reservoir (Walters et al., 1988; D'Amore etal., 1990). At The Geysers these high-temperature zones are at the same pressure as the vapor
- 6cr03
, 4co
coocouD)O
O
-2
-4
o G-2C> G-4A G-6
pH Na K Ca Mg Fe Cl S04 B SÍ02
Figure 5. Schoeller diagram of analyses (in meq/1) of water collected in condénsate traps froma low-Cl (G-2), and high-Cl (G-4) normal-temperature wells and from a high-temperature,high-Cl (G-6) well at The Geysers, USA.
429
reservoir and 50 to 100°C higher in temperature. Fluid in these zones must be superheatedsteam with only absorbed liquid or high-Cl brines possibly present. These zones containdistinct, high-gas, high-B, high oxygen-18 steam and HC1 that is either an original constituent(if a volcanic origin is assumed) or has resulted from boiling of brine or from halite-silicatereactions. In most areas of The Geysers where HC1 occurs, steam is sufficiently superheatedthat the first condensation occurs at the wellhead and corrosion can be prevented by injection ofalkaline solutions.
HC1 in superheated steam occurs in many places, mostly related to active volcanoes. In wellKG-12 at Krafla, Iceland, superheated steam containing HC1 appeared in late 1979 anddisappeared in 1982 (table 1, analyses 19 and 20 before and after superheating) with HC1content closely proportional to the degree of superheating and the rate of flow (Truesdell et al.,1989). The KG-12 acidity was mitigated by mixing the steam with two-phase discharge fromanother well. The Krafla occurrence was clear evidence that HC1 could appear in any high-temperature (>325°C?) geothermal field in which superheated steam appeared. The Clconcentration in waters from KG-12 (table 1, analysis 19) and nearby wells is normallyrelatively low (<100 ppm) but brines are occasionally encountered and evaporation in thereservoir may be the origin of the brine or possibly halite, which produced HC1 in KG-12. Asat The Geysers and Larderello, the appearance of superheated steam is related to the local orwidespread depletion of liquid in the reservoir and a decrease in reservoir pressure. Injection ofliquid water will recharge liquid in the reservoir and in most cases raise pressures and eliminateHC1 in steam.
Los Humeros, Mexico, an Unusual Case
The Los Humeros geothermal field in the state of Puebla, Mexico, is located in a large calderaat the eastern end of the Mexican volcanic belt. The productive reservoir is mainly developed inandesites and andesite tuff underlain by hornfels and calcareous skarn. There are severalpetrographic and fluid inclusion studies of Los Humeros reservoir rocks (Arnold andGonzales-Partida, 1987; Prol, 1988; Viggiano and Robles, 1988; Prol-Ledesma and Browne,1989). Chemical analyses of fluids (tables 1 and 2) and some downhole-temperaturemeasurements were provided by the Comisión Federal de Electricidad (CFE) and the InstitutoInvestigaciones de Eléctricas (HE). A report by Gutierrez-Negrin and Viggiano (1990) and oneby HE (1990) provided additional data on the history of well H-16.
Most wells of Los Humeros have high total fluid enthalpy and produce all or almost all steam atthe wellhead. Of well fluids collected before 1990 only those from well H-l (table 1, analysis22, and figure 6) were low enthalpy and originated as almost entirely liquid water in thereservoir (about 0.7 liquid fraction when produced to 4 bars). Other wells had liquid fractionsfrom zero to 0.4 but most were less than 0.1. With high total enthalpy and small liquidfractions the reconstruction of reservoir fluid concentrations is difficult because boiling isalmost certain to have caused some mineral precipitation in the reservoir or wellbore.
Well H-l is one of the shallowest of the field (1450 m TD) and produces relatively little excesssteam (fluid enthalpy, 1311 ± 100 J/g). The measured temperature at bottomhole was 260°C inagreement with geothermometer calculations (quartz saturation 260 ± 10°C; Na/K (Fournier)265 ± 10°C). It is probable that H-l fluid is typical of the main aquifer at Los Humeros (table2, analyses 1-5). Fluids from six other wells have average flashed Cl concentrations similar(±25 ppm) to that of well H-l (95 ± 28 ppm) and give similar geothermometer temperatureindications (figure 7). Other wells have a wide range of measured and calculated Clconcentrations but also have similar indicated reservoir temperatures (figure 7). Measured pHvalues of all (except H-4) fluids after flashing are generally neutral to alkaline (pH 7.5 to 8.5,rarely as low as 6.5).
430
TABLE 2. SELECTED ANALYSES OF FLASHED WATER FROM LOS HUMEROS WELLS H-l, H-16AND H-4 (in mg/L unless otherwise specified)Source: Comisión Federal de Electricidad (1990)
No. Date Enthalpy pH Na K Ca Cl HCOj SO4 B
1 82/06/102 85/01/103 89/06/06
Well H-16
86/02/1486/03/18
6 86/06/127 87/07/148 88/06/249 88/08/11
10 89/01/2011 89/02/1412 89/09/0913 89/10/13
1220 8.5 250 43 2 74.4 293 116 225 6941240 8.5 294 49 0.2 96.4 173 123 240 6751340 8.24 290 48 1.9 95.1 255 116 187 803
26602660
266023102330
2500
7.797.767.737.77.548.219.249.128.648.92
540600695494372430636632420586
28.826.632.2285440292944.232
77.710.37.21.861——1.20.9
20820830926329821221021021299.3
39345090014454.'73.:524523279464
150 1680 944197 1880 1020
1900 1080191 1660 651
54.7 95.2 1180 1080166 691 534192 248 19419794.6
142
187270220
367875551
1415161718192021
82/08/1982/08/3082/08/3083/01/2483/03/0883/03/0883/03/1583/04/21
7.22.92.96.26.68.58.54.95
6617015028537.45330
130
15.3267247
6.77.54—
4963370.20.00.00.0
63.3506472
—1064026.9
333
61——
97951585
115036.6
21611296941425232
.3
.2
.2
.7
46523204230400
18.417.310.8
376
35011.618.9
400481287303372
Notes: 2-inch orifice plates were used for H-l samples after 1983 and all H-16 samples presentedhere; orifice plate sizes for H-4 samples are not known. "--" indicates not available, not analyzed,or not detected.
10
cra£ 6
1 42+->g 2o
8 oD)
-4
a H-4O H-16A H-1
pH Na K Ca Mg Cl S04 B HC03
Figure 6. Schoeller diagram of analyses (in meq/1) of waters from two wells affected by deepHCl and of a normal well at Los Humeros, Mexico.
431
3000
* Total dischargea Aquifer liquida Flashed liquid
100 200 300 400
Chloride, mg/kg
500 600
Figure 7. Enthalpy-Cl diagram of average fluids from wells at Los Humeros, Mexico, withlines showing steam loss for average H-l discharge and for the highest chloride well (H-l 1,data for H-2, H-4 and H-19 are not sufficient to plot).
Well H-l6 was described in detail by Gutierrez-Negrin and Viggiano (1990) and IIE (1990).This well was drilled to 2048 m total depth and showed several likely production zones asindicated by intensive faulting and lost circulation, a definite deeper zone from 1750 m to totaldepth, and shallower, less well-defined zones above 1650 m to possibly 1300 m or above. Thelower zone (in andesite and hornfels) was entirely in the amphibole facies characterized by theappearance of amphiboles and garnet. The measured temperatures in the lower production zonewere 300-319°C but temperatures of 380-400°C have been measured in other wells. The upperproduction zone (in andesite) was mainly epidote facies but extended to the amphibole facies.Measured temperatures were 230-290°C. Quartz-saturation geothermometer temperatures offluid from this zone were variable but centered on 270°C similar to those from well H-1.
After initial production in 1985, the flow of this well progressively decreased and in 4 yearsnearly stopped flowing. The partially slotted liner installed from 1400 m to total depth wasraised and found to be highly corroded in its lower part (1420-1432 m depth) with only 50 mof unslotted and 20 m of slotted liner recovered. The middle part of the liner (1415-1420 mdepth) was scaled completely with Fe oxides and sulfides in the outer part and amorphoussilica with some Fe minerals in the inner part. The upper part of the liner was lightly scaled.
The chemistry of H-l6 well fluids (table 2, analyses 6-16, and figures 6 and 8) went through aremarkable evolution (Gutierrez-Negrin and Viggiano, op. cit.) in which HCOs, which wasoriginally the major anión, decreased from 450 to 55 ppm and then increased to 520 ppm whilechloride increased from 210 to 300 ppm and returned to 210 ppm. During this period the pH(after flashing) decreased from about 8.5 to 7.5 and then increased to 9.0, B decreased from1900 to 600 ppm and Ca decreased from 7 to about 1 ppm. Other species (Na, SÍÜ2) showedless definite decreases.
In December 1989 the lower zone was isolated by a plug at 1570 m and a new slotted liner wasinstalled, after which the well produced entirely from the upper zone and the fluid chemistrystabilized. The flow after December 1989 was about that measured when the well was firstdrilled but the enthalpy decreased from near 2660 to 2400 J/g (Luis Quijano León, pers.commun., 1990) suggesting that the lower zone had contributed heat but little fluid.
432
g'(o•M(Uocoo05O
3.5
2.5
1.5
0.5
-0.585.5 86.5 87.5
Year88.5 89.5
Figure 8. Changes in concentrations (in mg/1) of selected constituents of flashed dischargefluids from well H-16, Los Humeros, Mexico.
The chemical reactions that led to the corrosion and scaling may be interpreted from thewellhead chemistry and the character of the scale. The initial production was from both deepand shallow zones. The shallow water was similar to that of well H-l (table 2, and figures 6and 8) and is represented by samples taken in early 1989 after the lower zone was sealed off byscaling or had been cemented. The deep zone produced superheated steam containing HsBOs,HC1, and H2S along with CÜ2 and other gases. In the upper part of the slotted liner thesuperheated steam mixed with water from the shallow reservoir and partially condensed toproduce an acid high-Cl, high-B solution that attacked the casing and rock minerals includingplagioclase, other feldspars, and possibly calcite.
These reactions increased brine concentrations of Fe, SÍO2, Na, and Ca, and neutralized someof the acid. Before complete neutralization, sulfide was present as H2S and did not react withFe++, and SiO2, present in high concentrations from solution of rock silicates, was stabilizedby H+ ions so that amorphous silica did not precipitate although it was highly supersaturated.With further neutralization by reaction with casing, rock minerals, and HCC»3 from the upperreservoir, H2S was converted to HS and stabilization of supersaturated amorphous silicaended, producing the scale composed mostly of Fe sulfide and amorphous silica. Oxidation ofFe+2 to Fe+3 caused by fluid mixing resulted in minor precipitation of Fe oxides. In themixture, SC»4 from the shallow water had been present as HSÓ4 along with elevated Ca++
(from acid attack of rock minerals) and with neutralization, HSO4 became SO4 and anhydriteprecipitated. With sealing off of the deep zone, acid attack of rock minerals ended so that Cadecreased strongly (to equilibrium with anhydrite) and Na decreased moderately.Concentrations of B and Cl decreased as contributions from the deep fluid ended. Thecorrosion and scaling had continued from the first tests in 1986 with a decreasing contributionof deep fluid caused by progressive scaling until the deep flow was cut off entirely betweenSeptember 1988 and January 1989. The lowest pH values and the highest B and Clconcentrations were near the beginning when the proportion of deep flow was greatest. Thisinterpretation is similar to those of HE (1990) and Gutíerrez-Negrin and Viggiano (1990).
433
Other wells in the central zone probably produce from both upper and lower zones and mayhave similar problems. Well H-4, just south of H-16, produced HC1 to the wellhead with somuch corrosion that it was necessary to plug and cement it shut (table 2, analyses 17-24, andfigure 6). The short history of well H-4 is similar to that of well H-16 but more extreme(figure 9).
raE
03
ocouO)o
-182.5 83 83.5 84
Year
Figure 9. Changes in concentrations (in mg/1) of selected constituents of flashed dischargefluids from well H-4, Los Humeros, Mexico.
Well H-4 was drilled to 1880 m total depth. There were two permeable zones between 900 and1400 m and between 1700 and 1800 m. Measured temperatures were about 300°C at 1100 m,significantly higher at this depth than in most other wells. Measured temperatures below 1100m seemed to decrease but petrology of samples from 1400 m depth did not indicate lowtemperatures (Prol-Ledesma and Browne, 1989). Initial fluid production in mid-August 1982was neutral, but within 2 weeks, pH had dropped below 3.0. The well was flowed andsamples were taken intermittently until April 1983 but rapid wellhead corrosion was observedand the well was finally cemented and abandoned. During the first 2 weeks of production,flashed Cl concentrations immediately increased from 60 to 500 ppm and pH, initially 7.2,decreased to 3.7 and stabilized at 2.9. Over the same period B increased from 460 to 4000 ppmand HCOs, initially 60 ppm, disappeared entirely. Other species showed irregular changes; Na,K and Ca increased and SiÓ2, NH4 and SO4 decreased. When sampled in January and March1983, these changes had generally reversed, the fluid was neutral or alkaline (pH 6.6 to 8.5)with B (18 ppm) and Ca (0.2 ppm) much lower and HCO3 (800-1100 ppm) much higher thanin any 1982 samples. Again other changes were irregular but Na, K and Cl generally decreasedwhile SiO2, NH4 and 864 generally increased. After March 1983, most of these changes wereagain reversed, with lowered pH and HCOs ar>d increased B and Cl.
The changes in H-4 fluid with time are similar to those in H-16 fluids and undoubtedly H-4went through the same history of liner corrosion and progressive plugging leading toelimination of deep fluid influence. In H-4 the deeper fluid was blocked by scale in March1983 but reappeared later, possibly having broken through or bypassed the scale.
434
Conditions in the Deep Los Humeros Reservoir
During the initial production from wells H-16 and H-4 a mixture of shallow and deep fluidswas produced. The wellhead enthalpy of early H-16 production, combining deep and shallowfluids, was 2662 J/g and the enthalpy of the upper fluid alone (after scaling had blocked thedeeper fluid) was 2400 ±100 J/g. From mass balance calculations (using 2500 J/g shallowdischarge), the deep contribution would be 65% if it were saturated steam (2750 J/g at 300°C),50% if it were superheated to 2830 J/g, and 25% if superheated to 3150 J/g. The observationthat the flow of the repaired well (producing from the upper zone only) was nearly equal to theinitial flow from both zones probably rules out major production from the deeper zone andsuggests that the deep fluid was highly superheated steam.
This high enthalpy steam probably was not produced by equilibrium boiling from liquid. Steamfrom evaporation of 30% NaCl brines at 350°C would have an enthalpy of only 2890 J/g; andhigher temperature evaporation would produce steam with lower enthalpy (calculations by J.L.Haas, Jr., quoted in Truesdell and White, 1973). If the deep zone fluid was 25% of the totaland had an enthalpy of 3150 J/g, then it would have a temperature of about 410°C at thepressure of the upper reservoir (about 85 ba). This steam would not be compatible with liquidwater unless the water were highly saline or tightly adsorbed on rock surfaces. This suggeststhat the lower reservoir contains only superheated steam and that temperatures near 400°C existnot far below the bottom of well H-16. A temperature near 400°C has been observed at LosHumeros (Gutíerrez-Negrin and Viggiano, 1990) although the well and depth are not available.
The high-temperature reservoir found at Los Humeros beneath a normal temperature reservoiris analogous to the high-temperature zones underlying vapor-dominated reservoirs in thenorthern part of The Geysers. The main difference is that the shallower reservoir at LosHumeros is liquid dominated, and therefore, pressures in both reservoirs are higher.
The Origin of Deep Fluid Constituents
In the earlier discussion of changes in fluid chemistry it was shown that the constituentscontributed from the deep reservoir were all soluble in superheated steam or resulted from acidattack of casing and wallrock. The "primary" constituents HC1, B and H2S are also present inthe shallow reservoir (with HC1 represented as NaCl). It is also observed that in the naturalstate before drilling there is no evidence of acid fluids. The andesite contains calcite, feldspars,and mica; the hornfels and skarn are composed in part of carbonate rocks and normally containcalcite. If superheated vapor in the deep reservoir contained significant HC1 and B in its naturalstate, then it would be expected that minerals such as calcite and feldspars would not occur andthat boron minerals such as tourmaline would be found. The lack of mineralogical evidence fordeep acid or high boron suggests that the deep fluids observed in H-16 and H-4 may be in partartifacts of drilling formed by flows of shallow water down the wellbore.
A shut-in drill hole penetrating both the shallow, liquid-filled reservoir and the deep reservoircontaining only high-temperature steam would be a conduit for downward flow of liquidbetween the two zones. Liquid flowing down the well would quickly vaporize as it reached thehigh-temperature zone and it would precipitate salts and evolve gases. Some constituents suchas B (as HsBOs) would partition between solid and vapor. The continued downflow of upper-zone water could cause accumulation of significant quantities of alkali chlorides (mainly halite)and sulfates, boric acid, and amorphous silica and release CC>2 (from HCOs) and NH3 (fromNELO into the steam. The steam in contact with this deposit of salts would have high H3BÜ3and significant HC1 formed by halite-silicate reaction. With production of the well, gases andvolatile salts would be drawn upward to mix with the shallow reservoir water with the resultsdiscussed earlier. If this idea is correct, then shallower drilling at Los Humeros could haveavoided the formation of acid steam as well as its introduction into drill holes.
435
Summary
Acid reservoir fluids in geothermal fields seem to originate from either the introduction ofvolcanic fluids or from the volatilization and transport of HC1 in superheated steam. A minorsource may be infiltration of surface acid-SO4 waters formed by oxidation of H2S. With theexception of the high-Cl waters of the Tatun field, acid reservoir waters of volcanic origin seemto exist on the margins of geothermal reservoirs and do not represent a major part of the totalfluid. Acid from volatilization of HC1 is expected to appear in boiling high-temperaturereservoirs as they lose reservoir liquid and start producing superheated steam. The occurrenceof superheated, high-HCl steam at Los Humeros, Mexico, is unusual because the steam isproduced by flow from a deep dry reservoir to a shallower water-saturated reservoir withstrong corrosion and scaling resulting from fluid mixing and reaction with casing and rock. Itis suggested that the acid steam is an artifact of exploitation and could be avoided by shallowerdrilling.
Acknowledgments. The author wishes to thank Luis Quijano León and Enrique Tello Hinojosaof the Comisión Federal de Electricidad de Mexico for the use of unpublished data for LosHumeros and for enlightening discussions of well H-16. He also wishes to thank AlfredoMainieri P. and Leonel Vaca C. of the Instituto Costarricense de Electricidad for discussionsand data for Miravalles well PGM-2, and Mitsubishi Metals Corp. (Japan) for data onSumikawa well S-2. Partial support of the International Atomic Energy Agency in the LosHumeros study is gratefully acknowledged. He also thanks his colleagues, in particularDonald White and Werner Giggenbach, for many discussions of acid fluids in the earth.
References
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Arnold, M., and Gonzales Partida, E., 1987, Le systéme hydrothermal actuel de Los Humeros(México): Etat du systéme SC«4~-SH2 a 300°C, origine du soufre et phénoménesd'oxydation associés a l'ébullition du fluide ascendant: Mineralium Deposita, v. 22, p. 90-98.
Chen , C.-H., 1970, Geology and geothermal power potential of the Tatun volcanic region,U.N. Symposium on Geothermal Resources: Geothermics, v. 2, part 2, p. 1134—1143.
Comisión Federal de Electricidad, 1990, unpublished data.D'Amore, F., Celati, R., Ferrara, G.C., and Panichi, C., 1977, Secondary changes in the
chemical and isotopic composition of the geothermal fluids in Larderello field:Geothermics, v. 5, p. 153—163.
D'Amore, F., Truesdell, A.H., and Haizlip, J.R., 1990, Production of HC1 by mineralreactions in high-temperature geothermal systems: Proceedings, 15th Workshop onGeothermal Reservoir Engineering, Jan. 23-25, 1990, Stanford, California (in press).
Ellis, A.J., and Mahon, W.A.J., 1964, Natural hydrothermal systems and experimental hot-water/rock interactions: Geochimica et Cosmochimica Acta, v. 28, p. 1323-1357.
Ellis, A.J., and Mahon, W.A.J., 1977, Chemistry and Geothermal Systems: Academic Press,392 p.
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Giggenbach, W.F., 1981, Geothermal mineral equilibria: Geochimica et Cosmochimica Acta,v. 45, p. 393-410.
Giggenbach, W.F., 1988, The interplay of magmatic and hydrothermal processes in theformation of volcanic and geothermal fluid discharges: Preprint, Kagoshima InternationalConference on Volcanoes, Kagoshima, Japan, July 19-23, 1988.
Giggenbach, W.F., and Corrales Soto, R., 1990, The isotopic and chemical composition ofwater and steam discharges from the Guanacaste geothermal province, Costa Rica: thisvolume (in press).
Gutíerrez-Negrin, L.C.A., and Viggiano Guerra, J.C., 1990, Corrosion and scaling in well H-16 of the Los Humeros geothermal field: Geothermal Resources Council Transactions, v.14, p. 1591-1598.
Haizlip J.R., and Truesdell A.H., 1988, Hydrogen chloride in superheated steam at TheGeysers geothermal field: Proceedings, 13th Workshop on Geothermal ReservoirEngineering, Stanford, California, p. 93-100.
Instituto Investigaciones de Eléctricas, 1989, Characterizacion del fenómeno corrosión—obturación de pozos de Los Humeros: Informe IIE/11/3753A, 15 p.
Mainieri P., Alfredo and Vaca C., Leonel, 1990, Costa Rica: country update report:Geothermal Resources Council Transations, v. 14, p. 23-29.
Ozawa, T., Kamada, M., Yoshida, M., and Sanemasa, I., 1973, Genesis of hot springs, PartI. Genesis of acid hot springs: Journal of the Japan Geothermal Energy Association, v. 10,p. 31-40.
Prol, R.Ma., 1988, Reporte de los estudios petrográficos y de inclusiones fluidas en núcleosde pozos de exploración en el campo geothermico de Los Humeros, Puebla, México: Inst.de Geofísica, UNAM. Común. Technicas Serie 86, 75 p.
Prol-Ledesma, R.M., and Browne, P.R.L., 1989, Hydrothermal alteration and fluid inclusióngeothermometry of Los Humeros geothermal field, Mexico: Geothermics, v. 18, p. 667-690.
Robinson, B.W., Villaseñor, L.B., and Clemente, V.C., 1987, Preliminary stable isotopeinvestigations of acid fluids in geothermal systems of the Philippines: Proceedings, 9thNew Zealand Geothermal Workshop, Auckland, New Zealand, p. 73-78.
Rowe, J.J., Fournier, R.O., and Morey, G.W., 1973, Chemical analysis of thermal waters inYellowstone National Park, Wyoming, 1960-65: U.S. Geological Survey Bulletin 1303,31 p.
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Thompson, J.M. and Yadev, Sandhya, 1979, Chemical analyses of waters from geysers, hotsprings and pools in Yellowstone National Park, Wyoming, from 1974 to 1978: U.S.Geological Survey Open-File Report 79-704, 49 p.
Truesdell, A.H., Nathenson, M., and Rye, R.O., 1977, The effects of subsurface boiling anddilution on the isotopic compositions of Yellowstone thermal waters: Journal ofGeophysical Research, v. 82, p. 3694-3704.
Truesdell, A.H., Haizlip, J.R., Armannsson, H., and D'Amore, F., 1989, Origin andtransport of chloride in superheated steam: Geothermics, v. 18, p. 295-304.
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Viggiano, J.C., and Robles, J., 1988, Mineralogía hidrothermal en el campo geotérmico deLos Humeros Pue., Usos como indicadora de temperatura y del régimen hidrológico:Geotermia, v. 4, p. 15-28.
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438
IAEA INTERLABORATORY COMPARATIVE GEOTHERMALWATER ANALYSIS PROGRAM
W.F. GIGGENBACH, R.L. GOGUEL, W.A. HUMPHRIESDivision of Chemistry,Department of Scientific and Industrial Research,Petone, New Zealand
Resumen-Abstract
PROGRAMA DEL OTEA PARA ANÁLISIS COMPARATIVOS DE AGUAS GEOTERMALES ENTRELABORATORIOS.
La composición química de tres muestras de aguas geotermales de NuevaZelanda se determinó en 22 laboratorios, incluyendo los que participan en elprograma coordinado de investigación para América Latina sobre "Aplicación detécnicas isotópicas y geoquímicas en la exploración geotérmica". Aquí se pre-sentan y discuten los resultados de esta intercomparación. Con arreglo a loprevisto, los resultados indican que algunos laboratorios tienen capacidadpara realizar análisis químicos de elevada calidad, mientras que otros debenmejorar considerablemente su rendimiento para obtener resultados que puedanutilizarse en la exploración geoquímica de zonas geotérmicas. No obstante, engran medida corresponde a cada laboratorio evaluar su propio rendimiento yadoptar decisiones con respecto a las posibles maneras de mejorarlo.
IAEA INTERLABORATORY COMPARATIVE GEOTHERMAL WATER ANALYSIS PROGRAM.
The chemical composition of three samples of geothermal waters fromNew Zealand was determined by 22 laboratories, including thoseparticipating in the coordinated research programme for Latin America on"The application of isotope and geochemical techniques in geothermalexploration". The results of this intercomparison are presented anddiscussed. As expected, the results show that some laboratories have thecapacity to produce high quality chemical analyses, while others mustimprove their performance considerably in order to obtain results whichcan be used for the geochemical exploration of geothermal areas. To alarge degree, however, it is up to individual laboratories to assessperformances and decide on possible avenues of improvement.
439
INTRODUCTION
Application of recently developed techniques for the evaluation of chemical and
physical conditions over the deeper part of a geothermal system relies heavily on theavailability of sufficiently complete and accurate analyses of the fluid discharges. The
range and sophistication of these techniques can be expected to increase considerablywithin the near future putting great demands on the reliability of laboratoriesinvolved in geothermal fluid analysis. The most promising way to check on thepresent state of reliability of laboratories involved in geothermal projects, and to
identify possible lines of improvement, consists of a comprehensive interlaboratory
analysis program.
A number of national or regional interlaboratory water analysis programs have beencarried out. Most of these, however, were restricted to little mineralised potable orirrigation waters or waters of interest in environmental pollution studies. Thenumber and type of constituents covered by these programs are generally quite
different from those determined in geothermal investigations.
The only interlaboratory chemical analysis program involving many countries and
including geothermal water samples was organised by Chemistry Division, DSIR. Theresults are reported by A J Ellis (1976). For a number of constituents (Li, SO4, As,
NH3, Rb, SiO2) more than half of the laboratories reported values outside anacceptable range, for the rest only 80% of the data were considered acceptable. Thescatter exposed during this study revealed serious deficiencies in analytical accuracy
and the need for general improvement and standardisation of analytical procedures.
In view of this and its involvement in many geothermal projects, the InternationalAtomic Energy Agency decided to initiate a purely geothermal intercomparison wateranalysis program within the framework of the Coordinated Research Program on theApplication of Isotope and Geochemical Techniques in Geothermal Exploration in
Latin America. Chemistry Division, DSIR, New Zealand was asked to organise thecollection and distribution of suitable samples and to carry out an initial assessment
of the results.
440
TIMETABLE
13.08.85 - Distribution of announcement of program to some thirty laboratoriesinvolved in geothermal projects, together with a questionnaire as to the range ofspecies able to be analysed. Of the 25 replies received by the end of 1985, all statedthe ability to analyse for Na, K, Mg, Ca, Cl and SO4; 23 were able to determine pH,Li, F, HCO3; 22 to determine Fe and SiO2; 21 B and NH3; 18 Rb, Cs, Al, As; 12Hg, Br, I, D, 180.
30.12.85 to 1.1.86 - Collection of three sets of twenty liter, filtered and two preserved
one litre water samples.
15.01.86 - Distribution of three 500 ml samples by airmail to 25 laboratories togetherwith covering letter and reply forms to acknowledge receipt of samples (by end ofFebruary) and to report analytical results (by end of April 1986). As expected, only afraction of laboratories acknowledged receipt or submitted results by the deadlinesand a reminder was sent on
02.07.86 - enquiring about the fate of the samples sent. In due course, it was learnt
that all had arrived safely, with some leakage reported for two or three bottles. By
the extended deadline of
31.10.86 - twenty more or less complete lists of analytical results had been received.
LIST OF PARTICIPATING LABORATORIES
The selection of laboratories participating in the Intercomparison Program is largelybased on their present or past involvement in IAEA projects.
Facultad Ciencias, Universidad Mayor de San SimonCalle Sucre Final, Cochabamba, BOLIVIA.
Coordinador Área Materials Primas, Instituto de Asuntos NuclearesApartado Aereo 8596, Bogotá D.E., COLOMBIA.
441
Instituto Costarricense de ElectricidadPO Box 10032, San José, COSTA RICA.
Insti tuto Ecuatoriano de Electrificación (INECEL)
Casilla 565-A, Quito, ECUADOR.
Comisión Ejecutiva Hidroeléctrica del Rio Lempa
Avenida Norte, San Salvador, EL SALVADOR.
Institute Mixte de Recherches Geothermiques, BRGM
BP 6009, 45060 Orleans Cedex, FRANCE.
Laboratoire de Geochimie des Eaux, Universite Paris 7
F 75 251 Paris Cedex 05, FRANCE.
Instituto Nacional de Electrification
6a Ave. 2-73 Zona 4, Guatemala City, GUATEMALA.
Geological Survey of IndiaGhandi Bawan, Lucknow 226 001, UP, INDIA.
P T Geoservices
JL Setiabudhi 81, Bandung, INDONESIA.
Oil and Gas Technology Development Center, LEMIGAS
PO Box 89 JKT, Jakarta, INDONESIA.
Isotope Hydrology, IAEAPO Box 100, A-1400 Vienna, AUSTRIA.
Laboraterio Chimico Físico, ENEL-UNG56044 Larderello (PI), ITALY.
442
Ocean Research Institute1-15-1 Minami-Dai, Nakano-ku, Tokyo 164, JAPAN.
Comisión Federal de ElectricidadApartado Postal 31-C, Morelia, 58000 Midi., MEXICO.
Geothermal Research Centre, DSIR Chemistry WairakeiPrivate Bag, Taupo, NEW ZEALAND.
DSIR ChemistryPrivate Bag, Petone, NEW ZEALAND.
Electropem S.A.Edif. "La Torre" of. 903, Centro Cívico, Lima, PERU.
PNOC Energy Development Corpoation, GeothermalPort Bonifacio, Metro Manila, THE PHILIPPINES.
Geological Survey Division, Dept. of Mineral ResourcesRama 6 Road, Bangkok 10400, THAILAND.
Vernadsky Institute of Geochemistry, Academy of Sciences of the USSRKosygin St. 19, 117 975 Moscow, 5-334, USSR.
Ministerio de Energía y Minas
Torre Oeste - Piso 17 - Parque Central, Caracas 105, VENEZUELA.
Four other laboratories had agreed to participate, but had failed to supply results bythe extended deadline of 31.10.86.
DESCRIPTION OF WATER SAMPLES
The three water samples used in the Intercomparison Program were obtained from
geothermal areas on the North Island of New Zealand.
443
TABLE 1
Chemical analyses reported for sample 001 collected from a clear, colorless,boiling (97.5°) spring at Waikite on 31.12.85 (in mg/kg).
Lab No.01020304050607080910111314151718192021222325
median"true"
PH8.17.68.07.78.08.47.68.28.18.18.28.37.77.97.77.97.88.28.08.48.48.38.08.0
Li2.02.01.92.52.00.8-2.42.22.22.02.42.32.02.0.2.02.32.12.11.62.12.02.0
Na1972.3200178200186183220197191202200199192206-193182186210144203196196
K7.26.08.02.56.95.87.88.07.57.26.88.09.810.37.1-6.99.47.96.46.86.87.27.2
Rb0.09.
0.10.
0.70<.10
---
0.060.140.200.30-
0.09.
0.100.080.09<.10
--
0.100.10
Cs0.290.30..
2.60<.10
---
0.110.170.400.80-
0.40-
0.26-
0.280.27
--
0.280.28
Mg0.210.200.211.001.250.540.212.700.230.170.214.00<.100.200.22.
0.180.070.280.223.730.230.210.21
Ca7.98.08.28.07.916.48.014.37.67.88.76.07.07.08.3.7.39.610.08.010.27.98.08.0
Al<.05-------
<.030.040.02-
<.10-
0.01---
<.10---
0.020.02
Fe<.01<.50.1.200.51...
<.020.020.45
-0.010.800.01.
<.06.
<.02.
0.01-
0.04<.01
As<.200.31.
-0.92.
0.39--
0.38.-.-
0.32-..
0.23..-
0.320.30
F1.82.0.-1.9--1.32.21.92.02.02.21.92.3-2.42.12.2-2.8-2.02.0
Cl138128133298137136125250130125133140153109129124133136152135121131133133
Br-<.5-------0.40.4--0.80.4-----0.1-0.40.4
SO,34293523293633-32313631269632333429253656323333
HCOj278-344303307287259299299299293298299301300302278287315295254305300280
B1.21.31.21.13.5---1.31.61.35.41.5-1.4-1.1-0.11.5.1.41.31.3
SiO,15057149-283142-185154141144146134-146-15213014616025143146146
TABLE 2
Chemical analyses (in mg/kg) waters collected from the weirbox of well WK66 and Wairakei on 30.12.85. The enthalpy of the wellat the time of sampling was 1020 J/g, steam was separated at 166° and 100°C.
Lab No.01020304050607080910111314151718192021222325
median"true"
PH8.56.59.58.58.68.98.28.38.88.58.38.78.48.58.48.68.58.08.78.57.78.48.58.5
Li10.711.010.712.69.41.211.010.011.410.910.412.011.910.310.7-
10.210.411.210.87.811.210.710.7
Na115811301170105014421187120012251150109011701100118013081170
-11301045109912007149118011701170
K16914916413174176230200167168186160156187163-16320416617186168167167
Rb2.2-2.8-1.84.5-2.3-0.23.32.52.5-2.1-2.12.62.02.114.2-2.22.2
Cs2.01.9--2.52.05.0--0.32.22.53.2.2.0-1.9-2.02.018.4•2.02.0
Mg0.004
-.0102.7019.820.05
-.
<.010.160.20-<.100.040.002
-<.01-
0.08<.010.040.050.050.004
Ca20.220.019.718.018.941.812.021.418.719.021.612.017.316.520.0-
16.219.622.120.0.16.819.720.0
Al0.5-0.3---2.0.4270.50.6.0.2,0.4--0.30.4..-0.40.4
Fe<.01<.50.
1.000.34--.
<.01<.010.04.
0.051.000.004
-0.06.
<.02..-
0.04<.01
As3.84.1--4.1.2.0..4.7..-.4.2.-.2.8..-4.14.0
F3.37.0..1.4..3.77.69.57.68.06.91.57.8.9.57.58.4.9.3
7.67.6
CI195019701960271023632010248727001975193319601955196519911960197019601969199519701630195219701970
Br.5.3.----.-4.35.3.-1.65.4..-4.1.2.9-4.34.3
SO,32343430273834.34373739359037373533283765353535
HCO3<5-54493927494974929292396<14848581<5413340<5
B2533252352--.2623362925.28.24-2726.272626
SiO,596283588-234298683647481110135497-590-61017510558052582382590
TABLE 3
Chemical analyses (in mg/kg) reported for waters collected at Morere, New Zealand, on 1.1.86from a 47° spring associated with a natural gas discharge. The waters contained suspended
iron hydroxides, removed by filtration.
Lab No.01020304050607080910111314151718192021222325
median"true"
PH7.26.97.07.27.07.47.07.17.27.07.06.97.07.06.96.97.07.17.27.17.37.37.07.0
Li4.55.04.820.85.00.85.14.83.34.65.96.012.03.54.8-4.32.44.94.8-4.64.84.6
Na66907025666060007208606452006406770654067806800640076926770
-65906364649770001580688066606700
K8465721119867162108838285906058382-74110838271908284
Rb0.07
-0.40
-2.10<.10
-0.50
-0.040.170.506.20
-0.10
-0.100.060.130.12--
0.130.10
Cs.004<.10
--
1.16<.10
---
<.001-
0.2022.50
-<.10
-0.15-
<0.01<.10
--
0.100.004
Mg97581406901988116876767763736575-748183100174788080
Ca27503000277026202893249280032742660273027903506273822252720
-26902856285623002882260027602760
Al<.05.--...-
<.030.020.02.
0.40--._
0.110.10
---
0.10<.05
Fe0.15<.500.144.600.34-
0.12.
<.100.030.81-
0.14200.0<.01.
0.280.29<.02.
0.06-
0.140.15
As<.01.--3.1..-.
<.01.---
0.01-_-
<.01---
<.01<.01
F0.91.0--1.7-1.00.41.41.80.90.20.61.51.3.1.10.81.0_1.1-1.01.0
CI156701647015800266801607215537159002000016320147751560015750154081717615650158001566015755159641580013461153741580015800
Br-57---.---5758..263...51..-5757
SO4<3.26447<1075-<1<16.1-<1606-_<578-6<3
HCO,28-4473442973404035414639498542383931764539424230
B4442443884-40-4854454637-46.42.4245.474444
SiO228.413.730.0-
50.1583.03.656.027.734.027.626.023.0-
25.0.
25.525.426.5<50.055.035.027.027.0
The lowest salinity samples was collected on 31.12.85 at Waikite from a naturalboiling spring (97.5°). The waters are clear and colorless and deposit minor amountsof silica. The springs are located at the NW- margin of the Taupo Volcanic Zoneand are possibly associated with the Waiotapu - Te Kopia geothermal systems.
The intermediate salinity samples represents the weirbox discharge from well WR66at Wairakei collected on 30.12.85. The discharge enthalpy at the time of samplingwas 1020 J/g, steam was separated at 166° and 100°C. The water was clear andcolorless, minor amounts of amorphous silica accumulated on the cellulose filters.
The highest salinity sample was collected at Morere, an area of warm springs (47°) onthe East Coast of the North Island. The waters are associated with copious dis-charges of methane and deposit considerable amounts of ferric hydroxide (FeOOH)in flow channels. The waters are also characterized by high iodide contents which
impart a yellowish color to the samples after prolonged exposure to air.
Three samples were taken at each location. A twenty litre samples, pressure (N2)filtered through cellulose acetate (Millipore) filters, two one litre samples eachfiltered and acidified with 5 ml of cone. HNO3 and preserved with 5 ml 30% formalinrespectively. All sample containers, including sample bottles sent to participants, hadbeen cleaned by treatment with half-concentrated HNO3 and rinsing with distilled wa-ter. The samples bottles were number with the first two digits representing the labo-ratory, the last digit the type of water; Waikite (001), Wairakei (002), Morere (003).
RESULTS
The analytical results for the three geothermal water samples as received from 21laboratories by mid November 1986 are given at least seven results were reported,some laboratories reported results for a number of other species, they are shown inTable 4.
Excess significant figures given are rounded off, otherwise the data are shown asreceived in spite of some obvious typing, transcription or dilution errors such as avalue of 427 mg/kg of Al reported for 092, a Na-content of 640 given for 083, a Ca-
447
TABLE 4
Chemical results for constitutents analysed only sporadically(in mg/kg and °/» (SMOW))
Lab. I
Waikite (1)15 44.017 <0.4
medianWairakei (2)11 0.115 60.017 <.421 0.3
median 0.2Morere (3)1 1 26.915 30.017 30.021 29.5
median 30.0
Lab. NH,
01 0.0405 0.2506 <0.0209 <0.0211 0.0414 0.0415 0.7017 <.10
0.04
01 0.2305 1.2506 0.3009 0.3211 0.3414 0.8415 1.0017 0.22
0.33
01 10.2006 8.1009 11.5011 12.1017 11.40
11.40
Lab. Sr
01 0.0610 49.0021 0.05
-
11 0.1010 88.5021 0.11
-
01 23108 20810 20021 213
210
Lab. Hg
14 <.000221 .0009
-
14 <.000221 .0006
-
14 .007621 .0023
-
Lab. SD SIS0
01 -44.6 -6.5309 -42.9 -6.3210 - -6.1016 -41.8 -6.4518 -45.7 -6.70
- -45.2 -6.45
01 -41.8 -4.7209 -38.8 -4.6510 - -4.6016 -39.8 -4.6518 -40.5 -5.00
- -40.2 -4.65
01 -15.7 +3.9009 -12.0 +3.9310 - +3.6016 -35.4 +4.11
- -15.0 +3.90
content of 249 for 063, a HCO3-value of 7340 for 073, a B-value of 583 for 063, and aK-value of 583 for 153. Also Na-values for 232 and 233 are likely to have beeninterchanged, Sr-values for 101 and 102 are likely to be ng/kg. The occurrence of
such errors even in data sets submitted to an intercomparison program illustrates theneed for rigid checking procedure of analytical data and for good "book-keeping"
practices.
Samples Oil, 012 and 013 were analysed at Chemistry Division, DSIR, New Zealandby use of an iterative procedure. The waters were first analysed by comparison withindividual standards for each species. On the basis of these results, three artificialwaters were recreated gravimetrically with chemical compositions closely matchingthose of the original waters. These artificial waters were then used as standards for
448
the re-determination of the original samples thus eliminating matrix effects andchemical interferences. The results obtained can be expected to be as close to "true"values as possible for the species Li, Na, K, Rb, Cs, Mg, Ca, Sr, Al, Fe, As, B andSiO2 determined by atomic absorption or emission spectrophotometry.
The remaining parameters pH, F and NH3 were determined by use of specificelectrodes against gravimetric standards on samples stabilised by HNO3 or formalin,sulfate was determined gravimetrically as BaSO4, HCO3 by blank-corrected alkalinitytitration (Giggenbach and Goguel, 1986).
Samples 001 and 002 were chemically stable and analytical values for all three sub-samples (filtered, acidified, preserved with formalin) agreed closely. Both filteredand formalin treated samples of 003 (morere) continued to precipitate minoramounts of iron hydroxide, the filtered and acidified ones turned yellow due to theoxidation of iodide, the formalin treated one remained colorless.
EVALUATION
Due to the large diversity of analytical techniques applied by a wide range of
laboratories of possibly highly variable standards, statistical techniques based on theassumption of normal distribution around a mean are of little value. In order tominimise the effects of far outliers, a technique based on the distribution ofobservations around the median value of a given samples has been chosen. The "box-whisker"-plots, as proposed by Tukey, (1977) were modified as shown in Figs 1 to 3.There the analytical data for a given species, normalised to the median value arepresented on a logarithmic scale. The boxes extending to the left and right representeach 25% (quartile) of the data deviating to higher or lower values thusencompassing 50% of all data. Values outside this range are indicated by theirlaboratory number. The size of the boxes *'c an indication for the error associatedwith the determination of a given species by the "better half of laboratories.
Of the species considered in Figs 1 to 3, for some the scatter of results is within areasonable range (pH, Li, Na, K, Ca, F, Cl and B) for all three samples, for some,however, the range of results exceeds the two order of magnitudes covered (Rb, Mg,
449
1/5 1/2 median 2x 5x
H*
Li
No
K
Rb
CsIVg
Co
FeF
asq
MOO,
B
so2
o
(§M2><SX!3EH
© ——————— ©-©
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@@ ———— © ——— I
<ix v¿J vc5!ü(_
(ÍISSí
1 — ®~ ©-©©
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———— © ——————— ©21 06 1 —————— © — © ——— 1 06 23 J3
]-@@d5i — ®-@-*-§U 17 -«-09 1021 11 02 05 15 oT
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to ———————————————— ©it
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median8020
196
72
010028
021
80OO4
20
133
330
300
130146(mg/kg)
1 0.2 03 04 ' 06 ' ' 10 2 3 4 ' 6 ' ' ' 10 x
Fig. 1 - Comparison of analytical results for sample 001 (Waikite) in terms of
deviations from median. Best estimates of "true" values are indicated by
heavy bars Boxes contain 25% of values deviating to higher or lowervalues. Arrowed boxes indicate data outside the range of the figure.
1/5
Ca
Fe ••+J0910 _ J±?_L __ _ __ 11 U_ __ __ __ __ 0 2 W I 5
0.1 Q2 03
nedian85107
1170
16722
20
005197
0.04
76
197O
35
40
26382
Fig. 2 - Comparison of analytical data reported for sample 002 (Wairakei).Dashed lines enclose values too widely scattered to permit meaningful
statistical evaluation. Arrowed number indicate detection limits given.
450
1/5median
Csl'OHJ -"020617-22 19 13
Fig. 3 - Comparison of analytical results for sample 003 (Morere). For details seecaptions Fig. 1 and 2.
Fe, SO4, HCO3). In these cases any statistical treatment with the aim to arrive at the"true" composition of the samples is futile. In order to be able to select a range ofanalytical results most likely approaching this "true" value, the laboratories wereclassified in terms of the number of results outside the two quartiles shown in Figs 1
to 3 for pH, Li, Na, K, Cs, Ca, F, Cl, B and SiO2. Four groups of laboratories aredistinguished:
TABLE 5
Number of outliers:
Laboratory:
0-1
0911
1719
25
2-40203102022
5-100607
1314
21
10-200405081523
By use of the data reported by the first, most reliable group, together with thoseobtained at Chemistry Division, DSIR (01) an attempt was made to derive a bestestimate of the "true" composition of the three samples.
451
For sample 001, these estimates are very close to the median values derived earlier,except for iron for which a value below the detection limit of 0.01 mg/kg wasaccepted. The same conclusion for iron was reached for water 002. In addition, the
DSIR value of .004 nig/kg was accepted for Mg and a value of <5 mg/kg for HCO3,
well below the median values. A HCO3 value of 40 mg/kg was obtained here atDSIR as the result of a simple alkalinity titration. After correcting for the effects ofthe considerable amounts of the weak acids H3BO3 and H4SiO2 present, by "back
titrating" the samples with NaOH from pH 3.8 to their original pH (Giggenbach and
Goguel, 1986), the amounts of HCO3, however, were found to be below the detection
limit of the technique employed. Most of the low values reported for SiO2 are likely
to represent monomeric silica contents. As most silica geothermeters are based on
total silica contents, the much higher total silica contents as determined by AA areaccepted as "true" values.
A special effort was made to determine Cs in sample 003, therefore, the very lowvalue of 0.004 mg/kg as measured by DSIR is adopted. Similarly an upper limit of
0.05 mg/kg is accepted for Al, of 0.15 mg/kg for iron. The iron content of theacidified sample was 0.90 mg/kg but had dropped to 0.15 mg/kg in the filteredsamples by the time they were sent out. They may have decreased further during
shipment. Again a blank corrected lower value of 30 mg/kg was taken for HCO3,
with sulfate likely to be well below the limit of 3 mg/kg.
These best estimates of the true composition of the three waters are shown in Figs 1to 3 as solid bars, they may be used to judge the accuracy of individual values.
DISCUSSIONA detailed point by point discussion of the results is beyond the scope of this general
evaluation. Some general conclusions, however, may be drawn.
One of the major uses of chemical data is geothermal investigations consists of the
valuation of water-rock equilibration conditions at deeper levels. Application of such
geo-indicators relies largely on Na, K, Mg, Ca and SiO2-contents of the discharges. Alook at the results reported by the "better half of laboratories , as indicated by thesize of "boxes" in Figs. 1 to 3, shows that the errors for Na, K and Ca are within
452
reasonable limits (5-10%) for all three samples. For Mg, however, scatter increasesrapidly with decreasing concentrations. For sample 003 containing 80 mg/kg, thescatter is within 10-15%, it increases to a factor of three for sample 001 (0.21 mg/kg)and spans several orders of magnitude for sample 002 (0.004 mg/kg).
The effects of these errors on the evaluation of water-rock-equilibration temperaturesby use of a recently proposed technique (Giggenbach, 1986) are shown in Fig. 4. ForWaikite (001) the apparent range of K/Na-temperatures extends from 100° to 200°,K/Mg-temperatures from 60° to 140°. For wairakei (002), most data points are closeto the measured and the full equilibrium temperature of 260°. The results for 042,
052, 232, however, would give a completely unrealistic picture. Even for the high Mgwater 003, an unacceptable scatter is observed. The range of silica contents reportedfor the low (27 mg/kg) to intermediate (146 mg/kg) silica waters (003, 001) arereasonable.
CI/100
Li B
Fig. 4 - Evaluation of K-Na and K-Mg-water-rock equilibration temperatures(Giggenbach, 1986) by use of analytical results reported for Na, K, Mg for
Waikite (001), Wairakei (002) and Morere (003) geothermal discharges.
453
There silica is largely present in the monomeric form and colorimetric and atomic
spectroscopic values agree. In the case of the high silica water from Wairakei (002),however, the discrepancy of values obtained by the two techniques becomesunacceptable.
Another application of geochemical data consists of the evaluation of possibly source
components by use of "conservative" components such as Cl, B and one of the alkalisLi, Rb, Cs In Fig. 5 relative Cl, Li and B contents are shown for all three waters.Again considerable uncertainties are introduced by the scatter in the data reported by
some laboratories.
The main reason for likely errors in the analysis of low HCO3 waters has beendiscussed above (lack of back titration with NaOH to account for the presence of B,
Na/1000
K/100 200* K / Mg - terrperatunt (*C)300*
Fig 5 - Relative Cl, L» and B - results reported for Waikite (001), Wairakei (002)
and Morere (003) geothermal discharges.
454
SiO2 and other weak acids). Sulfate results for the two higher sulfate waters (001,002) are quite reasonable, most of the laboratories could not cope with the very lowsulfate water 003 for which much too high values were reported.
Accurate Al data are required in many computer studies of water-aluminium silicateequilibration, but only less than half of the laboratories report results for thisconstituent and then often only detection limits. The few values given are within anacceptable range, in contrast to results for iron, another species required in thecomputer simulation of mineral-water interactions. All three waters are likely tocontain iron in very low amounts (<.05 mg/kg), values reported, however, rangebeyond 1.0 mg/kg.
Of the constituents determined only by a few laboratories, iodide values for Morere(003) are very consistent but appear to be unreliable for the other two waters. Thevariations in NH3 reported may be due to some interference from biological activityin the unstabilised samples. The agreement of the results by many laboratories forWairakei (002) and Morere (003) with the DSIR results, obtained on acidified andsterilised samples, suggests that biological effects were not important. Strontiumvalues agree for the Morere water (003), as do Hg-data.
Interlaboratory checking procedures are obviously much better developed for isotopicmeasurements. Most deuterium and oxygen-18 values are well within acceptableranges of ±27°° and ±0.27°° respectively, except for 003. There some problems mayhave been introduced by the high salt content of this water.
RECOMMENDATIONS
The major purpose of the present intercomparison program was the identification of
deficiencies in the analytical procedures of laboratories involved in the geochemistryof geothermal systems. The results obtained clearly show that there is ample roomfor improvement. To a large degree, however, it is up to individual laboratories toassess their performance and to decide on possible avenues of improvement. In some
455
cases this may involve upgrading of existing facilities, eg, analysis of Mg and SiO2 byatomic absorption spectrometry rather than wet chemical techniques; the
introduction of more suitable techniques or the application of stricter calibration and
quality control procedures. Only very minor errors are likely to have been introduced
by the use of the simple sampling technique. Considering eg, the excessive range ofiron values reported, only little would have been gained by distributing a second
acidified sample.
REFERENCES
Ellis A.J. (1976) The IAGC interlaboratory water analysis comparison programme.
Geochim. Cosmochim. Acta 40, 1359-1374.
Giggenbach W.F. (1986) Geothermal solute equilibria, derivation of Na-K-Mg-Cageoindicators. Geochim. Cosmochim. Acta 52, 2749-2765.
Giggenbach W.F. and Goguel R.L. (1986) Methods for the collection and analysis ofgeothermal and volcanic water and steam samples. Chem. Div., DSIR, Report, pp.60.
Tukey J.W. (1977) Exploratory Data Analysis. Addison-Wesley Publ. Comp. 27-56.
456
LISTA DE PARTICIPANTES
Eduardo ALMEIDA, Jefe del Proyecto Geotérmico, Instituto Ecuatorianode Electrificación, Quito, Ecuador
Peter BLATTNER, Institute of Nuclear Sciences, Department ofScientific and Industrial Research, Lower Hutt, Nueva Zelandia
Egizio CORAZZA, Istituto di Geocronologia e Geochimica Isotópica,Pisa, Italia
Werner GIGGENBACH, Chemistry Division, Department of Scientific andIndustrial Research, Petone, Nueva Zelandia
Roberto GONFIANTINI, Organismo Internacional de Energía Atómica,División de Ciencias Físicas y Químicas, Vienna, Austria
Pablo HERNÁNDEZ PANAMEÑO, Jefe del Centro de InvestigacionesGeotérmicas, Comisión Ejecutiva Hidroeléctrica del Rio Lempa,San Salvador, El Salvador
Julieta JAUREGUI, Proyecto Geotérmico, Ministerio de Energía > Minas,Caracas, Venezuela
Alfredo LAHSEN, Decano de la Facultad de Ciencias Físicas y Naturales,Universidad de Chile, Santiago, Chile
Alfredo MAINIERI, Jefe del Proyecto Geotérmico, InstitutoCostarricense de Electricidad, San José, Costa Rica
Karla MIRANDA, Proyecto Geotérmico, Instituto Nicaragüense de Energía,Managua, Nicaragua
Rafael MURILLO, Jefe del Laboratorio Químico de Miravalles, InstitutoCostarricense de Electricidad, San José, Costa Rica
David NIEVA, Jefe del Departamento de Geotermia, Instituto deInvestigaciones Eléctricas, Cuernavaca, Morelos, México
Pietro NOTO, Istituto Internazionale per le Ricerche Geotermiche,Pisa, Italia
Hector PANARELLO, Instituto de Geocronologia y Geología Isotópica(INGEIS), Universidad de Buenos Aires, Argentina
Costanzo PANICHI, Istituto Internazionale per le Ricerche Geotermiche,Pisa, Italia
Javier RODRÍGUEZ C., Departamento de Química, Universidad Mayor de SanSimón, Cochabamba, Bolivia
Alberto RODRÍGUEZ M., Decano de la Facultad de Ciencia y Tecnología,Universidad Mayor de San Simón, Cochabamba, Bolivia
Alfredo ROLDAN M., Jefe de la Sección Geoquímica, Instituto Nacionalde Electrificación, Ciudad de Guatemala, Guatemala
Giovanni SCANDIFFIO, Unita Geotérmica Nazionale, Ente Nazionale perl'Energia Elettrica (ENEL), Pisa, Italia
José Luis SIERRA, Dirección de Desarrollo Geotérmico, Ente Provincialde Energía del Neuquén, Neuquén, Argentina
Paolo SQUARCI, Director, Istituto Internazionale per le RicercheGeotermiche, Pisa, Italia
Enrique TELLO, Oficina de Geoquímica, Comisión Federal deElectricidad, Morelia, Michoacán, México
Alfred H. TRUESDELL, Geological Division, Branch of ExperimentalGeochemistry and Mineralogy, U.S. Geological Service, MenloPark, California, USA
Leonel VACA, Proyecto Geotérmico, Instituto Costarricense deElectricidad, San José, Costa Rica
Pablo VÁRELA, Coordinador General del Proyecto Geotérmico, Ministeriode Energía y Minas, Caracas, Venezuela
Eulalia VIVES, Proyecto Geotérmico, Instituto Costarricense deElectricidad, San José, Costa Rica
Antonio YOCK, Proyecto Geotérnr-,o, Instituto Costarricense deElectricidad, San José, Costa Rica
Gian Maria ZUPPI, Dipartimento di Scienze della Terra, Universitá diTorino, Torino, Italia
457
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