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PAGINA PRINCIPAL CONTENIDO INDICE DE AUTORES

ANTERIOR SIGUIENTE C O L E G I O

D E G E

O L O G O S

D E B O L I V I A

SEQUENCE STRATIGRAPHY OF THELATE SILURIAN - DEVONIAN SUBANDEAN BASIN,

SOUTHERN BOLIVIA & NORTHERN ARGENTINA

MIRANDA, A. P.1

, SOUZA CRUZ, C. E.2

, OLLER, J.3

(1) Petrobrs Internacional S.A., BRASPETRO, Rio de Janeiro, Brazil (e-mail: aloisio@braspetro.petrobras.com.br )(2) Petrobras S.A., CENPES, Rio de Janeiro, Brazil

(3) Consultant geologist, Santa Cruz de la Sierra, Bolivia

INTRODUCTION

Due to the recent discovery of several giant gas fields inthe Subandean Basin in both Argentina and Bolivia, it hasbeen noticed an increasing interest in detailing thestratigraphic setting of the Devonian reservoirs in theseareas. Although extensive regional geological studies hadbeen already performed, they basically consisted in alithostratigraphic description. The absence of a suitablechronostratigraphic control was a major constraint for theunderstanding of the late Silurian - Devonianpaleogeography (Figs. 1 and 2).

The present work intends to apply a modern sequencestratigraphic approach on the late Silurian - Devoniansection within the study area, in order to provide morereliable chronostratigraphic correlations between outcropand well data. In this sense, selected outcrops werestudied, regarding their palynological content, sedimentaryfacies and related processes, depositional systems andsequence stratigraphic elements. Well data, some of themrelated to recent gas discoveries, were also regionallyintegrated, providing a more consistent stratigraphic viewthroughout the Subandean Basin.

STUDY AREA AND METHODOLOGY

The remarkable compressional deformation presentlyobserved in the phanerozoic section of the SubandeanBasin is a result of the Andean Orogeny, which recordedits maximum activity during Miocene Pliocene times. Thisdeformation propagated eastwards through the studiedSubandean Basin, creating three major physiographicfeatures: (1) the Eastern Cordillera, mainly composed byCambrian Devonian outcrops, (2) the Subandean zone,mainly composed by Carboniferous Tertiary outcropsand (3) the Beni Chaco Plains, mainly composed by

Quaternary outcrops (Fig.1).The Eastern Cordillera and surrounding areas presentexcellent outcrops of Silurian and Devonian rocks,observed through narrow roads and canyons along fewmajor rivers. Fieldwork logistics in such kind ofenvironment are very complex, mainly related to transportand accommodations. In this difficult terrain, two fieldcampaigns were performed during the present study,aiming at describing selected outcrops between Tarija and

Sucre, in southern Bolivia. In the first campaign, fromOctober to December 1997, seven outcrop sections weremeasured and preliminarily described. During the secondcampaign, in October, 1998, a detailed facies analysis,reconnaissance of depositional systems and sequencestratigraphic key elements were carried out along theseven previously measured sections, together withscintillometer measurements in more easily accessibleoutcrops (Sella, La Yesera and Sobo Sobo sections). Amodified Selley type composite stratigraphic column,including available scintillometric data, sedimentary faciesand environment description and sequence stratigraphicelements, was performed for each studied outcrop section(Souza Cruz et al., 2000). Also, 94 rock samples werecollected in both campaigns for palynological studies inPETROBRAS RESEARCH CENTER (CENPES) (Melo etal., 2000).

A field trip guide for each outcrop section was prepared inBRASPETRO, Rio de Janeiro, Brazil, including extensivephotographic data and detailed facies description. Lateron, several electric log data from exploratory wells wereintegrated with the outcrop data already calibrated withpalynological information, generating regional stratigraphicsections, with a sequence stratigraphic approach,throughout the studied area (Souza Cruz et al., 1999). Themain constraint for a proper sequence stratigraphicanalysis of the Subandean Basin is the restoration of thepaleogeographic setting of most of phanerozoicsediments, due to the intense tectonic deformationpromoted by the Andean Orogeny. Therefore, any regionalstratigraphic sections and paleogeographic maps musttake into account such limitation.

DATABASE

The following 7 outcrop sections were studied in southern

Bolivia: Sella, La Yesera, Balapuca, Pilaya, Huacareta,Sobo Sobo and Tarabuco (Fig.1).

Electric log data were analysed from the following 10exploratory wells: Chango Norte 1 (ChN X-1), Porcelana

1001 (Po X-1001), Aguarague profundo 1 (Ag Xp-1), in Argentina; Bermejo 44 (BJO - X44), San Alberto 10(Sal - X10), Sabalo 1 (Sbl - X1), Huayco 1 (HYC - X1),Caigua 11 (CAI - X11), Cumandairenda-1 (CMD - X1)and Iau- 2 (IAU X2), in Bolivia (Fig.1).

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boundaries are not always clearly recognized, since theyare commonly recorded within a more homogeneoussand-prone interval.

The criteria for identification of sequence boundaries arelisted as follows: evidence of a regionally defined erosionaltruncation; basinwards shifts of facies and environments;evidences of regional exposure and subsequentravinement by transgression; changes in parasequencestacking patterns, from progradational to retrogradational,and hiatuses indicated by biostratigraphy.

Several schematic cross-sections were performed in thisstudy, detailing the stratal surfaces, system tracts,sequence orders and facies relationships. The selecteddatum for the mentioned cross-sections (see example inFig.3) is the first marine flooding surface above the latestrepresentative sand body of the Huamampampa Fm. .

The 2nd order sequences present their maximum floodingsurfaces (MFS) in the more distal parts of the Kirusillas,Icla and Los Monos Fms., within radioactive shaleintervals. At least in the Icla and Los Monos Fms., the MFSis found close to their basal portions, indicating a rapidsea-level rise, due to an eustatic and/or strong tectonicsubsidence effects.

The 3rd order sequences observed in a pericratonic setting(ramp-type basin), such as the Subandean Basin, aregenerally thin and poorly differentiated when compared totheir passive margin equivalents. Slow subsidence rates,low depositional slopes or ramps and shallow water depthscollectively result in reduced sediment accommodationwithin the basin. Thin sheet-like sequences arepredominant, although laterally persistent, and only few ofthem present recognizable progradational geometries anderosional boundaries.

During sea-level lowstands in a ramp-type basin, little orno accommodation space is available. Sediments mainlybypass the proximal areas towards the basin and,consequently, lowstand deposits may be poorlydeveloped. Thus, the studied Silurian - Devoniansuccessions are commonly comprised by stackedtransgressive to highstand deposits, separated by near-planar unconformities or paraconformities, and floodingsurfaces commonly coincide with sequence boundaries.

Seaward of the mouth of incised rivers, shorelineregression takes place during lowstands. In opposition to

normal regressions, common in sea-level highstands, thesource of sediments is provided by the cannibalization ofthe exposed platform and coarse-grained sediments maybe then provided to a more basinal setting. This processhas been referred to as forced regression (Posamentieret al., 1992), and explains the long-distance shorelineregression across the shelf observed in lowstand depositsof the Huamampampa Fm..Paleocurrents observed from sedimentary structuresformed by unidirectional currents (tabular and trough

cross-stratification; sigmoids) show a predominantsedimentary supply direction from south to north in thestudied area. As a consequence, most of the thicker andcleaner fluvial to paralic sand bodies are predominant innorthern Argentina and southernmost Bolivia, whilethinner, more argillaceous sand packages mainlydeposited in a storm-dominated shallow marine platform,are found in south to central Bolivia (Souza Cruz et al.,2000).

Fig. 2 Simplified Stratigraphic Chart Late Cambrian toDevonian Subandean Basin.

MEGASEQUENCE DESCRIPTIONS

TAK Megasequence

Only the upper part (HST) of the TAK megasequence wasobserved in few outcrops and logs within the studied area.It is predominantly constituted by wave-dominated shelfdeposits. Its uppermost shale-prone portion is related tothe top of the Tarabuco Fm., informally called cuello

pelitico or Jumbate Fm. (Fig. 2).ISR Megasequen ce

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The ISR megasequence may be regionally correlatedwithin the studied area. Its sequence boundary (SB) isrepresented by a regional unconformity. A thick sandysection, deposited in fluvial to paralic environments,represents the lowstand tract (LST) (Santa Rosa Fm.). Aravinement surface (TS) is regionally found at the base ofthe Icla Fm.. The transgressive tract (TST) is thin, sincethe subsequent rapid relative sea-level rise led to theestablishment of the maximum flooding surface (MFS)during Pragian. The highstand tract (HST) is composed ofcoarsening-upward parasequence cycles, deposited in astorm-dominated shallow marine platform (Figs. 3 and 4).

LMH Megasequence

The sequence boundary of this megasequence isrepresented, in northern Argentina and southern Bolivia,by a regional unconformity, recognized in both outcropsand well logs, formed in response to a sea-level fall duringthe Early Emsian (Melo et al., 2000). In the northernmostpart of the studied area, the occurrence of an almostcontinuous shaly section from Pragian to Early Givetianage makes difficult the traditional lithostratigraphic divisionbetween Icla and Huamampampa fms., although theproper palynological control allows the distinction between3rd order sequences within the same interval, in the LMHmegasequence (Fig.3). A regional transgression started inthe Early Givetian (TS is found in uppermostHuamampampa Fm.), rapidly reaching the MFS during theMiddle Givetian (basal part of the Los Monos Fm.). Theintense erosion promoted by the Early Hercinian Orogenyhas removed most of the transgressive and highstanddeposits in the studied outcrops (Figs.3 and 4).

The large dataset related to the LMH megasequenceallowed a better application of the sequence stratigraphyconcepts. The LMH-1 to 4 sequences are recognized inboth outcrop and well log data (Fig.3). The LMH-1 and 4

sequences clearly show the basinward sediment bypass.During the sea-level lowstand, forced regressions led tothe deposition of fluvial-estuarine or fluvial-deltaicsediments to the north, in the distal portions of the basin.This model explains the occurrence of thick isolated sand-prone intervals deposited in predominant muddy offshoreenvironments northwards. The LMH-2 and 3 sequencesbasically comprise stacked transgressive to highstanddeposits separated by sequence boundaries, representedby ravinement surfaces. The maximum flooding surface issituated close to the sequence boundary (SB). Lowstanddeposits, if present, might be found northwards from thestudied area. The LMH-4 sequence presents theuppermost lowstand deposits related to theHuamampampa Fm.. Its basal ravinement surface definesthe start of the transgressive succession that reached itsmaximum expression through the deposition of the shale-prone Los Monos Fm. (Figs.3 and 4).

CONCLUSIONS

The results of this work clearly show that the sequencestratigraphic concepts can be applied to the late Silurian Devonian section in the Subandean Basin. The analysesof sedimentary facies, palynological dating and recognitionof key surfaces provide the understanding of the geometryof the sequences and their related controlling factors. Gasreservoir distribution can be regionally assessed by thecorrelation of 3rd order sequences. In general, theHuamampampa gas reservoirs are mainly represented byparalic widespread highstand sand bodies depositedduring normal regressions. To the north, they may belocally present, mostly related to paralic isolated lowstandsand bodies deposited during forced regressions. TheSanta Rosa gas reservoirs are related to fluvial - deltaiclowstand sand bodies, widespread in the studied area.

REFERENCES

Haq, B.U.; Hardenbol, J.; Vail, P.R. 1988 , Mesozoic andCenozoic chronostratigraphy and cycles of sea-levelchange. In: Wilgus, C.K.; Hastings, B.S.; Ross, C.A.;Posamentier, H.W.; Van Wagoner, J.C.; Kendall,C.G.St.C. (Eds.) , Sea level changes: an integratedapproach: Society Economic Paleontologists MineralogistsSpecial Publication 42, p.71-108.Melo, J. H. G., 2000 , Palynological Evaluation and

Correlation of Some Silurian-Devonian Sections ofSouthern Bolivia; in this edition.Mitchum, R.M.Jr.; Van Wagoner, J.C. 1991 , High-frequency sequences and their stacking patterns:sequence-stratigraphic evidence of high-frequencyeustatic cycles. Sedimentary Geology, v. 70, p. 131-160.-Posamentier, H.W., and G.P. Allen, D.P. James, and M.Tesson, 1992, Forced regression on a sequence

stratigraphic framework: concepts, examples, andexploration significance: AAPG Bulletin, v. 76, p.1687-1709.Souza Cruz, C.E., Miranda, A.P., Oller, J., 1999 , Facies

Analysis and Sequence Stratigraphy of the Late Silurian-Devonian Subandean Basin, Southern Bolivia andNorthern Argentina; Internal report, PETROBRASINTERNACIONAL S.A. - BRASPETRO.Souza Cruz, C.E., Miranda, A.P, Oller, J., 2000 , Facies

Analysis and Depositional Systems of the Late Silurian-

Devonian Subandean Basin, Southern Bolivia andNorthern Argentina; in this edition.Vail, P.R.; Audemard, F.; Bowman, S.A.; Eisner, P.N.;Perez-Cruz, G. 1991 , The stratigraphic signatures oftectonics, eustasy and sedimentation. In: Einsele G.;Ricken W.; Seilacher A. (Eds.), Cycles and events instratigraphy. Springer Verlag, Hilderberg, p. 617-659.

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Fig. 3 - Figure 3 - Regional Dip (S-N) Stratigraphic Cross Section through Studied Outcrops(see Fig.1 for location).

Fig. 4 - Regional Dip (S-N) Stratigraphic Cross Section through Exploratory Wells andStudied Outcrops (see Fig. 1).