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AAPG Bulletin, V.
Depositional and diagenetic controls on the reservoir quality of Lower
Cretaceous Pendência sandstones, Potiguar rift basin, Brazil
Sylvia M. C. dos Anjos,1 Luiz F.
De Ros,2 Rogério Schiffer de Souza,3
Carlos Manuel de Assis Silva,4 Cristiano L. Sombra5
1Petrobrás Research Center, Ilha do Fundão, Cidade
Universitária, Q. 7, 21949900, Rio de Janeiro, RJ, Brazil; email: [email protected]
2Universidade Federal do Rio Grande do Sul, Instituto de Geociências, Av.
Bento Gonçalves, 9500, Agronomia, 91501970, Porto AlegreRS, Brazil; email: [email protected]
3Petrobrás Research Center, Ilha do Fundão, Cidade Universitária, Q. 7,
21949900, Rio de Janeiro, RJ, Brazil; email: [email protected]
4Petrobrás Research Center, Ilha do Fundão, Cidade Universitária, Q. 7,
21949900, Rio de Janeiro, RJ, Brazil; email: [email protected]
5Petrobrás Research Center, Ilha do Fundão, Cidade Universitária, Q. 7,
21949900, Rio de Janeiro, RJ, Brazil; email: [email protected]
AUTHORS
Presently at the head of the Reservoir Section of Petrobrás Research Center, Sylvia dos Anjos has developed research on the x-ray diffraction and electron microscopy characterization of clay minerals and on the construction of predictive models of reservoir quality. She graduated in geology at the Federal University of Rio de Janeiro in 1978 and received an M.Sc. degree in shale petrology in 1984 and a Ph.D. in reservoir geology in 1986 from the University of Illinois in Urbana.
Luiz De Ros worked for nine years on reservoir geology and clastic diagenesis in Petrobrás before joining the Rio Grande do Sul Federal University in 1990. He received his B.Sc. degree from the same university, his M.Sc. degree in reservoir geology from Ouro Preto University, Brazil, and his Ph.D. in sedimentary petrology from Uppsala University, Sweden. He has published on the characterization and quality modeling of clastic reservoirs from several basins in Brazil, Norway, Tunisia, and other countries.
Rogério Schiffer de Souza received his B.Sc. degree in geology from Vale dos Sinos University, southern Brazil, in 1980. He worked as teacher and researcher at the Federal University of Rio de Janeiro and received his M.Sc. degree there in 1989. Since 1987 he has worked on sandstone diagenesis and petrologic reservoir characterization at Petrobrás Research Center, and he received a Ph.D. on predictive modeling of reservoir quality from the University of Texas at Austin in 2000.
Carlos Manuel de Assis Silva received his B.Sc. degree in geology from the Federal University of Minas Gerais, Brazil, in 1987 and an M.Sc. degree in reservoir geology from Ouro Preto University, Brazil, in 1991. He has worked at Petrobrás Research Center since 1993, where his main activities deal with the sedimentologic and petrographic characterization of sandstone reservoirs and their application to integrated geological modeling.
Cristiano Sombra graduated in geology in 1977 from the Federal University of Bahia, Brazil, and worked from 1978 to 1985 for Petrobrás as well-site and development geologist. After receiving an M.Sc. degree in reservoir geology from the Federal University of Ouro Preto in 1987, he moved to Petrobrás Research Center, where he works on statistical and geological methods of reservoir quality prediction and on the combination of sedimentologic, log, and petrophysical characterization of clastic reservoirs.
ACKNOWLEDGMENTS
We thank Petrobrás for access to data and samples and permission to publish this work. LFDR acknowledges the support of the Brazil National Research Council (CNPq) and Rio Grande do Sul State Research Foundation (Fapergs). This article was substantially improved by the suggestions of S. Morad and of AAPG reviewers J. P. Hendry and L. Sanchez-Barreda, as well as of editors N. Hurley and M. Longman.
ABSTRACT
The quality of the reservoirs of the Lower Cretaceous Pendência Formation of the Potiguar basin, northeastern Brazil, is directly controlled by depositional facies-related carbonate cementation and compaction. The study of the interplay of these processes in the reservoirs offers an opportunity to unravel the diagenetic patterns of clastic sequences in interior rifts and, in particular, the role of carbonate cementation in poorly understood continental systems.
The Pendência Formation is a thick sequence of fan-deltaic, fluvial-deltaic, turbiditic, and lacustrine sandstones, conglomerates, and shales deposited during the rift stage of the basin. The sandstones are predominantly arkoses (average Q49F40L11), with subordinate plutonic and volcaniclastic feldspathic litharenites. Compaction and cementation had similar importance in the destruction of porosity, with a dominance of cementation in the turbidites and of compaction in the fluvial deposits.
Carbonate cementation in Pendência reservoirs increases progressively with depth and is facies controlled. Eodiagenetic, nonferroan calcite I (d18OPDB 10.7 to 4.0; d13CPDB 17.5 to +8.5), mesodiagenetic rhombohedral ferroan dolomite/ankerite (d18OPDB 9.3 to 3.9; d13CPDB, 1.7 to +1.1), and ferroan calcite II (d18OPDB 17.2 to 6.8; d13CPDB 13.6 to +2.3) were precipitated at three distinct temperature intervals calculated from the d18O values: 21 to 58°C, 70 to 79°C, and 85 to 150°C, respectively. According to the d13C values, dissolved carbonate for calcite I was derived from oxidation and methanogenic fermentation of organic matter and from methane oxidation. Ferroan mesogenetic cements were derived from thermal decarboxylation of organic matter. The shales were a major source of dissolved (Begin page 1720) carbonate, as indicated by the isotopic similarity between their calcite (d13CPDB 0.2 to +1.8; 87Sr/86Sr ~ 0.719) and most cements in the sandstones and by the peripheral cementation along the contacts of interbedded sandstones. As a result of this cementation pattern, thin turbiditic and deltaic sandstone beds are pervasively cemented.
The best reservoir quality potential is encountered in the partially cemented fluvial sandstones at moderate depths. Deltaic and turbiditic sandstones are more pervasively cemented by carbonate derived from the interbedded shales. Alluvial-fan conglomerates and sandstones were flushed by telogenetic meteoric waters close to the borders of the basin and to the proximity to the postrift unconformity. However, porosity enhancement was very limited, due to the precipitation of kaolinite and the intense compaction related to the compositionally immature detrital framework.