(Begin page 1719)
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.