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Abstract

AAPG Bulletin, V. 83, No. 11 (November 1999), P. 1729-1758.

Stratigraphic Control on Facies and Diagenesis of Dolomitized Oolitic Siliciclastic Ramp Sequences (Pinda Group, Albian, Offshore Angola)1

Herbert Th. Eichenseer, Frederic R. Walgenwitz, and Patrick J. Biondi2
 

©Copyright 1999.  The American Association of Petroleum Geologists.  All Rights Reserved

1Manuscript received December 19, 1997; revised manuscript received January 28, 1999; final acceptance May 25,1999.
2Elf Exploration and Production, CSTJF, 64018 Pau Cedex, France; e-mail: [email protected]

We wish to thank Elf Exploration Angola and the management of Elf Exploration and Production for permission to publish this paper. Special thanks go to our colleagues Y. Kalck, P. Allix, T. Boisseau, L. Jacquelin-Vallée, E. Rubio, and G. Haddad for their help and fruitful discussions. We especially acknowledge the help of E. Magon de St. Elier, who made the cathodoluminescence analysis. We are also grateful to C. Moore for suggesting this publication. The manuscript benefited from constructive comments of the AAPG reviewers A. Lomando, J. D. Humphrey, and an anonymous reviewer. Our thanks are given also to J. L. Berger for preparing the drawings. 

ABSTRACT

Pervasively dolomitized, oolitic to siliciclastic ramp sequences are prolific oil reservoirs in the Albian Pinda Group of northern Angola. During the Late Cretaceous, this reservoir series was segmented by salt tectonic gravitational gliding into halokinetic "turtle-back" and raft structures that provide hydrocarbon traps today. The Albian ramp sequences contain a large variety of reservoir facies, reaching from littoral sandstones with interparticle porosity to various types of dolomitized marine shelf to shoreline carbonates with moldic, vuggy, and intercrystalline porosity.

Reservoir prediction is substantially improved by an integrated approach combining high-resolution sequence stratigraphy with carbonate geochemistry. As high-resolution stratigraphy documents, most of the important sandstone reservoirs formed at the base of stratigraphic sequences during shoreface transgression. Pisolitic dolostones with moldic porosity are the best carbonate reservoirs. They developed as transgressive shoreface sand sheets above the basal sandstone reservoirs. Regressive facies tracts contain oolitic and pisolitic dolomites with intercrystalline and moldic porosity that originated in progradational barrier and sand-belt settings.

The combined stratigraphic and geochemical approach allowed us to determine the Pinda Group dolomites as early diagenetic in origin, whereas fluid inclusion and oxygen isotope analysis alone suggested a late burial dolomite formation. Dolomite neomorphism at higher burial temperatures modified most of the original geochemical signatures of early, near-surface dolomitization. Cathodoluminescence data and trace element analysis suggest that dolomitization was associated with gradually decreasing pore water salinity. Accordingly, barrier oolites were completely dolomitized and developed intercrystalline porosity during the earliest stages of diagenesis. At a later phase of early diagenesis, as waters became less saline and undersaturated with respect to calcium carbonate, fabric-selective dissolution of the remaining calcareous components occurred. The decrease in pore water salinity and associated porosity generation are genetically linked with high-frequency events of relative sea level lowering. This possible relationship implies that high-resolution sequence stratigraphy can be a valid reservoir prediction tool even in pervasively dolomitized series. 

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