--> Abstract: Sedimentology, Mineralogy and Organic Geochemistry of Pre-Salt Formations, Congo Basin, West Africa: Implications for the Deposition of Lacustrine Source Rocks, by N. B. Harris; #90933 (1998).

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Abstract: Sedimentology, Mineralogy and Organic Geochemistry of Pre-Salt Formations, Congo Basin, West Africa: Implications for the Deposition of Lacustrine Source Rocks

Harris, Nicholas B. - The Pennsylvania State University

The Lower Cretaceous pre-salt (or syn-drit) sequence of the Congo Basin, West Africa, contains the primary source rocks for oils in onshore and shallow water areas, in addition to important hydrocarbon reservoirs. The sedimentology, mineralogy and inorganic geochemistry of the pre-salt section have been analyzed to develop models for lake hydrology and controls on deposition of organic-rich shales.

The Barremian Marnes Noires Formation is the most important petroleum source rock, averaging more than 5% TOC. It is carbonate-rich and siliciclastic-poor relative to other pre-salt shales, containing subequal amounts of calcite and dolomite. The presence of halite-cemented dolomites deposited near the margins of the lake indicates that conditions there were evaporitic; clasts of these dolomites preserved in sediment gravity flow deposits suggest that the deep lake was at or near halite saturation and that the lake was saline-stratified. This stratification may have been instrumental in preserving organic matter from degradation. Less abundant carbonate and the absence of evaporite minerals in other pre-salt Formations indicates that this stratification was unique to the Marnes Noires and suggests a primary factor in its remarkably high organic content.

Other indicators of lake chemistry employed in this study include sulfur content and carbon isotopic composition of calcite and dolomite. Sulfur is particularly high relative to other lake deposits, averaging 2 to 3% in all pre-salt formations. These indicators will be discussed in the context of the origin of lacustrine source rocks.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil