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Abstract

The Toca Formation is a Barremian lacustrine carbonate in the late rift section of the Congo Basin, equatorial west Africa. It is an important oil reservoir in Cabinda, Angola; noncommercial discoveries exist elsewhere in the basin. However, the Toca is a difficult target to explore and develop due to major sedimentary facies changes over short distances. The Toca occurs at multiple stratigraphic levels, here designated Toca 1, Toca 2, and Toca 3 in ascending order. Toca 1 and Toca 2 are coeval with the Marnes Noires Formation (Middle Bucomazi Formation), an organic carbon-rich marl. Toca 3 is probably coeval with the lowermost Argilles Vertes Formation (Upper Bucomazi), an organic carbon-poor siltstone.

The Toca shows systematic stratigraphic variations that reflect decreasing lake salinity and probably climate change. Toca 1 consists primarily of allochthonous algal-derived carbonate clasts, including oncolites, algal grainstones, and gastropods. Toca 2 contains both in situ and allochthonous deposits of algal-derived clasts and pelecypod coquina formed by filter-feeding organisms. Toca 3 consists almost entirely of pelecypod shells and lime mud (no algal-derived bioclasts) and forms in situ carbonate banks. Diagenesis and reservoir quality are related to depositional facies. Porosity and permeability were enhanced by early dolomitization and early dissolution related to subaerial exposure. Porosity and permeability were adversely affected by early compaction (particularly in allochthonous facies), calcite cementation, and late dolomitization.

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