Toca Carbonate, Congo Basin: Response to an Evolving Rift Lake
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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Petroleum Systems of South Atlantic Margins
“Giant oil and gas fields have been discovered through recent exploration in the deep and ultra-deep water regions of Angola, Brazil, and Nigeria. This volume includes 1. Descriptions of the petroleum geology of more than 12 basins along the two margins 2. Discussion of various components of the petroleum systems within these basins 3. Discussion on the use of the petroleum systems concept in the South Atlantic marginal basins to classify and characterize the diversity of the systems 4. Methodologies used in the assessment of petroleum system components 5. Overviews of the different components of the region's petroleum systems. Petroleum explorationists and researchers working in the South Atlantic will welcome this addition to their libraries.
The printed version of Memoir 73 was the Winner of the 2000 Dott Award for best AAPG Special Publication.”