Abstract: A New Pre-Salt Source Rock for Central West African Deep Water?

Bill Braumbaugh, Nick Cameron, Steven G. Henry

The syn-rift Organic Bucomazi is the source rock associated with the majority of oil accumulations in central West Africa. A younger pre-salt source rock is postulated to exist in early thermal subsidence sag basins. These basins have been observed to underlie deep water blocks in Angola, Congo and Gabon. Speculative data by Geco and TGSI characterize these sag basins' geometries and point to possible source-rock occurrences.

The younger source rocks in the sag basin are differentiated from the older Organic Bucomazi by their structural relationships. Unlike the syn-rift Organic Bucomazi, this new source rock is not offset by the rift faulting and is sub-parallel to the base of the salt rather than to the rotated basement blocks. Seismic facies define probable clastic shorelines, carbonate mounds and source rocks within this sag system.

Sag basin depocenters tend to lie just outside the 1000 meter isobath and are located between the major cross-basin basement arches. They developed in areas of high crustal extension and attenuation. This resulted in greater subsidence which produced more accommodation space for sediment deposition. Rift-associated heat flow would also be higher in these areas. Due to the combination of enhanced heat flow and the thickened syn-rift/sag basin sediments, the Organic Bucomazi is at risk for having passed through the oil window prior to deposition of the post-salt reservoirs. This proposed pre-salt sag basin source rock has the unique advantage of being capable of charging deep water prospects, where the Organic Bucomazi is overmature and the post-salt is immature.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994