Abstract: From Pre-Salt Sources to Post-Salt Traps: A Specific Petroleum System in Congo Coastal Basin

Rene Vernet

The Bas Congo basin extends from Gabon to Angola and is a prolific oil province where both pre-salt and post salt sources and reservoirs have been found.

In the northern part of the basin referred to as the Congo coastal basin, the proven petroleum system is more specific: mature source rocks are found only in pre-salt series whereas by contrast 99% proven hydrocarbon reserves are located in post-salt traps.

Such a system is controlled by the following factors:

- Source rocks are mostly organic rich shales deposited in a restricted environment developed in a rift prior to the Atlantic Ocean opening.

- Migration from pre-salt sources to post-salt traps is focalized by local discontinuities of the regional salt layer acting otherwise as a tight seal.

- Post-salt reservoirs are either carbonates or sands deposited in the evolutive shelf margin developed during Upper Cretaceous.

- Geometric traps are linked to salt tectonics (mostly turtle-shaped structures).

- Regional shaly seals are related to transgressive shales best developed during high rise sea level time interval.

Stratigraphically, the age of hydrocarbon fields trends are younger and younger from West to East: lower Albian in Nkossa, Upper Albian and lower Cenomanian in Likouala, Yanga, Sendji, Upper Cenomanian in Tchibouela, Turonian in Tchendo, Turonian and Senonian in Emeraude.

AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France