Abstract: Post-Salt Petroleum Systems in the Lower Congo Basin (Congo-Angola)

Reynaud, F. - Elf Exploration Production; Amaral, J. - Elf Exploration Angola; Drapeau, D. - Elf Congo; Grauls, D. - Elf Exploration Production

The Lower Congo Basin, bounded to the south by the Ambriz high in Angola and to the north by the Casmaria-Kaba ridge close to the Gabon-Congo border, is emerging as a new major prolific oil province in Africa. To date, up to 12 billion barrels of initial recoverable reserves have been discovered in this province, in several plays within Pre-Salt series (Neocomian to Aptian in age), and Post-Salt series (Albian to Tertiary in age).

The exploration results, the regional structural knowledge and a large volume of geochemical data from this basin, allowed us to define different major Post-Salt petroleum systems, characterize the key elements, and delineate the expected extension.

The reservoirs of the Post-Salt petroleum system are located in the Albian carbonate platform (Pinda and Sendji formation), in the Upper Cretaceous and Paleogene sandy deposits (labe formation in Angola), and in the Upper Tertiary (Malembo and Paloukou formations) as turbidite meandriforme channels.

Traps are mainly of a structural type in the Albian and Upper Cretaceous plays due to the gravity sliding tectonism whereas the stratigraphic component, in relation to the sedimentological environment, is also present in the Tertiary play.

The other key parameters of the Post-Salt petroleum systems (source-rock characteristics, maturity, pressure regime, geothermicity, schematic migration pattern, charge timing and oil-source correlations) are illustrated through examples from the Congo and Angola.

Three main source-rocks are involved in the Post-Salt petroleum systems:

- the marine Albo-Cenomanian interval (upper Sendji to Likouala fm in Congo), a mixed II-III O.M. with a low to fair potential,

- the marine Upper Cretaceous (labe and Madingo fm) mainly amorphous type II O.M. with a high to very high initial petroleum potential,

- the deltaic and basinal Upper Tertiary (Malembo and Paloukou fm) series, mainly amorphous type II-III with relatively low to medium initial petroleum potential.

Two different pressure domains exist in this basin: an overpressure domain, consequence of the high rate of Tertiary sedimentation and thickness which is located in the major Tertiary depocenter in Northern Angola (> 8500m of thickness); and a hydrostatic domain, characterized by a less Tertiary sediment thickness in the other basin areas.

These different illustrated cases which may be extrapolated to other parts of the Lower Congo Basin, allow us to better evaluate the petroleum potential, not only of prospective areas in our permits but also in deep offshore acreages.

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