Geology of Oil Fields and Future Exploration Potential in West African Aptian Salt Basin

Roger D. Bignell, Alan D. Edwards

The Aptian Salt basin of west Africa, extends from Equatorial Guinea southward to Angola, contains recoverable reserves estimated at nearly 4 billion BOE, and is currently producing 600,000 BOPD. The basin developed as a result of tensional forces between west Africa and South America initiated at the end of the Jurassic. The prospective sedimentary sequence ranges in age from Early Cretaceous (uppermost Jurassic in places) to Holocene and is divided by the Aptian transgressive sand and salt into a pre-salt, nonmarine, syn-rift sequence and a post-salt, marine, post-rift sequence.

Both the pre- and post-salt sequences contain several successful exploration plays, the most prolific of which are the Early Cretaceous nonmarine sandstone fields in tilted fault blocks of Gabon and Cabinda; Early

Cretaceous carbonate buildups on the margins of basement highs in Cabinda; Early Cretaceous transgressive marine sandstone fields in anticlines draped over basement highs in Gabon; Late Cretaceous shallow marine sandstone and carbonate fields in salt-related structures in the Congo, Zaire, Cabinda, and Angola; Late Cretaceous dolomites in structural/stratigraphic traps in Angola; Late Cretaceous/early Tertiary deltaic/estuarine sandstone traps formed by salt movement in Gabon, Cabinda, and Angola; and Tertiary marine turbidite fields in Cabinda and Angola.

Despite the exploration success in these trends, much of the basin is under or poorly explored. The major problems for exploration are the poor quality of seismic definition beneath the salt, which makes it difficult to predict pre-salt structure and stratigraphy, and the importance of a stratigraphic element in many of the post-salt traps, also difficult to detect on seismic. Exploration must therefore place a strong reliance on examination of the major factors controlling the formation of the fields in the basin to provide a series of geologic models for locating future reserves in this and other related passive margin basins.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.