ABSTRACT: Geology of the Lower Cretaceous Viodo Carbonate Offshore Congo

HARRIS, NICHOLAS B., Conoco, Inc., Ponca City, OK, and DONALD F. TOOMEY, Albuquerque, NM

The Neocomian Viodo Carbonate and its coeval facies, the Marnes Noires Formation and Toca Carbonates, were deposited in large lakes formed in the evolving rift as South America separated from Africa in the Early Cretaceous. The Marnes Noires is an organic-rich marl that sources much of the oil in the Congo basin. The shallow water Toca Carbonates are reservoirs in the southern Congo basin. The Viodo Carbonate, a largely allochthonous deposit of shallow water components that were redeposited in nearby deeper water, hosts Conoco's Viodo oil discovery.

The Viodo Carbonate comprises three units. The Lower Viodo Carbonate consists of coated grain, gastropod, and ostracod grainstones deposited as turbidites. Deposition of the Lower Viodo Carbonate was triggered by a fall in lake level. The Mid-Viodo shale comprises marl and minor thin carbonate grainstones (distal turbidites), deposited in quiet, deep water because of a rise in lake level. The Upper Viodo carbonate is a complex unit, consisting of carbonate conglomerates (dominant), grainstones, calcareous shales, carbonate mudstones, and minor siliciclastic sandstones. The conglomerates contain sand-to-boulder-size clasts of molluscan coquina and form discontinuous lenses encased in shale; they are interpreted as subaqueous debris flow deposits adjacent to a shallow water platform. Le ching and cementation in the coquinas suggest the sourcing platform was periodically exposed.

All facies in the Viodo carbonate have undergone complex diagenesis, including early dissolution, cementation of coquinas, silica cementation, late physical compaction, stylolitization, and dolomitization. Intensive dolomitization is necessary for good reservoir quality.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)