ABSTRACT: West Africa Lower Congo Coastal Basin: Petroleum Geochemistry and Source Potential of the "Bucomazi" Formation

BURWOOD, RALPH, PETROFINA S.A., Brussels, Belgium, and GORDON A. MCLANACHAN, PETROFINA Canadian Branch, Calgary, Alberta, Canada

Oil production in the prolific Lower Congo basin derives from a range of accretionary wedge, carbonate platform, and pre-salt clastic reservoirs, each representative of an evolving stage of the South Atlantic passive margin. A diversity of oil types and character indicates a complex hydrocarbon habitat drawing on multiple sourcing.

Prominent among potential source systems, often reaching thicknesses in excess of 2 km of organic-rich bituminous shales, is the rift phase Early Cretaceous (pre-salt) "Bucomazi" Formation. Here, localized graben/half-graben initiated lacustrine sedimentation evolves upward into that of a larger, coalesced water-body. Preempting irreversible oceanic ingress and the Loeme Salt forming desiccation event, a transition to marine-influenced depositional conditions ensued.

Representative of such diverse depositional conditions, "Bucomazi" Formation reduces to a generic sedimentological term. However, geochemical characterization confirms a high level of vertical and lateral inhomogeneity, embracing both source potential and organofacies development, in these sediments. Definition of these trends has been examined by the systematic geochemical and biostratigraphic study of the type section penetrated in Cabinda well 86-1 (Bucomazi-1).

Source potential plus kerogen type and kinetic character was determined by conventional Rock-Eval analyses. Kerogen carbon

isotope and biomarker-based parameters allowed segregation of the section into a succession of lacustrine through marine depositional events within a chronostratigraphic framework.

Zones of enhanced source richness could be correlated to specific environmental events marked by strong carbon isotopic excursions. The accompanying biomarker signatures showed potential as stratigraphic tools in the wider delineation of the "Bucomazi" depositional event and in unraveling aspects of basin architecture.

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