ABSTRACT: Aptian to Cenomanian Post-Salt Sediments of the South Atlantic Salt Basin: Major Controls on Reservoir Quality

CRlPPS, DAVID W., and DAVID O. WILLIAMS, Simon-Robertson, Llandudno, Gwynedd, U.K.

Aptian to Cenomanian Post-Salt sediments contain important hydrocarbon reservoirs in several South Atlantic pericratonic basins (Pinda Group, Lower Congo/Kwanza basins; Macae Formation, Campos Basin), while in others their reservoir potential is underexplored (e.g., Madiela Formation, Gabon basin). In each basin, deposition occurred in an overall transgressive regime. Nonmarine to nearshore siliciclastic (locally evaporitic) deposits pass basinward through shallow marine carbonates into deeper marine terrigenous and carbonate mudrocks. Reservoirs occur in sandstones and carbonates deposited in nonmarine to shallow marine settings.

Reservoir character is determined by both depositional environment and diagenesis. The main reservoir facies comprises oolitic-oncolitic shoal carbonates. Reservoir facies distribution is controlled both by large-scale (eustacy basin subsidence, paleogeography) and localized (siliciclastic sediment supply, basement structure, growth faulting, halokinesis) factors. In the Campos and Kwanza basins, salt doming was apparently a major control on shoaling but may have been less important further north. Throughout the West African Salt basin, basement highs at a structurally controlled shelf edge also appear to have influenced shoaling. Usually, porosity in shoal carbonates has been modified by diagenesis. The main porosity-creating processes were dolomitization, carbonate dissolution, and racturing, whereas the principal porosity-destroying processes were calcite, dolomite, and anhydrite cementation. Since macroporosity in limestones is commonly occluded by calcite cement, the best reservoir quality typically occurs in dolomitized shoal sediments.

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