Unraveling Complex Albian Reservoir-Trap-Seal Systems, Lower Congo Basin, Cabinda, Angola

Bryan R. Bracken
Chevron Petroleum Technology Company, San Ramon, CA

Rapidly accumulating sediments deposited over thick, low-density sedimentary sequences commonly leads to inverted, density stratification and formation of complex, syndepositional "growth" structures. Tectono-stratigraphic growth systems contain world-class reservoir/trap/seal combinations in West African and other petroleum basin of the world. This paper presents the characteristics of, and controls on, Albian shallow-marine reservoir/seal systems in the petroleum-rich, Cretaceous, Lower Congo Basin.

Continental rifting ended in the Lower Congo Basin in the late Aptian with the deposition of 100's to 1000's m marine-derived evaporates in restricted, Proto-Atlantic basins. By early Albian water of normal marine salinity was circulating across shallow ramp-like shelves onto which carbonate and siliciclastic sediment accumulated. When -1000 ft Albian, shallow-marine sandstone and carbonate sediment accumulation, a critical density instability threshold was reached and salt movement commenced.

In an on-going effort to understand uncertainties associated with Albian reservoir/seal systems ChevronTexaco, operator of Gulf Oil Cabinda Association (Block 0, Cabinda, Angola), has developed a sequence stratigraphic based, tectono-stratigraphic model for Albian hydrocarbon systems. The model is the product of the integration of 3D seismic, well log, and core data covering 3000 sq km into a dynamic, geological framework (sequence stratigraphic framework and synthesized tectonic history). It has been found that critical first-order controls on reservoir architecture, quality, and gross volume are proximity to major active salt-withdraw sub-basins and bounding faults and stratigraphic position (systems tract). Second order controls are geomorphic position (paleogeomorphic) and proximity to active sediment fluvio-deltaic sediment input points.