Ephemeral River Reservoirs, Albian Petroleum Systems, Offshore Angola and Democratic Republic of Congo, Africa

Bryan Bracken¹ and Jessica Allen^2
1Energy Technology Company, Chevron, San Ramon, CA
²Geology and Geophysics, University of Utah, Salt Lake City, UT

Models for predicting reservoir architecture and quality of Albian reservoirs in central West Africa (Congo to Democratic Republic of Congo) are now emerging from decades of petroleum activity. Regionally, Albian depositional systems consist of shallow-marine carbonate sediments deposited onto unstable ramps on the margins of the Proto-Atlantic Ocean basin. Along the eastern parts of the basin, where continent-draining river systems reached the coast, mixed carbonate/siliciclastic sequences formed. At points of highest fluvial input coastal deposits were dominated by sand-rich fluvial and shallow marine systems. This poster presents a sub-regional synthesis of characteristics of Albian fluvial reservoirs. They consist of bedsets, up to 10 meters thick, composed of sharp-based, fine-grained sandstone composed of planar laminations and ripple cross-laminations (low- to high- angles of climb). Mudstone rip-up clasts commonly line erosional surfaces. The scarcity of dune-scale cross bedding and abundance of graded, upward-fining event beds indicate deposition by high-energy, unconfined sheetfloods. Presence of anhydrite, complete lack of vegetation, and red beds suggest sediment were deposited by flashy, arid (ephemeral), river systems. Where floodwater reached bays and lagoons, sediment formed subaqueous “terminal” fans or deltas. Based on modern and ancient, dryland, fluvio-deltaic systems these reservoirs characteristically have amalgamated, sand-rich, proximal facies, moderately good vertical connectivity, and extensive lateral continuity (>5 km). Reservoir quality, bed thickness, and connectivity/continuity decreases in the distal facies. Examples from the Sanha and Mafumiera fields in offshore Cabinda, Angola, and Thsiala field in offshore DRC will presented in this poster to illustrate reservoir architecture and quality.

AAPG International Conference and Exhibition, Cape Town, South Africa 2008 © AAPG Search and Discovery