Abstract: Evolution of Late Cretaceous and Tertiary Depositional Fill and Structural Trends along the West African Continental Margin

Swanson, W. A.; D. A. Hartman, P. R. Smith, M. P. Campbell, K. C. Hood and West Africa Regional Team-Exxon Exploration Company

During the past 25 years, major exploration emphasis has focused on the larger fluvio-deltaic systems of Niger, Congo and Ogooue rivers where thick Upper Cretaceous to Tertiary strata and abundant structures are present. In recent years, deepwater exploration has increased significantly with the extension of proven plays and the development of new plays.

Ten offshore basinal depocenters exist in the eastern South Atlantic between Nigeria to southern Angola. These include the Nigeria, Douala, Ogooue, Congo, Kwanza, Benguela and Mocamedes depocenters. Important differences in crustal type, provenance and the absence or presence of a mobile substratum all impact the depositional fill and structural character within these continental margin depocenters.

Integration of gravity, magnetic and seismic data indicates deposition from West African rivers occurred on crust ranging from continental to oceanic. Crustal character influenced the regional geometry of the basins. The Niger and Congo River drainage areas are the largest in West Africa and their associated depositional fills are controlled by pre-existing basin architecture. The offshore Niger depocenter is deposited primarily on oceanic crust. This, coupled with heavy sediment load, has created the thickest sedimentary section in West Africa. Sediment within the Congo basin is deposited on attenuated continental crust. The sediment is relatively thick; however, much of the recent sedimentation from the Congo River bypasses the continental margin and is directed to the abyssal plain via the Congo canyon. This creates the modern Congo fan. Smaller river systems deposited sediment on more stable crust and resulted in smaller depocenters such as the Oogoue, Outer Kwanza, Benguela and Mocamedes.

Regional provenance analysis allows relative comparison of reservoir between areas with limited well data. Large drainage areas generally provide access to more mature, quartz-rich sediments. Local drainage areas can provide adequate sources of sands for reservoirs although the sediment composition is often less mature.

The distribution of Aptian evaporites or mobile shale within individual depocenters also strongly impacts the sediment fill. Areas with thicker salt deposition generally exhibit a higher degree of active deformation. In some areas this deformation is expressed on the seafloor and acts as a buttress trapping sediment updip.

Two primary structural episodes are evident along the West African margin. The first was initiated in the Albian and the second in the early Tertiary. Both vertical and horizontal processes are present and the nature of this deformation varies both spatially and temporally. Structures with a dominantly vertical component result from salt and shale deformation. Structures with a horizontal component result from thermal subsidence, uplift and loading. In general, extensional regimes exist updip while vertical and contractional regimes exist downdip.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil