Relating Petroleum System & Play Development to Basin Evolution: West African South Atlantic Basins
Sedimentary basins can be classified according to their structural genesis and evolutionary history and the latter can be linked to petroleum system and play development. West African South Atlantic basins between Cameroon and Namibia have experienced similar tectonic and sedimentary basin evolution and are genetically related. Four basin cycles have been identified (pre-rift, syn-rift, transitional and post-rift), each associated with at least one type of petroleum system (PST). Two PSTs are regionally extensive and very productive: the lacustrine syn-rift and marine post-rift PSTs, while three minor PSTs, the fluvio-marine transitional, restricted hypersaline transitional and deltaic post-rift PSTs, are locally developed. Play development is closely related to basin tectonic and sedimentary evolution: syn-rift plays are associated with lacustrine/fluvial facies and trap geometries related to graben development, while post-rift plays include deltaic and shallow to deep marine clastic and carbonate facies in combination with traps which formed due to salt withdrawal. The number and variety of plays increases with basin evolution, as tectonics and sedimentary patterns become more complicated. Three basin families have been identified. Basin family I includes a lacustrine syn-rift section, followed by a fluvio-marine sand/shale and a restricted hypersaline evaporite unit (the transitional section), and ultimately by a marine post-rift section. It contains the lacustrine syn-rift, fluvio-marine transitional and marine post-rift PSTs. Basin family II is characterized by the same basin evolution, but has a thick deltaic wedge in the latest post-rift phase, which gives rise to an additional PST: the deltaic postrift PST. Basin family III is defined by the same basin evolution as basin family I, except that the transitional section is dominated by shales instead of evaporites, which are the source for the restricted hypersaline transitional PST. The Douala Basin, the pre-salt section of the Rio Muni and North Gabon Basin and the (ultra)-deep waters of the Ogooue Delta, and Lower Congo, Congo Fan and Kwanza Basin are under-explored areas with high exploration potential. In conclusion, the tectonic and sedimentary evolution of the individual West African South Atlantic basins has led to the development of similar PSTs and plays, which can be used as analogues for evaluating exploration opportunities in basins with similar geodynamic and sedimentary evolution.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009