Passive Continental Margins of West Africa and Unusual Features of the Oil and Gas Potential of Their Deep-Water Part

摘要

The West African region is bounded by a passive continental margin confined to a steadily subising zone. It covers sedimentary basins that extend to the deep-water areas of the Atlantic Ocean; they are the Gulf of Guinea, Cuanza-Cameroonian, and Namibian basins. The oil-and gas-bearing basins of this region have much in common in the history of their geological development and tectonic-stratigraphic evolution. Within the sedimentary filling of the basins one can distinguish three structural units commonly separated by discordances or major stratigraphic gaps. The lower unit corresponds to the sequence of the rift sediments mostly represented by the red-colored continentical rocks of the Late Jurassic and Aptian age. The intermediate structural unit is composed of the rocks of the carbonate and terrigenous-carbonate sequences of the Late Jurassic-Paleo-gene. The upper structural unit involves the sediments covering the shelf and numerous slope areas and corresponding to the Miocene and later periods. The overall recovered reserves in this region exceed 3.1 Gt. About 3 half to these reserves is confined to the deep-water areas of the continental slopes with a sea depth of 900-3000 m. The deposits of the Late Cretaceous and Middle Paleogene are considered to be the source rocks for oil and gas. Enormous hydrocarbon accumulations were discovered in the deep-water fans or in the turbidite sequences.The reservoir rocks of the Oligocene-Miocene and Aptian-Cenomanian age feature a high porosity (up to 30%) and enhanced permeability (up to 3-4 darcy). The fields have a multilayer character and are represented by stratigraphic, tectonically screened, and combined structures. The studies showed that all the findings are located over a line running from the deep-water regions off Nigeria in the north to the South African Republic in the south. However, in the north, this trend does not terminate off Nigeria; rather, it extends over the deep-water parts off North and Northwest Afria to the areas off Sengal, Mauritania, and Morocco.