Deep-Water Petroleum Systems of Africa's Major Rivers
Published:December 01, 2013
D. G. Evans, M. J. Cope, Dr., J. A. English, 2013. "Deep-Water Petroleum Systems of Africa's Major Rivers", Shelf Margin Deltas and Linked Down Slope Petroleum Systems–Global Significance and Future Exploration Potential, Harry H. Roberts, Norman C. Rosen, Richard H. Fillon, John B. Anderson
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Africa's major rivers are associated with some of the largest deep-water petroleum systems in the world. The most successful of these in terms of hydrocarbon discoveries, the Congo, Niger and Nile, are currently sites of intense exploration activity, while others (such as the Kwanza, Zambezi, and Rufiji-Ruvuma) have yet to yield a significant number of finds and remain under explored. The theme of this paper is to review examples of these systems, attempt to show why this has occurred and where some of the future offshore hydrocarbon provinces of Africa will be.
Seismic data will be used to review the deep water plays and prospectivity associated with each major river system. The deep-water systems reviewed include the Congo, Niger, Nile, Kwanza, Zambezi, and Rufiji-Ruvuma. The more mature deep-water hydrocarbon provinces will be compared with the less explored areas. Suggestions as to the reasons for limited exploration of some areas will be made, concluding that visible structuring is as least as important as the perceived presence of source rock in the initial decision to explore in a basin. Visible structuring in the form of salt or mud diapirs, anticlines, etc., undoubtedly makes exploration “easier” in choosing locations and “selling” prospects in the early stages of basin exploration. Seeking stratigraphic traps when there are few or no structures is very difficult in an unexplored basin.
The future exploration potential of the deep water provinces will be reviewed with particular reference to the currently less explored or less mature areas to show their untested prospectivity. As exploration has moved into deeper water in known hydrocarbon provinces such as Niger and Congo deltas, then the targets have been the deeper water analogies of existing plays; i.e., Niger delta mud supported anticlines and Congo fan channelized sands. In areas in which there are few or no existing working plays, extension by analogy into deep water breaks down or does not exist. So often in these areas new plays and source rock postulations have to be developed. Examples of new untested plays that do not exist in shallower water are shown from Tanzania Rufiji-Ruvuma and Mozambique deep water systems where there has been no drilling. Many of these involve structuring that does not exist in the shallower explored areas and are beginning to attract interest.
In areas such the Kwanza Basin where there is a very structured sedimentary section the (correct) perception that the play type is not the same as farther north in the Congo Fan has caused the area to be downgraded, despite the fact that hydrocarbons have been found here. This downgrading is partially due to recent wells targeting the same play as farther north and failing to find oil. In this case, the primary reservoir targets are not highly visible Tertiary channel sands but Cretaceous sands having a more subtle seismic expression, a play that has proven to work by the Semba-1.
The potential of the ultra deep water (i.e., water depths >3km) and abyssal areas beyond the Congo and Niger fans will be examined to demonstrate that as yet untested plays and prospectivity exist in these future exploration provinces.