Tectonic Evolution and Hydrocarbon Prospectivity of Nigerian Frontier Grabens of Dahomey Basin from Interpretation of New Aeromagnetic and Existing Seismic and Geophysical Data

Olufemi Oladapo Babalola

Three major oceanic fracture zones, interpreted from new aeromagnetic data, intersect the Dahomey basin and extend farther inland as northeast-trending faults. Aeromagnetic anomalies running northwest across the Dahomey embayment delimit two main grabens separated by a horst; the seaward end of all of these are terminated by a northeast-trending offshore trough. The northwest-southeast grabens are interpreted as being underlain by continental crust basements but the northeast-southwest graben seems to be underlain by oceanic crust. The northwest graben (the Ikorodu or Ogun River Trough) is intersected farther seaward by a northeast-trending graben (the offshore Lagos-Badagry Trough) in the shape of a southwest-facing reflection of the letter L. The smaller and shallower I e graben (inferred by previous investigators) continues colinearly with the major northeasterly graben.

The main problems in the exploration of the Nigerian portion of the Dahomey basin include lack of drilling density, especially in the poorly mapped coastal area, and difficulty of acquisition of marine data in the shallow-water area of the Gulf of Guinea cul-de-sac.

Kerogen-rich source rocks are expected in the organic-rich sediments of a reducing rift environment, which theoretically would be thermally mature. The paleogeographic location of the area since the Late Cretaceous indicates that sand deposits from fan deltas, alluvial and turbidite fans, canyon fills, and onlap fills would probably constitute the potential reservoirs. The hydrocarbon reservoirs would probably be dominantly stratigraphic pinch outs, facies changes, and faulted traps, in contrast to the growth-faulted rollovers of the Niger delta. If crude oil prices support the economic feasibility, the existing production infrastructure for the adjoining Niger delta and the increasing knowledge of these grabens would in future encourage active exploration and possible production in the Nigerian part of the Dahomey basin.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.