Hydrocarbon Potential Assessment of the Maastrichtian Araromi Shales in the Dahomey Basin, Southwestern Nigeria

Olabisi A. Adekeye¹ and Samuel O. Akande^2
1 University of Ilorin, 240093, Ilorin, Nigeria
² University of Ilorin, Ilorin, Nigeria

Dahomey basin is an extensive sedimentary basin in the Gulf of Guinea. It extends from the southeastern Ghana through Togo and Benin Republic on the west side to the Okitipupa ridge/Benin Hinge line on the east side in the southern Nigeria. Cretaceous-Tertiary sediments outcrop in the basin in roughly parallel belt to the ancient coastline.

Organic geochemical and organic petrologic studies of the Maastrichtian Araromi Shales (penetrated by the Araromi borehole and Bode Ashe well) were carried out. The Araromi Shales have total organic carbon of 0.34-2.75 wt %, hydrogen index of 1-202 mgHC/gTOC, vitrinite reflectance of 0.45-0.66%, source potential of 0.03-5.46 kgHC/ton and Tmax of 391-428 C in the Araromi borehole. Kerogen Type II-III, III and IV with higher percentage of terrestrially derived arid reworked organic matter were observed. In the Bode Ahse well, Araromi Shales have total organic carbon content of 0.28-6.37 wt %, hydrogen index of 55-199 mgHC/gTOC, vitrinite reflectance of 0.50-0.68%, source potential of 0.68-12.22 kgHC/ton and Tmax of 393-430 C. Kerogen Type II, II-III and III with dominance terrestrially derived organic matter were observed.

The study suggests that the shale facies of the Araromi Formation have low hydrocarbon potential, gas and mixed gas/oil prone organic matters that are generally thermally immature to marginally mature with increasing depth. This trend suggests mature source beds in the deep offshore.