Paleoecologic and Organic Geochemical Assessment of Cretaceous Hydrocarbon Source Rocks in the Gulf of Guinea: New Insights from Eastern Dahomey and Benue Rift Basins with Implications for the Cenomanian-Coniacian Petroleum System

Abstract

An integration of foraminifera paleoecology and organic geochemical studies of shale samples from shallow onshore boreholes and a deepwater X well in the Eastern Dahomey Basin coupled with selected exploratory wells and exposed Cretaceous sections in the Benue Basins were carried out. Black shales of the Afowo and Eze-Aku Formation (Cenomanian- Turonian) in the Dahomey and Lower Benue Basins respectively contain a large abundance of planktonic foraminifera e.g. Rotalipora greenhornesis, Hedbergella delyioensis, Heterohelix moremani, Heterohelix cenomana, Heterohelix globulosa, Hedbergella planispira, Whiteinella inonata, Praeglobotruncana sp.(?) and no significant benthic foraminifera. This assemblage was deposited under a deepwater, euxinic environment of the neritic to upper bathyal probably not deeper than 250 m. Paleogeographic interpretations indicate a northeasterly shoaling of marine conditions characterized by the dominance of benthic foraminifera e.g. Ammobaculites sp., Ammotium sp., Lenticulina sp. and Bolivina sp. in the time equivalent shales of the Yolde Formation (Upper Benue Basin). Fifteen shales of the Afowo Formation from the deepwater well have Total Organic Carbon (TOC) contents from 0.62 to 2.31% (mean-1.07%), S2 values from 3.27 to 11.63 (mean-5.41), Hydrogen index (HI) values from 331 - 653 (mean-525), Vitrinite reflectance (Ro) of 0.68 to 0.95% and dominated by Type II oil prone kerogen. Ten outcrop samples of the Eze-Aku shales exhibited TOC from 3.05 to 5.47% (mean-4.07), S2 values from 8.4 to 21.2 (mean-13.9), HI range from 237 -387 (mean-333), Ro of 0.43 to 0.58 with a prevalence of Type II oil and gas prone assemblage. These contrast with the time equivalent shales of the Yolde Formation where thirty four samples from an exploratory Nasara well (TD-2,100 m) in the Upper Benue Basin exhibited TOC contents from 0.1 to 1.1% (mean-0.4%), S2 from 0.16 to 1.39 (mean -.43), HI values from 38-200 (mean-94), Ro of 0.64 to 0.77% and dominated by Type III gas prone kerogen. Burial history construction suggests maturity for these sequences since the Santonian age. We conclude that the Cenomanian - Coniacian transgressive sequences in the Eastern Dahomey and the Lower Benue Basins have good to excellent quality source rocks related to the world wide Oceanic Anoxic Event - 2 (OAE 2) thus forming an integral element of the petroleum system of the regionally extensive coastal basins in the West African Gulf of Guinea.