Liquid hydrocarbon source-rock potential of mid-Cretaceous coals and coal measures in the Middle Benue Trough of Nigeria

The Middle Benue Trough of Nigeria is filled with approximately 4000 m of C retaceous sediments. Among these sediments are the Turonian-Santonian organ ic matter-rich and coal-bearing strata of the Awgu Formation. Maceral analy ses on 32 samples obtained from four boreholes and two outcrop sections fro m the Awgu Formation allow the subdivision of the coal beds and interseam s ediments into three different petrographic/coal facies, namely: a vitrinite -fusinite coal facies, a trimaceritic coal facies,and a shaly coal facies. Vitrinite reflectance (VR) measurements gave values from 0.74 to 1.25% VR. These high VR values in such shallow boreholes arise as a result of the mid -Santonian tectonic episode during which the sediments were folded and faul ted, accompanied by uplift. Coalification to the present rank was attained prior to this deformation. All the VR values fall within the zone of oil ge neration in source rocks. In the study area, however, oil generation potent ial can be expected only in the trimaceritic coal facies and some parts of the shaly coal facies which are rich in liptinite macerals. The vitrinite-f usinite coal facies with its high content of humic organic matter (vitrinit e+inertinite) is likely to generate gas (wet+dry) only, within the given th ermal maturity range.