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 Cretaceous sediments. Among these sediments are the Turonian-Santonian organic matter-rich and coal-bearing strata of the Awgu Formation. Maceral analyses on 32 samples obtained from four boreholes and two outcrop sections from the Awgu Formation allow the subdivision of the coal beds and interseam sediments 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 faulted, accompanied by uplift. Coalification to the present rank was attained prior to this deformation. All the VR values fall within the zone of oil generation in source rocks. In the study area, however, oil generation potential 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-fusinite coal facies with its high content of humic organic matter (vitrinite+inertinite) is likely to generate gas (wet+dry) only, within the given thermal maturity range.