First discovery of oil shale in the Benue Trough, Nigeria

Abstract

An oil shale deposit, possibly of high economic value and corresponding to the Turonian Ezeaku shale (lower Nkalagu formation) of the Lower Benue Trough, was found in a 1.5 × 1.0 km belt in Lokpanta near Okigwe, in Imo State, Nigeria. The characteristically dark-grey, laminated and fissile marlstone contains total organic carbon (TOC) > 7% in some locations and total extractable organic matter generally in excess of 10 000 ppm. The kerogen is type I–II (oil-prone) and at the updip rim it has attained intermediate thermal maturity status. An initial appraisal of the economic potential of the fossil fuel deposit by pyrolysis (modified Fischer assay) indicates an average oil-yield of more than 42 tonne −1, which is the lower acceptable threshold for economic exploitation. A total thickness of > 25 m was established by drilling/coring for one location. Apart from being the first recorded occurrence of oil shale in the Benue Trough, the Lokpanta discovery is important because it provides an authentic model for the deposition of anoxic sediments in the mid-Cretaceous Benue Trough. The most appropriate depositional setting for this highly carbonaceous shale seems to be sedimentation in a restricted euxinic platform (sub-basin) of the epeiric Benue sea.