Hydrocarbon-Generative Potential of Coal and Interbedded Shale of Mamu Formation, Benue Trough, Nigeria

Abstract

Coal and its interbedded shale samples were collected from Okaba, Onyeama, and Okpara seams in the Mamu Formation, Lower Benue Trough, Nigeria. In the present study, source rock evaluation and biomarker distribution in the samples were investigated by Rock-Eval pyrolysis, vitrinite reflectance measurement, and gas chromatography–mass spectrometry (GC-MS) analysis. All of the samples analyzed contained the minimum of 0.5 wt.% and 2 mg/g of total organic carbon (TOC) and genetic potential (GP) respectively of organic matter required to serve as good source rock for oil and gas. Several plots from the Rock-Eval pyrolysis classified the organic matter in the samples as type II/III kerogen. The abundance of hopanes, homohopanes (C31–C35), benzohopanes, and C29 steranes in most of the samples indicates terrigenous materials, phytoplankton, and cyanobacteria contributions to the organic matter that formed the coal. High Pr/Ph ratio (1.73–12.47) and n-alkane distribution in the samples showed that Mamu samples consisted of terrestrial organic matter with marine incursion deposited under oxic/suboxic-oxic in lacustrine-fluvial/deltaic environments. The distribution patterns of C32–C35 benzohopanes in the samples confirmed the redox condition of organic matter deposition within the formation. The occurrence of olean-18-ene, olean-13 (18)-ene, and olean-12-ene in Okaba samples favors terrestrial organic matter deposited in lacustrine-fluvial/deltaic environment. The vitrinite reflectance values (0.48–0.60% Ro) and all the maturity parameters derived from the Rock-Eval analysis and biomarker distributions showed that Okaba samples are immature, whereas Okpara and Onyeama are at the beginning of the oil window.