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Fossil Fuel Deposits of Benue Trough: A Potential Source of Future Hydrocarbon Reserves in Nigeria

Abstract

Integrated geochemical approach of analysis can be a useful tool in hydrocarbon exploration success. Elemental analysis, Rock-eval pyrolysis, Gas chromatography coupled with flame ionization detector (GC-FID), Gas chromatography-mass spectrometry (GC-MS) and Gas Chromatography–Combustion–IsotopeRatio Mass Spectrometer (GC–IRMS) were used to characterize coal, shale and ditch-cutting samples collected from Lower (Mamu Formation), Middle (Awgu Formation) and Upper (Gongola Basin), Benue Trough, Nigeria. All the samples analyzed contained the minimum of 0.5 wt.% of total organic carbon (TOC) organic matter required to serve as good source rock for oil and gas. Several plots from the Elemental analysis and Rock-Eval pyrolysis classified the organic matter in the samples as type II/III, III and III/IV kerogen in Lower, Middle and Upper Benue Trough respectively. 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 in most of the samples from Lower and Middle Benue Trough while biomarker distributions in the samples from Upper Benue Trough are characterized by dominant medium molecular weight n-alkanes (n-C18 to n-C24), abundant hopanes, homohopanes, gammacerane and predominant C28, C29 regular and rearranged steranes which indicate organic matter derived majorly from phytoplankton e.g. marine algae and bactrioplankton. The occurrence of olean-18-ene, olean-13 (18)-ene, and olean-12-ene in Okaba samples (Mamu Formation, Lower Benue Trough) and samples from Gombe Formation (Gongola Basin, Upper Benue Trough) indicate immaturity. The pristane/phytane ratio, distributions of polyaromatic hydrocarbons and carbon isotope composition of individual n-alkanes depicts different depositional environment and oxicity of organic matter contained in the samples. Various maturity parameters computed from Vitrinite reflectance, Rock-eval, biomarkers, polyaromatic aromatic hydrocarbons and carbon isotope compositions reflect, immaturity to oil-window, main phase of oil-window and immaturity to early oil-window in Lower, Middle and Upper Benue Trough respectively. Benue Trough can complement the present crude oil reserves in Nigeria in the near future if given proper attention.