Abstract
The present work deals with a study based on the geochemical techniques such as biomarkers, Rock-Eval pyrolysis, and detailed petrographic study to evaluate hydrocarbon generation potential of coal by collecting nine coal and carbonaceous shale samples from boreholes in Awgu Formation of Middle Benue Trough, Nigeria. The values vitrinite reflectance (0.94–1.15%VR) and Tmax (446–469°C) confirmed that samples are matured enough to generate liquid and gaseous hydrocarbon in coal. The coal samples also contain sufficient quantity of vitrinite and liptinite macerals varying from 70.28% to 74.10 wt%, which confirm the production of liquid hydrocarbon. The cross-plot between H/C and O/C atomic ratio indicates that samples were predominant in the bituminous rank and having kerogen Type III makes it suitable for hydrocarbon generation. Similar results were found in Rock-Eval pyrolysis analysis (Types II-III and Type III kerogen). The homohopane index (C35/C31 - C35) and homohopane ratio (C35αβS/C34αβS) range from 0.02 to 0.12 and 0.15 to 0.92 indicates oxic condition during organic matter deposition from Lafia-Obi samples. The Moretane/Hopane, Hopane/Hopane + Moretane, Ts/Ts + Tm, 22S/22S + 22RC32homohopane ratios range from 0.06 to 0.14; 0.88 to 0.94; 0.34 to 0.66; and 0.53 to 0.62 and 20S/20S+20R and αββ/αββ+ααα C29 ratios range from 0.43 to 0.58 and 0.42 to 0.55 indicate samples ar e within the late oil window/gas phase. Plots of 22S/22S+22R C32hopanes against C29αββ/αββ+ααα steranes show that Lafia-Obisamples are thermally mature.
Highlights
The shale limestone sequence formed during Late Cenomanian to Early Turonian includes Odukpani Formation of Calabar area, Eze-Aku/Makurdi Formation that extends from North of Ishiagu in the Lower Benue Trough and Dukul, Gongila and Pindiga Formations of Upper Benue Trough in the North Eastern Nigeria
Biomarker or geochemical fossils are organic compounds found in geosphere whose structure can be unambiguously linked to their biological origin, despite the possibility of some structural alteration due to diagenetic or other processes
Biomarkers are widely used in petroleum geochemical studies in source rock evaluation, oil-oil or oil-source rock correlations, basin evaluation and reservoir management (Peters et al, 2005)
Summary
Maturity of source rocks can be determined from the biomarker distribution in source rock extracts. Many maturity parameters have been developed from the distribution of terpanes, steranes and polyaromatic hydrocarbons in source rocks (Peters et al, 2005).In the present investigation, six coals, two carbonaceous shales and coaly shale; totaled nine samples were collected from borehole at Lafia-Obi, Middle Benue Trough Nigeria. The objective of this study is to evaluate the hydrocarbon generation potential of coal samples collected from boreholes in Awgu Formation of Middle Benue Trough, Nigeria. Shallower resulting in the formation of swamps in the Anambra basin. The swamps and associated vegetation were later buried under thick sediments to produce coal-bearing rocks
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