Prospective source rocks (shales) of the Bima Formation in the Gongola Sub-Basin, Northern Benue Trough Nigeria were studied in field and laboratory to increase our understanding on their hydrocarbon generating potential, thermal maturity and plaeodepositional environment. Results obtained from field studies suggest that the Bima Formation was deposited in a fluvial setting owing to the absence of marine fossils and presence of ferruginized beds. Other attributes of fluvial settings observed include mud cracks, poor sorting implying textural immaturity. Results of geochemical analysis obtained shows that the studied shales are rich in organic matter, and inferred to have mostly kerogen Type II-III and Type III which may be able to yield mixed oil and gas and gas respectively. The trisnorhopane thermal maturity (Ts/Tm) values of the studied shales varies between 0.88 and 1.12 implying that the studied shales are within the oil window. Also, cross plot of C29 steranes epimer ratios versus C32 homobopanes epimer ratios range from 0.46 to 0.48 suggesting that all the source rocks are thermally mature. The Pr/n-C17 versus Ph/n-C18 cross-plot and the steranes ternary plot revealed that oil extracted from the studied shale samples originated from organic matter of both marine and terrestrial sources (mixed organic matter) and deposited under anoxic and sub-oxic conditions.
Key words: Oil geochemistry, Bima Formation, Trisnorhopane thermal indicator, Anoxia, Paleoenvironment, Source organic matter
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