Oil geochemistry of Cretaceous Bima Formation shales in Gongola sub-basin, Northern Benue Trough, Nigeria

Abstract

Prospective source rocks (shales) of the Bima Formation in the Gongola sub-basin, 
Northern Benue Trough, Nigeria, were studied in the field and laboratory to increase 
our understanding on their hydrocarbon-generating potential, thermal maturity, and 
paleo-depositional environment. Results obtained from the 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 the fluvial settings observed include 
mud cracks and poor sorting, implying textural immaturity. Results of the geochemical 
analysis obtained show 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 (trisnorneohopane/trisnorho
pane) values of the studied shales vary 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 was deposited 
under anoxic and suboxic conditions.