Organic geochemical characteristics of Cretaceous Lamja Formation from Yola Sub-basin, Northern Benue Trough, NE Nigeria: implication for hydrocarbon-generating potential and paleodepositional setting

Abstract

An integrated geochemical and molecular characterisation of the Cretaceous Lamja Formation shale and coal sediments from the Yola Sub-basin, Northern Benue Trough, northeastern Nigeria, has been undertaken to provide an overview on the origin, richness, hydrocarbon generation potential and paleodepositional conditions. This study is based on geochemical analyses of whole rock (total organic carbon content, pyrolysis, bitumen extraction and biomarker distributions) and vitrinite measurements. The total organic carbon (TOC) contents of the Lamja Formation range from 0.8 to 63 % and 0.8 to 1.16 % for coal and shale samples, respectively, with an average TOC value of 43.87 %. The hydrogen index of these samples ranges from 93.1 to 228 mg hydrocarbon (HC)/g TOC. The kerogen is predominantly type III with a significant mixture of type II kerogens, indicative of mainly gas with limited liquid hydrocarbon-generating potential. The analysed Lamja Formation samples have vitrinite reflectance in the range of 0.57–0.82 %Ro and pyrolysis temperature at maximum (T max) in the range of 435–451 °C which indicate that the samples are thermally mature and entered early mature to peak oil window stage. The molecular geochemical biomarkers are characterised by dominant odd carbon numbered n-alkanes in the range of n-C23 to n-C33, moderately high pristane/phytane (Pr/Ph) ratios (1.77–4.16), very high C27 17α(H)-22,29,30-trisnorhopane/C27 18α(H)-22,29,30-trisnorneohopane (Tm/Ts) ratios (>10) and high concentrations of regular sterane C29, indicating suboxic to oxic conditions, typical of delta plain/coastal marine environment of deposition with prevalent contribution of land plants and minor aquatic organic matter input. The occurrence of oleanane in the analysed samples is also a strong indicator of a terrestrial angiosperm plant source input and the presence of marine influence.