Biomarker Geochemistry of Nkporo Shale from Ndi-Owerre in the Afikpo Basin, Southeast Nigeria

Abstract

Biomarker analysis was conducted on shale samples belonging to the Cretaceous Nkporo sediments of the Afikpo Basin in southeastern Nigeria to determine the distribution of n-alkanes, pristane and phytane ratios (Pr/Ph), odd-over-even preference (OEP), carbon preference index (CPI), transformation ratios. These parameters revealed the source, depositional conditions and maturity of the organic matter. Ts/Ts + Tm values of 0.9 to 0.12 and moretane/hopane ratio of 0.33 to 0.35 indicate shales are thermally immature to marginally mature. The presence of long-chain aliphatics (C19-35) in the shale matrix suggests shale can atleastgenerate oil and gas. Data show high Pristane/Phytane (Pr/Ph) values and preeminence of intermediate molecular weight n alkanes (nC12 - nC33). The predominance of Pr against Ph suggests humic, toxic to dysaerobic organic matter origin. Distributed n-alkane patterns of gas chromatograms are non-unimodal with varying peaks. They show maximal between nC12 - nC33 with no remarkable protrusion, indicating slight or no organic matter alteration. The prevalence of OEP values (0.23 -2.10) and CPI values ranging from 1.09 and 1.61 with average oxygen indexes of 53.33mgHC/gTOC suggests donation from mostly terrestrial organic matter poor in hydroxl groups.