Molecular Characterisation and Biomarker Evaluation of Crude Oils from Selected Oil Fields in the Niger Delta Basin, Nigeria

Abstract

Crude-oil molecular biomarkers provide reliable geochemical indicators for evaluating petroleum origin, thermal maturity, depositional environment and reservoir correlation. This study investigated the physicochemical characteristics and molecular biomarker composition of crude oils obtained from eight selected oil fields in the Niger Delta Basin, Nigeria. Physicochemical properties were determined using standard petroleum analytical methods, whereas saturated hydrocarbon fractions were isolated by column chromatography and analysed by gas chromatography–mass spectrometry (GC–MS). Biomarker parameters, including n-alkanes, isoprenoids, terpanes, hopanes, steranes and aromatic hydrocarbons, were used to evaluate organic-matter sources, thermal maturity, biodegradation and depositional conditions. Pearson correlation analysis, principal component analysis (PCA) and hierarchical cluster analysis (HCA) were used to establish relationships among the crude oils. The oils were light and sweet, with API gravities of 33.5–37.2°API, sulphur contents of 0.16–0.25%, wax contents of 3.09–4.18% and asphaltene contents of 1.51–2.16%. They were dominated by n-C₁₆–n-C₂₁ hydrocarbons and exhibited Pr/Ph ratios of 1.24–1.33, Ts/Tm ratios of 0.65–0.73, C₂₉ sterane 20S/(20S+20R) values of 0.49–0.53, MPI-1 values of 0.86–0.94 and CPI values close to unity. These parameters indicate mature oils generated within the principal oil window, with negligible biodegradation. Biomarker distributions indicate mixed terrestrial–marine organic matter deposited under deltaic sub-oxic to weakly reducing conditions. Oil–oil correlation coefficients of 0.980–0.999 indicate strong genetic affinity and derivation from a common petroleum system.