Biomarker Fingerprinting of Crude Oils from Niger Delta Depobelts, Nigeria.

Twenty-eight oil samples from four depobelts in the Niger Delta Basin were investigated utilizing gas chromatography and gas chromatography-mass spectrometry to determine their source, paleoenvironment, thermal maturation, and alteration mechanisms utilizing biomarkers. The oils generally have an oleanane/hopane ratio > 0.2, a lower homohopane index, and a predominance of C29 ααα steranes and C30 hopanes, signaling contributions from both marine and terrestrial sources. The substantial concentration of C27 steranes indicates marine algal input, while the prevalence of C29 steranes supports terrigenous organic materials originating from higher plants. Variability in sterane concentrations in the oils reflects complex origins and the dynamic interplay of marine and terrigenous processes within the deltaic-transitional settings. The thermal maturation ratios of sterane biomarkers suggest the crude oils belong to the early-to-peak oil range. Conversely, the hopane result suggests the oils are at peak maturity. Applying the criteria of oleanane, sterane/hopane, and isoprenoids/n-alkane, the oil sets were classified into 2 sub-families, ‘A and B.’ ‘A’ oils are characterized by elevated oleanane/hopane ratios with lower sterane/hopane ratios, sourced from clay-rich rocks in an oxic environment. Family B includes oils from mixed organic matter sources with low oleanane/hopane ratios and low to moderate sterane/hopane ratios. The established oil families are independent of thermal maturities, as the same source rock can generate different thermal maturity fluids based on burial depth and basin evolution. Biodegradations and/or evaporative fractionations were prominent in a few samples, consequence of the depletion of light-end n-alkanes and then the relative enrichment of aromatics in the saturate-aromatic profiles of oils, thereby elevating the isoprenoids/n-alkane ratios. The oils have a low homohopane index, a substantial presence of C27 steranes, which reveals an important contribution from marine algae, and a major presence of C29 steranes, which indicates a significant input from terrestrial organic matter from higher plants.