Biomarker fingerprinting of crude oils from Niger Delta depobelts, Nigeria

Abstract

Twenty-eight crude oil samples from four depobelts in the Niger Delta Basin were analyzed to evaluate source input, depositional environment, thermal maturity, and post-generation alteration. Analytical methods included gas chromatography (GC) and gas chromatography–mass spectrometry (GC–MS). Biomarker signatures—oleanane/hopane ratios >0.2, low homohopane indices, dominance of C₂₉ ααα-steranes, and abundant C30 hopanes—indicate contributions from both marine and terrestrial organic matter. Elevated C₂₇ steranes suggest algal input, while the predominance of C₂₉ steranes implies significantly higher plant contributions, characteristic of a deltaic-transitional setting. Sterane isomerization ratios suggest oil generation occurred within the early to peak oil window, while hopane parameters confirm peak maturity. Biomarker parameters, particularly oleanane, sterane/hopane, and isoprenoid/n-alkane ratios, delineate two oil families. Family A exhibits high oleanane/hopane and low sterane/hopane values, pointing to a predominantly oxic, terrigenous source. In contrast, Family B includes oils from mixed sources, with lower oleanane/hopane and variable sterane/hopane ratios. Oil family classification is independent of maturity level, as variations likely reflect differences in source rock burial history and basin evolution rather than thermal stress alone. Total ion chromatograms (TICs) and selected ion monitoring (m/z 191, 217) display no evidence of biodegradation, evaporative fractionation, or water washing in the biomarker assemblage. However, subtle effects of secondary migration or compositional mixing cannot be entirely ruled out. These findings underscore the complexity of hydrocarbon generation and accumulation in the Niger Delta, shaped by variable organic inputs, depositional settings, and post-generative geological processes.