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.本次研究针对尼日尔三角洲盆地四个沉积坳陷带（depobelts）采集的28个原油样品展开分析，采用气相色谱（gas chromatography）与气相色谱-质谱联用法（gas chromatography-mass spectrometry），借助生物标志物（biomarkers）解析其成因、古沉积环境、热成熟度及蚀变机制。整体而言，这批原油的奥利烷/藿烷比值大于0.2，同型藿烷指数较低，且以C29 ααα-甾烷与C30藿烷为主，指示其兼具海源与陆源双重成因贡献。高浓度C27甾烷表明存在海源藻类输入，而C29甾烷占优则印证了高等植物来源的陆源有机质供给。原油中甾烷浓度的差异，反映了三角洲过渡沉积环境中海陆沉积过程的复杂成因与动态相互作用。甾烷生物标志物的热成熟度比值显示，这批原油处于早成熟至成熟峰值阶段；而藿烷相关指标则表明原油已达到成熟峰值。基于奥利烷、甾烷/藿烷比值以及类异戊二烯烃/正构烷烃比值的判别标准，本次研究将原油划分为‘A’与‘B’两个亚家族：‘A’类原油具有较高的奥利烷/藿烷比值与较低的甾烷/藿烷比值，其源岩为氧化环境下的富黏土岩；‘B’类原油则源自混合有机质来源，奥利烷/藿烷比值较低，甾烷/藿烷比值处于低至中等水平。已确立的原油家族划分与热成熟度无关，因为同一套源岩可根据埋藏深度与盆地演化过程，生成不同热成熟度的油气流体。部分样品存在显著的生物降解与/或蒸发分馏作用，表现为轻端正构烷烃被消耗，饱和烃-芳烃色谱剖面中芳烃相对富集，进而推高了类异戊二烯烃/正构烷烃比值。本次分析的原油普遍具有较低的同型藿烷指数、显著的C27甾烷丰度（指示海源藻类的重要贡献）以及以C29甾烷为主的特征（表明高等植物来源的陆源有机质显著输入）。