Abstract
Comprehensive two-dimensional gas chromatography with time-of-flight mass spectrometry (GC×GC–TOFMS) was used to identify unusual steranes in branched-cyclic hydrocarbon fractions of crude oils from Sergipe-Alagoas Basin, Brazil and infer the geochemical settings of their Cretaceous source rocks. Hopane/sterane and sterane ratios, and the presence of gammacerane and β-carotane in the oil samples indicates a saline source rock depositional environment and predominantly marine organic matter (OM) input. Steroidal hydrocarbons were represented by alkyl steranes and several sterane series, including abundant short-chain steranes (C21 and C22), regular steranes (C27–C29), and diasteranes (C21–C23, C27–C29). The ring-A methyl sterane series contained 3β-methyl steranes (C22 and C23, C28–C30), abundant 3β-methyldiginane (C22), and 3β-methyldiacholestanes (C28), including 4-methyl steranes with abundant C30 4α,23,24-trimethylcholestanes in the branched-cyclic fraction of these crude oils. The C29 sterane isomers, 23,24-dimethylcholestane and 24-ethylcholestanes, were resolved by GC×GC–TOFMS, and a C29 dinosterane derivative was identified for the first time in Brazilian crude oils. The 23,24-dimethyl and 4,23,24-trimethyl steroids can be related to dinoflagellate OM and abundant dinosteranes suggests dinoflagellate blooms during source rock deposition in this Brazilian marginal basin. Additionally, the detection of C28–C32 3β-alkyl cholestanes, C29–C33 3β-alkyl 24-methylcholestanes, and C30–C34 3β-alkyl 24-ethylcholestane with ethyl to pentyl groups at C-3 resulted in a complex sterane fingerprint, as expected for petroleum generated from source rocks deposited under predominantly marine saline conditions with high microalgal input. These biomarker results provide additional insight into the character and geologic processes responsible for the source rocks of crude oils in the Brazilian marginal basins. This information will be a valuable exploration tool that can be used to constrain the distribution of different oil types in these basins.
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