Abstract
Crude oil from reservoirs in the onshore Niger Delta is a mixture of low and high maturity charge. Aromatic biomarkers in these oils, such as alkylphenanthrenes and alkylnaphthalenes, show relatively high thermal maturity, whereas the saturated sterane and hopane biomarkers indicate lower maturity close to the peak of the oil window. The high predicted maturity of the crude oils based on alkylphenanthrene and alkylnaphthalene indices is probably caused by mixing of high maturity condensate with less mature black oil. This correlates with the gas washing phenomenon of studied Nigerian oils caused by migrating condensate stream. The results of our study indicate that asphaltenes can be used to infer the organofacies of the low maturity petroleum charge in mixed crude oils. Compositional characteristics of the asphaltene pyrolysates indicate an origin of the related oils from Type II/III or Type III source rock kerogen. The n-alkyl-chain length distributions of the asphaltene pyrolysates predict source rocks that generated high wax paraffinic–naphthenic–aromatic oils or paraffinic oils. The n-alkyl-chain length of asphaltene pyrolysates changes in proportion to oxicity and the relative input of terrigenous organic matter to the source rock.
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