Sixty‐six oils from eleven offshore and onshore fields in the NW Niger Delta, Nigeria were analyzed geochemically for their biomarker and isotopic compositions. Multivariate statistical analysis was employed to distinguish generic oil families from the large, complex data set. Biomarker and isotopic source parameter distributions were used to group the oils into three generic families. Family A oils, located in the onshore swamp to transitional area, received charges from predominantly Late Cretaceous or younger marine source rocks laid down in a sub‐oxic to oxic depositional environment. Family B oils occur in the near‐offshore area and are derived from Tertiary source facies that received an input of mixed terrigenous and marine organic matter. Family C oils, which dominate the offshore area, were derived from Tertiary source rocks typical of those deposited in oxic, nearshore or deltaic settings receiving significant terrestrial organic matter. Biomarker maturity parameters showed that the onshore (swamp) oils were generated at the peak of the oil generating window, while the transitional to offshore oils were expelled at an earlier stage of oil generation.
Fifty‐eight out of sixty‐six oils from eleven onshore and offshore oilfields in the Niger Delta were found to contain three oleanene isomers: olean‐13(18)‐ene, olean‐12‐ene and 18α‐olean‐12‐ene. The geochemistry of these compounds reveals that the secondary isomers (olean‐13(18)‐ene and 18α‐olean‐12‐ene) form during diagenesis from the precursor isomer (olean‐12‐ene). The presence of other angiosperm markers such as oleananes, the absence of measurable amounts of other olefins characteristic of immature organic matter, and the thermal maturity of the oils analyzed are consistent with the hypothesis that the oleanenes were inherited directly from the source rock, and that they migrated with the rest of the oil. A good correlation exists between the abundance of oleanenes and the maturity status of the oils: oils generated at an early stage of hydrocarbon generation contain oleanenes, while those generated at peak of hydrocarbon generation do not contain oleanenes in identifiable quantities.
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