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 18a-olean-12-ene. The geochemistry of these compounds reveals that the secondary isomers (olean-13(18)-ene and 18a-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.