Abstract The oleananes, as markers for the angiosperms, provide valuable source and age information when present in an oil. Nevertheless, they are not quantitatively related to the land plant input and indeed their presence reflects only a small leak in diagenetic processes leading primarily to aromatic oleanoids. Because they are minor products, the abundance of oleananes in terrigenous oils and sediments may be highly sensitive to changes in early diagenetic conditions. Here we present evidence that contact of plant matter with seawater during early diagenesis enhances the expression of oleananes in a mature sediment or oil. Oleananes are absent or present at very low concentrations in samples from the base of an Eocene coal seam affected by postdepositional seawater intrusion. However, their abundance increases toward the top of the seam in correlation with % organic sulphur, dibenzothiophene/phenanthrene, and the homohopane index. Similarly, in deltaic sediments from the South Sumatra Basin, oleanane/hopane is strongly correlated with indicators of marine influence such as C 27 C 29 steranes and the homohopane index. In each case, increasing oleanane abundance is accompanied by a reduction in the extent of aromatisation and, for the South Sumatra Basin, the proportion of A-ring contracted oleananes. An angiosperm-derived Miocene coal from the Philippines, deposited under freshwater conditions, shows abundant aromatic oleanoids but no oleananes. These results show that oleananes need to be used with caution as age and source markers in fluvio-deltaic and lacustrine petroleum systems. On the other hand, their sensitivity to early diagenetic conditions may make them useful in locating effective source rocks in such systems.