OLEANANES IN OILS AND SEDIMENTS - EVIDENCE OF MARINE INFLUENCE DURINGEARLY DIAGENESIS

The oleananes, as markers for the angiosperms, provide valuable source and age information when present in an oil. Nevertheless, they are no t quantitatively related to the land plant input and indeed their pres ence reflects only a small leak in diagenetic processes leading primar ily to aromatic oleanoids. Because they are minor products, the abunda nce of oleananes in terrigenous oils and sediments may be highly sensi tive to changes in early diagenetic conditions. Here we present eviden ce that contact of plant matter with seawater during early diagenesis enhances the expression of oleananes in a mature sediment or oil. Olea nanes 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 s eam in correlation with % organic sulphur, dibenzothiophene/phenanthre ne, and the homohopane index. Similarly, in deltaic sediments from the South Sumatra Basin, oleanane/hopane is strongly correlated with indi cators of marine influence such as C-27/C-29 steranes and the homohopa ne index. In each case, increasing oleanane abundance is accompanied b y a reduction in the extent of aromatisation and, for the South Sumatr a 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 so urce markers in fluvio-deltaic and lacustrine petroleum systems. On th e other hand, their sensitivity to early diagenetic conditions may mak e them useful in locating effective source rocks in such systems. Copy right (C) 1997 Elsevier Science Ltd.