BIOMARKER AND N-ALKANE ISOTOPE PROFILES FOR TERTIARY OILS - RELATIONSHIP TO SOURCE-ROCK DEPOSITIONAL SETTING

Biomarker and n-alkane isotope profiles have been measured for 29 Late Cretaceous/Tertiary oils from SE Asia, China, Papua New Guinea, New Z ealand and the U.S.A. The results are interpreted with respect to six kinds of source rock depositional setting: fluvio-deltaic (FD), freshw ater transitional (TR), lacustrine (LAG), saline lacustrine (SAL), mar ine deltaic (MD) and marine carbonate (MC). A comparison of biomarker and n-alkane isotope results suggests that parameters such as the olea nane/hopane ratio may overestimate the higher plant contribution to ma rine oils. The abundance of bicadinanes is much more variable than tha t of oleanane, probably because of a specific association with the hig hly resinous, dipterocarp hardwoods of SE Asia. However, traces do occ ur in an oil from outside the paleogeographic range of the dipterocarp family, confirming that there is also a more general angiosperm sourc e. The bicadinane isomeric distribution is shown to depend on maturity , leading to new maturity parameters which are resistant to even very heavy biodegradation. Retention indices based on the regular hopane se ries are provided for the main bicadinane and methyl bicadinane isomer s and for a group of oleanane-related triterpanes of unknown structure . The presence of the latter compounds may indicate locally deposited as opposed to transported higher plant matter in the source. Source ro ck depositional setting is the primary control on the shape of the n-a lkane isotope profile, with negatively sloping curves being characteri stic of FD and TR oils and flat or positively sloping curves typical o f marine oils. The difference is probably related to the bacterial rew orking of higher plant matter in the FD environment. A slight isotopic anomaly at n-C-17 correlates with the abundance of algal-derived ster anes and may indicate a minor marine contribution to the source of an FD oil.