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 Zealand and the U.S.A. The results are interpreted with respect to six kinds of source rock depositional setting: fluvio-deltaic (FD), freshwater transitional (TR), lacustrine (LAC), saline lacustrine (SAL), marine deltaic (MD) and marine carbonate (MC). A comparison of biomarker and n-alkane isotope results suggests that parameters such as the oleanane/hopane ratio may overestimate the higher plant contribution to marine oils. The abundance of bicadinanes is much more variable than that of oleanane, probably because of a specific association with the highly resinous, dipterocarp hardwoods of SE Asia. However, traces do occur in an oil from outside the paleogeographic range of the dipterocarp family, confirming that there is also a more general angiosperm source. 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 series are provided for the main bicadinane and methyl bicadinane isomers 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 rock depositional setting is the primary control on the shape of the n-alkane isotope profile, with negatively sloping curves being characteristic of FD and TR oils and flat or positively sloping curves typical of marine oils. The difference is probably related to the bacterial reworking of higher plant matter in the FD environment. A slight isotopic anomaly at n-C₁₇ correlates with the abundance of algal-derived steranes and may indicate a minor marine contribution to the source of an FD oil.