Abstract
Many oils in Southeast Asia region exhibit non-marine characteristics. Among these oils, some indicate high oleanane index (oleanane/C30 hopane), which are supposed to generate in terrestrial source rocks deposited in fluvio-deltaic setting. Geochemical data on source rocks indicated that the rocks with high oleanane index are only found in prodelta shales. However, source rock potential of these prodelta shales is poor; TOC<2% and HI<200mgHC/gTOC. It is difficult for oils to be expelled out from the source rocks with these lean potential and the oils are cracked to be gas in the source rocks with additional heat energy. The oleanane index of the rocks appears to increase as the source rock potential decreases. The oleanane indices of the oils from Southeast Asia were plotted against Ts/Tm ratios. This diagram clearly separated the oils into two groups. With geological background and other geochemical parameters such as Pr/Ph and carbon isotope, these two groups are interpreted as; 1) algal oils which are supposed to generate in lacustrine source rocks in Oligocene, and 2) terrestrial oils which are supposed to generate in fluvio-deltaic source rocks in Miocene. In each group, the oleanane index increases as the Ts/Tm ratio increases. The Ts/Tm ratio is supposed to be affected by maturity and depositional environment. However, even in each group, the oleanane index increases as the Ts/Tm increases, which suggests that the oleanane index could be affected by maturity.
