Abstract
In order to investigate the origin of non-marine oils in southeast Asia, especially the oils which indicate high oleanane/C30 hopane index, the occurrence of unsaturated oleanenes, possible precursor of oleanane, was studied in two onshore Malaysian wells. Four isomers were identified in the same samples from these wells. Among these isomers, olean-13 (18) -ene was the most dominant in m/z=191, 218 and 410 mass chromatograms. Rapid decrease of olean-13 (18) -ene and 18α-olean-12-ene with increasing depth suggests that these isomers may be precursors of oleanane. These unsaturated oleanenes were still observed at 2500m depth with vitrinite reflectance of 0.75%. The conversion from oleanenes to oleanane has not completed in upper oil generation window.
Effect of molecular-sieving on the analysis of oleanenes was also investigated. This treatment is often applied to fluvio-deltaic source rock to remove n-alkanes. Experimental results indicated that oleanenes abundance decreased with increasing the amount of molecular sieve and the duration of the treatment. Degradation and/or conversion of unsaturated oleanenes can proceed faster due to catalytic effect of the sieve. Oleanenes are important compounds in the fluvio-deltaic petroleum system. However, the analysis of oleanenes with molecular-sieves should be conducted carefully.
