Novel C^sub 15^ sesquiterpanes and their origin in different crude oils from the Pearl River Mouth Basin of China

In the saturate fractions of crude oils from the Zhu 1 and Zhu 2 depressions of the Pearl River Mouth Basin, two novel C15 sesquiterpanes have been detected except for drimanes and rearranged drimanes. By comparison with spectra and retention times in previous studies, they have been identified as 2,2,4[alpha],7,8-pentamethyl-trans-decalin and 2,2,4[alpha],7,8-pentamethyl-cis-decalin, respectively, which are related to the D/E rings of 18[alpha](H)- and 18[beta](H)-oleanane. Results show that the novel C15 sesquiterpanes are relatively abundant in condensate oils that have high Pr/Ph ratios and are rich in terrigenous triterpanes such as oleanane and bicadinanes from the Zhu 2 depression, but evidently low in lacustrine oils that have low Pr/Ph ratios and are rich in 4-methylsteranes from the Zhu 1 depression. This suggests that the novel C15 sesquiterpanes could be related to terrigenous organic matter in origin and can help distinguish the origin and source of crude oils in the study area. In addition, there exist completely different correlations between the relative abundances of different C15 sesquiterpanes that have different sources and the Pr/Ph ratio, which indicates redox conditions in the crude oils. For example, the novel C15 sesquiterpanes and cadinane that indicate terrigenous input are positively correlated with the Pr/Ph ratio, but drimanes and rearranged drimanes having microbial origin are negatively correlated with the Pr/Ph ratio. This implies that the formation of the novel C15 sesquiterpanes requires relatively oxic conditions. The novel C15 sesquiterpanes are much more abundant in carbonaceous mudstone (Pr/Ph=7.39) than in dark mudstone (Pr/Ph=2.19) from the Enping formation, although their organic matter is humic. However, they are not common in sapropelic lacustrine source rocks (Pr/Ph=2.29) from the Wenchang Formation. This suggests that terrigenous organic matter is only a sufficient condition, but an oxic environment during deposition and diagenesis may be a prerequisite for the formation of novel C15 sesquiterpanes.