Triterpane and sterane biomarkers in the YA13-1 condensates from Qiongdongnan Basin, South China Sea

Abstract

Triterpanes and steranes in condensates from the YA13-1 gas field, Qiongdongnan Basin, were monitored. The YA13-1 condensates have unusual biomarker distributions dominated by terpanes and steranes derived from higher plants. Anomalously abundant 18α-oleanane and remarkably abundant bicadinanes are present in the YA13-1 condensates, whereas the 17α-hopane contents are extremely low. Taraxastane and significantly abundant 17α-diahopanes occur in the condensates. In addition, a number of unknown C29 and C30 pentacyclic triterpanes including previously unreported compounds were detected in the condensates, some of which are significantly abundant. The unknown compounds may be terrestrial biomarkers. C29 homologues are relatively predominant among the regular and rearranged steranes. The diasterane concentrations are markedly higher than those of regular steranes. The maturity of the YA13-1 condensates is relatively high, at the peak to late oil generation stage (corresponding to 0.85–1.10% Ro), based on sterane and terpane and including bicadinane maturity parameters (i.e. T/(T1+R) and T/R bicadinane ratios). The above maturity assessment result is different from that based on diamondoid maturity parameters (%Ro=1.60–1.70) [Org. Geochem. 25 (1996) 179], which can be explained by a contribution of hydrocarbons from two sources at different depths. The YA13-1 condensates were probably generated from the Yacheng and Lingshui coal-bearing source rocks buried both in the Qiongdongnan Basin (3400–5000 m) and in the Yinggehai Basin (>5000 m). The possible contribution of lower maturity hydrocarbons from the Yacheng and Lingshui Formations (3400–4100 m) in the Qiongdongnan Basin to the YA13-1 gases and condensates should not be neglected.