Geochemically Distinct Oil Families in the Gudong Oilfield, Zhanhua Depression, Bohai Bay Basin, China

摘要

Twenty crude oil samples were obtained from the Gudong Oilfield and their organic geochemical characteristics were analyzed. The oil samples were classified into three families by hierarchical cluster analysis and principal component analysis based on 13 source-related and depositional environment-related biomarker parameters. Oils in family I have low ratios of C_(19)/C_(23) tricyclic terpanes and C_(24) tetracyclic terpane/C_(26) tricyclic terpanes, and relatively high ratios of steranes/hopanes and C_(30) 4-methylsteranes/ααα20R C_(29) sterane, thus indicating that microalgae were the dominant organic matter input for the source rocks of family I. The gammacerane/C_(30) hopane ratios are higher than that of family II and family III, whereas the C_(35)/C_(34) homohopane ratios are lower, thus indicating a suboxic, brackish water environment for the source rocks. The inferred source rock is the first member of the Shahejie Formation in the Huanghekou Sag. Family II is characterized by high ratios of C_(19)/C_(23) tricyclic terpanes and C_(24) tetracyclic terpane/C_(26) tricyclic terpanes but relatively low ratios of steranes/hopanes and C_(27)/C_(29) αααR steranes. These findings suggest that the original organic matter of the source rocks had a greater contribution from terrigenous higher plants than from microalgal. The relatively low ratios of gammacerane/C_(30) hopane and C_(35)/C_(34) homohopane suggest that the source rocks were deposited in an oxic environment with a low salinity, thus corresponding to the Dongying Formation in the Huanghekou Sag. Family III oils have high C_(27)/C_(29) ααααR steranes ratios and low C_(30) 4-methylsteranes/ααα20R C_(29) ratios, which indicate the contribution of microalgae (especially zooplankton algae) to the source rocks. The relatively high abundance of C_(35) homohopane and low gammacerane/C_(30) hopane ratios suggest a weakly reducing condition with low salinity, which is in accordance with the third member of the Shahejie Formation in the Gunan Sag. The C_(31)S/(S + R) homohopane ratios imply that oil samples in this study are in the mature stage, although the ratios of C_(29)20S/(20S + 20R) and C_(29)ββ/(αα + ββ) steranes suggest that the maturity of family II is higher than that of family I and family III.