Effect of Depositional Environment on the Occurrence of 17α(H)-Diahopanes in Triassic Lacustrine Sediments from the Ordos Basin, NW China

In this study, an abundant 17α(H)-diahopanes has been observed in the sediments from the seventh member of the Triassic Yanchang Formation in the central part of the Ordos Basin. The results show that the maturity has a minor contribution to the relative abundance of 17α(H)-diahopanes as the maturity of the samples are similar and fall into a limited range in the selected samples from the study area. The C₂₇-C₂₉ regular steranes distributions of the samples are also similar and shaped a "V" pattern, indicating a mixture of aquatic organisms and terrigenous organic matter, which may be not responsible for the different 17α(H)-diahopanes distributions. Mineral compositions indicate clay present in those samples with a distinctly enhanced abundance, which likely suggests that catalysis plays a critical role on the formation of arranged hopanes. Moreover, the relative contents of 17α(H)-diahopanes appear to be affected by the depositional environment due to a positive correlation with those of gammacerane, indicating that a more reducing water body is more helpful for the formation of 17α(H)-diahopanes, which is different from the conclusions in the previous studies. Therefore, the biomarker 17α(H)-diahopane may be an effective indicator for oil-oil/source correlation in this study area.