Geochemical significance of 17α(H)-diahopane and its application in oil-source correlation of Yanchang formation in Longdong area, Ordos basin, China

Abstract

Relatively high abundance of 17α(H)–C30 rearranged hopane has been detected in some oils and rock extracts from Yanchang Formation of Longdong area, southwestern Ordos basin. Oil shale, silty mudstone and carbonaceous mudstone extracts from Yanchang Formation all have low abundance of C30 rearranged hopane, while high concentrations of C30 rearranged hopane were observed in the dark mudstone extracts. Analysis on the geochemical parameters of the organic matter origin, thermal maturity, and sedimentary organic facies were carried out. The result showed that the depositional-redox environment and lithology may be the critical factors for the rearrangement of hopanes which usually occurred in suboxic condition with clay-catalyzed reactions, as is consistent with the viewpoint of Peters and Moldowan (1993). According to the relative abundance of diahopane, three oil classes can be identified. Class 1 oil with low abundance of diahopanes was originated from oil shale of Chang 7 member, while Class 2 oil and Class 3 oil with middle to high abundance of diahopanes were derived from dark mudstones of Chang 4 + 5 to Chang 9 members of Yanchang Formation. Of the 151 analyzed oil and extracts samples from different stratigraphic intervals, 105 samples belong to Class 1 oil coming from oil shale, while 46 samples belong to Class 2 and Class 3 oil derived from mudstones. So oil shale of Chang 7 member is the main effective source rock of Longdong area, and dark mudstones of Chang 4 + 5 to Chang 9 member are also important source rocks.