Geochemistry and origin of crude oils of the Yanchang formation in Jiyuan area

The geochemical features, origin and genesis are studied systematically by using group composition, stable carbon isotope and saturated hydrocarbon analysis. The results show that oil is typical lacustrine-derived oil, with δ13CPDB value generally from -32.79 ‰～-30.69 ‰ . Sterane and terpane biomar kers of compounds in oil from the region and source rock from Ordos basin were systematically analyzed. The oil contains a certain amount of pregnane, homo-pregnane, rearranged sterane and enriches αβ and βα isomers,while ααα-20R sterane isomers display an asymmetric Vtype distribution pattern. Its terpane distribution is characterized by high content of pentacyclic terpane, low content of gammacerane and there are significant differences in relative abundance of 17α (H)-diahopanes. The biomarker distributions of the oils show that they were formed under a weak reducing and fresh water environment. The results of oil-source rock correlation showed that the oil was mainly derived from Chang-7 source rock.