Origin of Chang 8 Oil Reservoir in Jiyuan Region of Ordos Basin

Chang 8 oil reservoir is one of the most important hydrocarbon accumulation horizons in Jiyuan region of Ordos Basin.However,the research on genesis of the oil is still known little.In this paper,sterane and terpane biomarkers of compounds in oil from the region and source rock from Ordos basin were systematically analyzed.The oil is typical lacustrine-derived oil,with d13CPDB value generally from-31.11 ～-31.57‰.Its terpane distribution was characterized by relatively abundant 17α(H),21β(H) – hopanes.The relative content of C29 hopane of Chang 8 oil in NE migration-accumulation zone was higher,C29Ts takes second.The relative content of C30 rearranged hopane in SW migration-accumulation zone is higher,C30* hopane / C29 norhopanes is greater than 1.The oil contains rich pentacyclic triterpanes,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.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.