Geochemical Characteristics And Significance In Crude Oils From Deep Buried Reservoirs In The Shunbei Area,Tarim Basin

Assessment on the oil genetic types in deep buried reservoirs under high temperature and pressure condition,has been one of the main challenges in the deep petroleum exploration.The Ordovician oils from the Yingshan Formation and the Yijianfang Formation at the depth of 7300?8000m in the Shunbei area,Tarim Basin provide an opportunity to probe the genetic types within the geochemical background.The characteristics of the light hydrocarbons and the stable carbon isotopes of individual compounds,the stable carbon isotopes of n-alkanes,biomarkers and aromatic hydrocarbons of the crude oils in the Shunbei area,Tarim Basin are systematically analyzed.The highlights of this study are:?1?The crude oils in the Shunbei area,Tarim Basin are light oils and condensate.The biomarkers in the crude oils from the Shunbei area suggest that the features are similar to that of crude oils from Tahe area.They may have the same hydrocarbon sources.?2?The stable carbon isotopes of n-alkanes in the crude oils of Shunbei area are2?4‰ heavier than the typical end member oils that come from Cambrian source rocks in the Lower Paleozoic oil-gas system of Tarim Basin.The carbon isotopic fractionation is caused by the thermal maturation.It reveals that the deep crude oil in the Shunbei area,Tarim Basin may come from the Cambrian source rocks.?3?The map of source and maturity based on alkylbenzene and alkylnaphthalene is constructed.Although the crude oils from Well Shunbei 1 and Shunbei 5 have unified sources and high degree maturity,there are significant differences in the thermal maturity of crude oils between different fault zones,and even within the Well Shunbei 1 fault zone.?4?Based on the ratios of C₂-B/nC₈ and C₃-B/nC₉ and C₃-B/nC₉ v.s.?¹³CMCH,it suggests that there are some differences among different oils.The crude oils fromWell Shunbei 1,Shunbei 2,and Shunbei 5 are kerogen thermally degraded products,while the one from Well Shunbei 3 probably experiences slightly thermal alteration.As indicated by relatively heavy carbon isotopic compositions of methylcyclohexane and low aromatization characteristics,the crude oil from Well Shunbei 7 needs further study.