关键词: 柴达木盆地, 南八仙油气藏, 轻烃, 正构烷烃, 碳同位素, 蒸发分馏

Abstract: The geo-chemical characteristics and genetic origins of crude oil and gas of E₃¹ layer in Nanbaxian Reservoir were compared with those of N₂² and N₁layers.Based on the relationship between fractionation and compositional alteration of oil and gas,the evaporative fractionation in Nanbaxian Reservoir was analyzed.The genetic mechanism of the reservoir was investigated,according to the geological setting and geo-chemical features of hydrocarbon and source rocks.The research result shows a significant difference presented in their compositions of light hydrocarbon and distribution of n-alkane homologues,though crude oils of E₃¹,N₂² and N₁ layers in Nanbaxian Reservoir have the same genetic origin.The nC₅—nC₉ light hydrocarbon of crude oil in E₃¹ layer was depleted in n-alkanes and remarkably rich in aromatic compounds.The distribution curve of log molar fraction for n-alkanes has a slope break at C₁₀ point,which suggests that the evaporative fractionation happened in this reservoir.Whereas,the crude oils from N₂² and N₁ layers comtain light hydrocarbon dominated by n-alkanes and low amount of high molecular weight n-alkanes,which shows that the oils of N₂² and N₁ layers were derived from E₃¹ layer by evaporative fractionation.The petroliferous faults in the reservoir and high maturity of hydrocarbon and coal-bearing source rocks are the main factors for controlling the evaporative fractionation.

Key words: Qaidam Basin, Nanbaxian reservoir, light hydrocarbon, normal alkane, carbon isotope, evaporative fractionation