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

摘要： 对南八仙油气藏E₃¹与N₂²—N₁油层油气地球化学特征和油源的关系进行了分析,根据油气分异作用与其组成变化的相互关系研究了该油气藏的蒸发分馏作用,并从地质构造背景和油气及其烃源岩地化特征方面探讨了其成因机制.研究结果表明,虽然南八仙油气藏E₃¹与N₂²—N₁两层系的原油对比表明两者是同源,但它们在轻烃组成和正构烷烃系列分布上存在显著差异.E₃¹原油轻烃中正烷烃含量很少,而芳烃化合物含量异常高,其正烷烃摩尔分数对数曲线为折点在nC₁₀的折线,表明其经历了强烈的蒸发分馏作用.而N₂²—N₁原油轻烃中正烷烃占优势,高碳数化合物较少,说明该层原油是从E₃¹油气层通过蒸发分馏作用向上运移而来.该含油气构造带的断裂系统和高成熟的油气及其煤系烃源岩是导致蒸发分馏作用的主控因素

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