库车坳陷迪北凝析气藏流体地球化学特征及成藏过程解析

Analysis of Fluid Geochemical Characteristics and Accumulation Process of Dibei Condensate Gas Reservoir in Kuqa Depression

通过对迪北地区侏罗系储层中油、气、水等流体的地球化学分析,探讨了库车坳陷北部凝析气藏的成藏过程。同位素分析表明,迪北侏罗系储层存在两期成藏,早油晚气;油气均具有混源特征,主要来自下伏三叠系湖相烃源岩,也受到侏罗系煤型烃源岩产物的侵染。原油轻烃组分及正构烷烃组成特征表明,侏罗系圈闭中早期成藏的原油受到晚期充注天然气的气洗改造。气洗作用导致原油中同分子量的正烷烃相对贫化,而环烷烃和芳烃等化合物相对富集。侏罗系储层中油、气、水等流体具有油上气下以及局部气层中间产水等异常分布的特点,对其流体的地球化学特征进行分析,结果表明,流体的上述非重力分异与油气动态成藏过程密切相关。侏罗系储层致密化窗口期恰好发生于原油和天然气两期成藏之间,致使早期原油及部分残留水被致密化储层禁锢,在后期天然气充注时储层中流体无法按重力进行分异,这是导致侏罗系流体形成非重力分异的主要原因。

The forming process of condensate gas reservoir in the northern Kuqa Depression is discussed based on the geochemical analysis of oil,gas and water in the Jurassic reservoir of Dibei area.Isotope analysis shows that there are two periods of hydrocarbon accumulation in the Jurassic reservoir of Dibei area,early oil accumulation and late gas accumulation;the oil and gas have mixed source rocks,mainly from the underlying Triassic lacustrine source rocks,but also infected by the Jurassic coal type source rocks.The characteristics of light hydrocarbon components and n-alkanes in crude oil show that the crude oil formed in the early Jurassic trap was reformed by the natural gas filled in the late stage.The gas washing effect of natural gas on crude oil results in the relative lack of n-alkanes with the same molecular weight in crude oil,while the relative enrichment of cycloalkanes,aromatics and other compounds.There is the abnormal distribution of oil,gas and water in Jurassic reservoir:The oil is at the top,the gas is at the bottom,and there is water in the middle of local gas reservoir.The analysis of fluid geochemical characteristics shows that the non gravity differentiation of fluid is closely related to the dynamic hydrocarbon accumulation process.Jurassic reservoir densification happened between the two stages of oil and natural gas accumulation,resulting in the early crude oil and some residual water being confined by the densified reservoir,and the fluid in the reservoir not being differentiated according to gravity during the later natural gas charging,which is the main reason for the formation of non gravity differentiation of Jurassic fluid.