The Kekeya natural gas reservoir of lower Jurassic was discovered in northern piedmont zone in Turpan-Hami basin. In order to define the character, the origin and the accumulation mode of the gas during middle-upper Jurassic oil accumulation, geochemical and dissection of gas accumulation were studied. The results show that the gas is humic type deviated sapropel with the carbon isotope ratio of methane from -44.0%o to -38.7%o and carbon isotope ratio of ethane from -29.0%o to -27.5%o. The physical property and biomarker feature of association oil present that this coal measure mature oil has different origins from middle-upper Jurassic hydrocarbon. So the geologic conditions and the data of geochemistry prove that the gas is in mature phase with short vertical migration distance and is from coal measure source rock of Badaowan Formation. The tight sand reservoir distributed widely has been formed before natural gas filled in, and lower Jurassic abnormal overpressure developed, and the resource and the reservoir superposed intensely. Because of these tight sand gas reservoir forming conditions, this independent gas system which is the new natural gas exploration in Tuha basin is formed.%为了明确山前带柯柯亚下侏罗统气藏,对于这套位于中上侏罗统油气藏之下的天然气性质、来源及成藏模式,运用地球化学分析和天然气气藏解剖方法进行研究.研究结果表明:本区天然气甲烷碳同位素分布在-44.0‰～－38.7‰,乙烷碳同位分布在-29.0‰～－27.5‰,属于偏腐泥型的腐殖气,伴生原油的物理性质和生物标志物呈现出典型的成熟煤系油特征,并与中上侏罗统油气来源不同,即地质条件和地球化学数据都证明天然气母质处于成熟演化阶段,垂向运移距离短,来自八道湾组煤系泥岩；天然气大量充注前大面积致密储层的形成、下侏罗统异常超压的发育以及源储的紧密叠置,都是形成致密砂岩气藏的地质条件.
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