四川盆地元坝-通南巴地区须家河组天然气地球化学特征和成因

摘要

In order to investigate the origin and figure out the source of natural gas in the Upper Triassic Xujiahe Forma -tion in Yuanba-Tongnanba area of Sichuan Basin ,the composition and stable carbon and hydrogen isotopes were used to analyze the geochemical characteristics of the natural gas .The study indicates that the natural gas in the Xujiahe Forma-tion in the Yuanba-Tongnanba area is mainly composed of methane .It appears to be dry gas with a dryness coefficient generally higher than 0.97.The δ13 C1 and δ13 C2 values are in the range of -34.5‰~-29.3‰ and -35.4‰~-21.5‰,respectively.The δ13 CCO2 values are generally higher than -8‰,and the δDCH4 values are in the range of-181‰~-144‰without significant correlation with δ13 C1 values.The identification of gas origin and gas-source corre-lation indicate that the natural gas in the Xujiahe Formation is mainly coal-derived gas and sourced from the coal-measure source rocks in the Xujiahe Formation .The gas in the 1st and 2nd Members of the Xujiahe Formation in the Yuanba area and the Xujiahe Formation in the Tongnanba area contain a certain amount of secondary oil-cracking gas ,which are main-ly the product of the low-degree cracking of oil generated by the Wujiaping Fomation source rocks .Some gas samples from the Xujiahe Formation in the Yuanba-Tongnanba area display partial reversal of alkane carbon isotopic series , and the partial reversal of the gas from the 3rd and 4th Members in the Xujiahe Formation in the Yuanba area is resulted from the fact that they were generated by the Xujiahe Formation source rocks in the high-mature stage,while that from the 1st and 2 nd Members in the Xujiahe Formation in the Yuanba area and from the Xujiahe Formation in the Tongnanba area are attributed to the mixing of a small amount of oil cracking gas and a large amount of high-mature coal-type dry gas.%为了探讨四川盆地元坝-通南巴地区上三叠统须家河组天然气的成因,明确天然气的来源,综合利用了天然气组分和稳定碳、氢同位素等手段对天然气地球化学特征进行了分析. 研究表明,四川盆地元坝-通南巴地区须家河组天然气以甲烷为主,主体表现出干气特征,干燥系数普遍高于0.97,δ13 C1 和δ13 C2 值分别介于-34.5‰~-29.3‰和-35.4‰~-21.5‰,δ13 CCO2值多数高于-8‰,δDCH4值介于-181‰~-144‰,且与δ13 C1 值之间没有明显的相关性. 天然气成因鉴别和气-源对比研究表明,该区须家河组天然气主体为煤成气,来自须家河组煤系烃源岩,其中元坝地区须一、须二段和通南巴地区须家河组天然气中混入了一定量的原油二次裂解气,它主要为吴家坪组烃源岩生成的原油在裂解程度相对较低时的产物. 研究区须家河组烷烃气碳同位素系列普遍发生了部分倒转,其中元坝地区须三、须四段天然气的部分倒转主要源自须家河组烃源岩在高成熟阶段的产物,而元坝地区须一、须二段和通南巴地区须家河组天然气发生的部分倒转主要源自少量的原油裂解气与大量的高成熟煤型干气的混合.