塔西南坳陷山前带天然气地球化学特征和成因

Geochemical characteristics of natural gas and its genesis in piedmont zone of southwest Tarim basin

阿克莫木气藏,柯克亚—柯东气藏天然气地化特征差异明显,天然气类型及气源岩一直存在争议.通过对塔西南坳陷山前带天然气稳定碳氢同位素及轻烃组成特征的分析,研究了天然气成因类型,并在此基础上,进行了气源对比,明确了气源岩类型.综合运用轻烃三角图版、稳定碳同位素图版、Mango参数、Whiticar图版及碳氢同位素图版,认为阿克莫木气藏为来自于下侏罗统的煤成气,柯克亚—柯东气藏为来自于石炭系及侏罗系混源成因的天然气.研究结果表明:阿克莫木气藏干燥系数接近1,δ^(13)C_1值为-24.2‰^-23.3‰,是塔西南坳陷山前带最重的,为来源单一的煤成气;而柯克亚—柯东气藏干燥系数均小于0.90,且存在单项性碳同位素倒转,为混源成因的湿气.前者气源岩主要为下侏罗统康苏组,干酪根以Ⅲ型为主;后者气源岩为石炭系,同时混有侏罗系气源岩,干酪根以Ⅱ-Ⅲ型为主.阿克莫木气藏苯的体积分数高达31.10%~37.38%,甲苯的体积分数为9.63%~13.46%,天然气成熟度远大于柯克亚—柯东气藏.

There are obvious differences between Akemomu and Kekeya-Kedong gas reservoir in geochemical characteristics, and the types and source rocks of natural gas in the two reservoirs have been controversial. Through analyzing the characteristics of stable hydrocarbon isotope and light hydrocarbon composition, this paper studied the origin of natural gas in piedmont zone of southwest Tarim basin. On this basis, the gas source was compared and the types of source rocks were determined. Combining light hydrocarbon system triangle chart, stable car- bon isotope chart, Mango parameter, Whiticar chart and carbon-hydrogen isotope chart, it is considered that the coal-derived natural gas in Akemomu gas reservoir is from I.ower Jurassic and the mixed natural gas in Kekeya-Kedong gas reservoir is from Carboniferous and Jurassic. The results show that the drying coefficient of Akemomu gas reservoir, which is single-sourced and coal-derived natural gas, is about 1 and the δ13C1 value is between -24.2‰ and -23.3‰, the heaviest in piedmont zone of southwest Tarim basin. While the drying coefficient of Kekeya-Kedong gas reservoir, which is humid gas with mixed source, is below 0. 90 and the carbon isotope is partly inverted. The source rock of the former is Lower Jurassic Kangsu For- mation and the kerogen type is III. The source rocks of the latter are Carboniferous mixed withJurassic and the kerogen types are II- III. The relative content of benzene and toluene in Ake- momu gas reservoir is 31.10%-37.38% and 9.63%-13. 46% respectively, which indicates that the maturity of Akemomu natural gas is much higher than that of Kekeya-Kedong gas res- ervolr.