塔里木盆地西南山前坳陷恰探1井二叠系天然气成因和来源

Genesis and source of Permian natural gas in well Qiatan⁃1 of piedmont depression,southwestern Tarim Basin

近期,塔里木盆地西南山前坳陷恰探1井在西天山冲断带二叠系碳酸盐岩地层取得了天然气勘探的重大突破,标志着塔西南山前地区一个全新勘探层系的发现。然而,该区存在多套烃源岩,且具有异常复杂的沉积和构造特征,对于恰探1井天然气成因及来源的研究十分薄弱,制约了其下一步天然气的勘探部署。为此,基于区域地质背景、天然气地球化学特征、潜在烃源岩特征等系统开展了恰探1井天然气成因及其来源研究。实测结果表明,恰探1井天然气以甲烷为主(83.53%),气体干燥系数(C_(1)/C_(1-5))为0.992,具有较高含量的N_(2)(8.36%)、CO_(2)(7.28%)和He(0.110%)。天然气中甲烷、乙烷、丙烷和CO_(2)的碳同位素值分别为-27.8‰、-20.2‰、-18.4‰、1.7‰。基于天然气组分、烷烃碳/氢同位素组成等综合判识认为,恰探1井天然气为高—过成熟阶段的煤型气。结合潜在烃源岩的分布、有机质丰度、类型、热成熟度特征认为,该井天然气主要来源于二叠系棋盘组烃源岩,可能还混入了少量碳同位素组成更重的无机烃类气体。另外,N_(2)、CO_(2)等非烃气体及He同位素证据均表明恰探1井天然气有一定比例的无机气体混入,幔源氦比例约占14.6%,其氦含量已达到富氦天然气(He≥0.100%)标准。

Recently,a significant breakthrough in natural gas exploration was achieved in well Qiatan-1 in the Permian carbonate strata of the Western Tianshan thrust belt in the piedmont depression of southwestern Tarim Basin,marking the discovery of a new exploration layer in the area.However,this region is characterized by multiple sets of source rocks and extremely complex sedimentary and structural features.Research on the genesis and source of the natural gas in well Qiatan-1 is insufficient,restricting its further natural gas exploration.There-fore,the study systematically investigated the genesis and source of the natural gas in well Qiatan-1 based on regional geological background,geochemical characteristics of the gas,and potential source rock features.The measured results showed that the natural gas in well Qiatan-1 was mainly composed of methane(83.53%),with a gas dryness coefficient(C_(1)/C_(1-5))of 0.992,and contained relatively high levels of N_(2)(8.36%),CO_(2)(7.28%),and He(0.110%).The carbon isotope values of methane,ethane,propane,and CO_(2) in the gas were-27.8‰,-20.2‰,-18.4‰,and 1.7‰,respectively.Based on the natural gas composition and alkane carbon/hydrogen isotope composition,the natural gas in well Qiatan-1 was determined to be coal-type gas in the high to over-mature stage.Considering the distribution,organic matter abundance,type,thermal maturity of its potential source rocks,the gas in this well was mainly sourced from Permian source rocks in the Permian Qipan Formation,and may also be mixed with a small amount of carbon isotopes,forming heavier inorganic hydrocarbon gases.In addition,the components and isotopic evidence of non-hydrocarbon gases such as N_(2),CO_(2),and He showed that a certain proportion of inorganic gas had mixed into the natural gas in well Qiatan-1.The helium isotopic composition suggested that the proportion of mantle-derived helium was about 14.6%,and the He content had reached the standard for helium-rich natural gas(He≥0.100%).