Studies on gas isotopes of natural gas hydrates recovered from the Qilian Mountain permafrost is an effective way to understand the genesis and origin of the gas. In this paper, 8 gas hydrate bearing core samples have been collected from 2 scientific drilling holes ( DK2 and DK3 ) at the southern foot of the Qilian Mountain permafrost. The samples were decomposed under vacuum conditions, and the released gases were measured. The gas components and stable isotopic compositions ( I. E. Carbon and hydrogen) were measured using Gas Chromatography (GC) and Gas Chromatography-Isotope Ratio Mass Spectrometer (GC-IRMS) , respectively. The results show that the stable isotope compositions of the released gases varied significantly. The carbon isotope δ13 C values of CH4, C2 H6 and C3 H8 ranged from -52. 6‰ to -48. L‰, from - 38. 6‰ to - 30. 7‰ and from -34.7‰ to -2l.2‰, respectively. The lowest δ13C value for CO2 was -27.9‰, while the highest value was 16.7‰. The hydrogen isotope δD values of CH4 , C2H6 and C3H8 ranged from -285‰ to -227‰, from -276‰ to -236‰ and from -247‰ to - 198‰, respectively. The isotopic characteristics of carbon and hydrogen were comprehensively investigated by relevant diagrams, including a plot of C1/( C2 + C3 ) - δ13 C1 and a plot of δ13C -SD of CH4. The results indicate that the hydrate-bound gases in the Qilian Mountain permafrost are mainly thermogenetic gas originating from organic genetic gas formed in a freshwater environment.%开展祁连山冻土区天然气水合物气体同位素研究,是解决其气体成因、来源等科学问题的一个重要手段.本研究采集祁连山南麓多年冻土区水合物科学钻探DK2和DK3孔共8个含水合物的岩芯样品,采用真空顶空法收集样品中水合物的分解气,分别用气相色谱(GC)、气相色谱同位素比值质谱(GC-IRMS)测定其气体成分和同位素组成,测试结果表明:祁连山冻土区天然气水合物样品的气体碳氢同位素变化较大,甲烷、乙烷和丙烷的碳同位素(δ13C)变化范围分别为-52.6‰～-48.1‰、-38.6‰-30.7‰和-34.7‰～21.2‰,而二氧化碳的碳同位素(δ13C)最低为-27.9‰,最高为16.7‰;甲烷、乙烷和丙烷的氢同位素(δD)变化范围分别为-285‰～227‰、-276‰～236‰和-247‰～198‰.通过对这些碳氢同位素进行综合研究,包括气体分子组成与同位素的关系分析、甲烷的碳氢同位素之间的关系判断等,结果表明研究区天然气水合物的气体主要来源于热解气,而且是在淡水环境中形成的有机成因气.
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