祁连山冻土区天然气水合物分解气碳氢同位素组成特征

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。

Carbon and Hydrogen Isotopic Compositions Characteristics of the Released Gas from Natural Gas Hydrates in the Qilian Mountain Permafrost

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 δ13C values of CH4,C2H6 and C3H8 ranged from-52.6‰ to-48.1‰,from-38.6‰ to-30.7‰ and from-34.7‰ to-21.2‰,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)-δ13C1 and a plot of δ13C-δD 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.