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.