Measurement of Carbon and Hydrogen Isotopes of Natural Gas Hydrate-Bound Gases by Gas Chromatography-Isotope Ratio Mass Spectrometry

The isotopic compositions of natural gas hydrate-bound gases can provide valuable information about the origin, migration and accumulation process of the gases. At present, the instrumental analysis methods for carbon and hydrogen isotope of natural gas hydrate-bound gases mainly refer to that for the natural gases. However, there is a lack of comprehensive studies on the influences of preparation technologies such as decomposition, collection and storage. In this paper, a Gas Chromatography-Isotope Ratio Mass Spectrometry (GC-IRMS) method was established for the measurement of carbon and hydrogen isotope of natural gas hydrate-bound gases (C₁-C₃ , CO₂). The practical applicability of different gas decomposition and collection methods (i.e. headspace, syring and drainage), and the storage effect of different decomposed gases' storage vessels (i.e. aluminum air bag, glass bottle with butyl rubber plug) were comparatively studied. The results indicate that the best preparation methods are vacuum headspace-decomposition and in situ storage. Good precision of δ¹³C-(C₁-C₃, CO₂) and δD-(C₁-C₃) is in the range of 0.12‰-0.23‰ and 1.0‰-1.8‰ with relative standard deviations (RSD, n=6) of 0.38%-0.86% and 0.62%-1.00%, respectively. The method was successfully applied in the determination of gas isotopic compositions of natural gas hydrate-bound gases from the South China Sea, Qilian Mountain Permafrost and laboratory synthetic samples. This method is easy to operate, can be applied widely, and is suitable for the determination of isotopic compositions of hydrate-bound gases.