Effect of Oxygen and Methane on Stable Isotope Fractionation of Nitrogen between Gas and Hydrate Phases during Formation of Mixed-Gas Hydrates

Knowledge of the isotope fractionation of guest molecules during natural gas hydrate formation may contribute to the understanding of the gas hydrate formation process. In this study, the stable isotope fractionation of nitrogen between gas and hydrate phases during the formation of nitrogen + oxygen and nitrogen + methane mixed-gas hydrates in the temperature range of (226–271) K was investigated, and the nitrogen isotopic fractionation factor α_H–V of the guest gas was determined. In the nitrogen + oxygen system, the 10³lnα_H–V was +0.15 ± 0.03 in the case of air composition, and air hydrate was slightly concentrated ¹⁴N¹⁵N. As the composition of nitrogen decreased, 10³lnα_H–V changed from positive to negative. In contrast, in the nitrogen + methane system, 10³lnα_H–V increased with decreasing composition of nitrogen. The dependence of the nitrogen isotope fractionation factor on the composition of mixed gas may be due to the selective encapsulation of guest molecules in large and small cages comprising the crystallographic structure of gas hydrates. These observations suggested that light nitrogen (¹⁴N₂) and heavy nitrogen (¹⁴N¹⁵N) were tend to be encapsulated in large and small cages, respectively.