Hydrate equilibrium conditions formed from deuterated methane isotopologues (CH 3 D and CD 4 ) were measured over a temperature range of (273 to 278) K and a pressure range of (2.6 to 4.2) MPa. The phase boundary of CH 3 D and CD 4 hydrates was shifted to higher pressures and lower temperatures than that of the CH 4 hydrate. Crystal structure and cage occupancies of gas hydrate crystals formed from CH 3 D and CD 4 were characterized by Raman spectroscopy. The isotope effect of methane hydrates revealed in this study can be used to explain the isotopic fractionation of hydrogen in methane during the formation of clathrate hydrates observed in natural settings, as reported by previous studies.

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