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.

Hydrate equilibrium conditions for 13CH4 were measured for temperatures ranging from 270 to 278 K and for pressures ranging from 2.3 to 4.2 MPa. The equilibrium pressure values in a 13CH4–H2O system were nearly the same as those in a 12CH4–H2O system. The similarity between the equilibrium pressures for these two systems can be used to explain why an existing study found nearly no fractionation of CH4 carbon isotopes during the formation of synthetic CH4 hydrates. We also measured the hydrate equilibrium conditions for singly deuterated CH4 (CH3D) in a temperature range of 270–273 K and pressure range of 2.3–2.6 MPa. The quadruple points (the coexistence of ice, liquid water, solid hydrate, and gaseous methane) for the 13CH4–H2O, CH3D–H2O, and CH4–H2O systems were determined using phase equilibrium (p–T) data and the Clausius–Clapeyron equation. The difference between the equilibrium pressures for CH3D and CH4 hydrates below the quadruple point (∼0.02 MPa) was slightly smaller than above the quadruple point (∼0.04 MPa) as reported by an existing study. The results of the phase equilibrium p–T conditions for hydrates encapsulating CH4 isotopologues can be used to explain the difference in fractionation behaviors for carbon and hydrogen isotopes of CH4 during the formation of the CH4 hydrate. Moreover, we characterized the crystal structure and the cage occupancies of the 13CH4 hydrate using powder X-ray diffraction and Raman spectroscopy. Our results suggest that the effect of encapsulating 13CH4 molecules in hydrate crystals was nearly the same as for CH4.