  An accurate knowledge of cage occupancy of methane is central for
understanding the physical-chemical properties of gas hydrates, the actual
inventory of natural gas in hydrate deposits and the description of gas
exchange processes. Here we report the absolute cage occupancies, the cage
occupancy ratios and hydration numbers of the synthetic CH4-H2O and CH4-D2O
hydrates formed from the ice-gas system under different pressures and
temperatures. The results were obtained from Rietveld refinement using
high-resolution synchrotron X-ray powder diffraction patterns and from Raman
spectroscopic measurements. The small-cage occupancies of methane in the
deuterated hydrates are found to be slightly higher than in the hydrogenated
form, likely due to their different lattice constants. The CH4 occupancy in the
small cages agrees fairly well with the predictions of CSMGem at the formation
pressure of 3.5 MPa, but with the increasing formation pressure the
disagreement grows up to 11 percent. While some deficiency of the prediction
model cannot be excluded, the observed discrepancy may well be due to
experimental difficulties of reaching true equilibrium at higher pressures. The
experimentally determined large-to-small cage occupancy ratios of the synthetic
and natural CH4 hydrates formed from the water-gas system are consistently
higher than the results of CSMGem calculations. Possible reasons for these
discrepancies will be discussed.
