The results from novel in situ high-pressure nuclear magnetic resonance (NMR) spectroscopy, infrared (IR) spectroscopy, and X-ray diffraction (XRD) investigation of the interaction of the smectite hectorite with variably wet supercritical methane (scCH4) at 90 bar and 323 K (hydrostatic conditions equivalent to ∼1 km depth) show that CH4 occurs in the clay interlayers, in pores external to the individual clay particles, and as bulk fluid. The occupancy of each environment depends on the relative humidity (RH) of the CH4-rich fluid and the hydration energy and size of the charge-balancing cation. As RH increases, the fraction of interlayer and interparticle CH4 decreases, although with Cs+, addition of a small amount of H2O initially increases CH4 uptake. Maximum interlayer CH4 adsorption occurs when the mean basal spacing just permits methane intercalation (∼11.5 A) and never below this basal spacing. It is also higher with divalent cations than with monovalent cations. The data show that CH4 adsorption o...