Shale is composed of two distinct permeable media: (1) inorganic, and (2) organic. Both media may contribute to the amount of hydrocarbon and non-hydrocarbon species in shale. In this work, we investigate sorption in clay minerals which may constitute most of the inorganic matter in shale. We represent the inorganic matter by the montmorillonite clays with different charges for different atoms. Sorption of methane and carbon dioxide is investigated by Monte Carlo simulations. In this work, we assume that methane is structureless and CO2 is assumed to have structure and we assign partial charges to its atoms. Our results indicate that charge affects the orientation of CO2 molecules close to the surface and plays an important role in CO2 sorption. Methane sorption is found to be mainly a function of surface area. We also incorporate cation exchange in clay description and model its effect on sorption. Cation exchange increases CO2 sorption at low pressure significantly and as pressure increases, the effect becomes less pronounced. Cation exchange also affects orientation of CO2 molecules near the surface. Results from our simulation are expected to provide insight into phase behavior in clays, a major constituent of shale media. (C) 2013 Elsevier B.V. All rights reserved.