The organic matter in organic-rich shales has an important significance for the methane adsorption capacity on shales. Kerogen is simplified to ideal graphite, and oxygenated functional groups are grafted onto graphite surfaces to obtain different O/C atomic ratios, reflecting varying maturation levels of kerogen. The adsorption behaviors of methane in the pores of graphite with different O/C ratios were investigated by the. grand canonical Monte Carlo method. The results show that the isosteric heat of adsorption of methane is reduced with an increase in the pore size or a decrease in the O/C ratio. The methane adsorption capacity in micropores increases with. an increasing pore size, whereas it decreases with an increasing pore size in mesopores. The methane adsorption capacity in pores with the same pore size decreases with decreasing O/C ratio. The proportion of the adsorbed gas in the pores decreases either with increasing pressure with the same pore size or with-an increasing pore size under the same pressure. Methane in the organic pores of the organic-rich shales is mainly in the adsorbed state when the pore size is less than 6 nm. The adsorption sites of methane gradually change from lower energy adsorption sites to higher energy ones with increasing temperature, leading to the reduction of the methane adsorption capacity. The water molecules in the pore affected by van der Waals forces, Coulombic forces, and hydrogen-bonding interactions are close-to the oxygen-containing groups and occupy the adsorption space of methane molecules, leading to a decrease of the methane adsorption capacity. The reduction of the mole fraction of methane in the gas phase, change of adsorption sites of methane, and decrease of the adsorption space of methane generate the methane adsorption capacity for the methane/carbon dioxide binary, gas mixture adsorption system.