CO₂ injection in shale oil/gas reservoirs is a feasible method for CO₂ geological sequestration and enhanced oil recovery. However, the mass transfer mechanisms in inorganic pores and organic matter (kerogen) are still ambiguous. Thus, the mechanisms of diffusion and adsorption were experimentally and numerically investigated. A novel adsorption-diffusion model was proposed for mass transfer of CO₂ and oil in shale reservoirs. Mathematical models for hydrocarbon mass transfer in cylindrical shaped shale and limestone samples were derived. The predicted responses of the mathematical models closely matched the experimental data of CO₂ injection experiments performed on various shales and tight rocks under high pressure and high temperature conditions. Hydrocarbon recovery of shales shows a delayed effect compared to tight rocks due to the adsorption and diffusion in kerogen. Hydrocarbons were extracted out of the bulk kerogen and then diffused through the inorganic and organic pores. Hydrocarbon diffusion coefficients and mass transfer rate coefficients for shale samples were obtained by matching experiments and mathematical models. Hydrocarbon diffusion coefficients in limestone were obtained. Both diffusion coefficients and extraction rate coefficients decreased exponentially with carbon numbers. The extraction rate coefficients decreased exponentially because the diffusion coefficients of hydrocarbons in kerogen also decreased exponentially with carbon numbers. This research advances the mass transfer theories and promotes the CO₂ storage and enhanced oil recovery in shale formations.