For carbon dioxide (CO2) sequestration in depleted shale gas reservoirs or CO2 injection as an enhanced shale gas recovery technique, it is important to understand the adsorption mechanism in these reservoirs. In this study, experimental adsorption measurements for Dadas shale samples were conducted at 25 degrees C, 50 degrees C, and 75 degrees C up to approximately 2000 psia by using pure CO2 (maximum adsorption capacity 0.211 mmol/g at 25 degrees C) and pure methane (CH4) (maximum adsorption capacity 0.0447 mmol/g at 25 degrees C). By using Langmuir isotherm and Ono-Kondo lattice models (three-layer and monolayer), experimental adsorption results were evaluated and adsorption isotherms were constructed. It was concluded that Ono-Kondo monolayer model is really capable of fitting adsorption isotherms, especially at high pressures for CO2 adsorption. For initial gas-in place calculations, the equations used by the help of Langmuir isotherm were modified with Ono-Kondo monolayer model and proposed to calculate the amount of CO2 that might be stored as adsorbed and free gas in depleted shale gas reservoirs. For the case in this study, it was calculated that adsorbed gas concentration changes from 39.2% to 71.8% between 5000 psia and 500 psia. Moreover, binary mixture Ono-Kondo monolayer model was used to evaluate the adsorption isotherm of CO2 CH4 mixtures by using their pure adsorption experimental data. This data is useful if there is a purpose to inject CO2 as an enhanced shale gas recovery technique because of the adsorption capacity difference between CH4 and CO2. (C) 2016 Elsevier B.V. All rights reserved.