The recent rise in global shale gas production rates has renewed interest in making extraction more efficient. Enhanced recovery using CO2 injection can be a potential solution and achieve two simultaneous benefits: more CH4 recovery and secure CO2 storage. This would be accomplished by displacing CH4 in the shale reservoir by exploiting the preferential adsorption of CO2 on the shale surface. To properly assess the potential of enhanced recovery, a robust understanding of the Gas-in-Place (GIP) and the factors that influence it is required. In this study, high pressure adsorption measurements with CO2 and CH4 have been performed on two shales, the Longmaxi and the Marcellus shales, using a Rubotherm Magnetic Suspension Balance. To further evaluate the effect of shale composition on adsorption, the same measurements have been performed on a synthetic mesoporous carbon. The three sets of adsorption isotherms have been used to compute the GIP at subsurface conditions. The agreement between the shales and the mesoporous carbon data suggests that shale’s mesoporosity drives shale gas storage and that the organic matter is the main influence on gas adsorption capacities in shales.