This paper investigates the use of CO 2 as an EOR solvent for a heavy oil and high permeability naturally fractured reservoir complex in Mexico. The complex is under partial pressure maintenance by Nitrogen injection. First geological features and production performance are analyzed to discern peculiar pressure trends caused by natural depletion and N 2 injection in order to establish the nature of prevailing fluid communication and identify a confined site for CO 2 injection testing. An East Block in the North fields due to its unique dynamic faulting characteristics is found nearly compartmentalized to serve as a suitable site for CO 2 -EOR injection studies. Second, a finely-gridded dual permeability compositional simulation sector model with local grid refinement and boundary flux scheme is constructed and a calibrated 8-component EOS along with full tensor molecular diffusion is implemented to model CO 2 -EOR process mechanisms. CO 2 and N 2 injections into the gas cap at varying rates and huff-n-puff injection in the oil column are simulated. The impact of injection rate is illustrated, where injection of CO 2 at low rates promotes diffusion and is shown to drain more of the matrix oil. The huff-n-puff simulation cases also indicate increased oil recovery and reduced matrix oil saturation by CO 2 injection as compared with N 2 injection due to a combination of oil swelling, reduced oil viscosity and partial miscibility with CO 2 . The paper concludes that the efficiency of CO 2 injections is more pronounced at higher reservoir pressures and with no or less volumes of prior injected N 2 .