This paper investigates the use of CO2 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 N2 injection in order to establish the nature of prevailing fluid communication and identify a confined site for CO2 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 CO2-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 CO2-EOR process mechanisms. CO2 and N2 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 CO2 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 CO2 injection as compared with N2 injection due to a combination of oil swelling, reduced oil viscosity and partial miscibility with CO2. The paper concludes that the efficiency of CO2 injections is more pronounced at higher reservoir pressures and with no or less volumes of prior injected N2.