This paper presents a new perspective on modeling of CO2 and miscible gas injection into shale oil plays for potential enhanced oil recovery (EOR) and CO2 storage. Our major points are the conceptual understandings of the dominant trapping and the oil recovery mechanisms behind miscible gas injection. This paper investigates the efficiency of miscible gas (solvent) injection into shale oil reservoirs with a wide range of permeability (from 1 to 100 mu D). We set up a large-scale numerical model to simulate and capture the important mechanisms behind various miscible gas injection and geological storage scenarios. This numerical study demonstrates that injecting miscible gas such as CO2 and recycled gas rich in ethane substantially increases oil recovery in shale oil reservoirs. Numerical simulation models reveal that miscibility and CO2 adsorption, along with gas diffusion, are important physical mechanisms. However, recycled-enriched gas injection demonstrated a larger oil recovery rate compared to miscible CO2 injection. On the other hand, CO2 trapping is considerable, because of adsorption and other traditional trapping mechanisms in shale plays. The amount of CO2 trapped in unconventional reservoirs can be a significant fraction of the total injected amount (similar to 25 to50% including the important and dominant trapping mechanisms, e.g. CO2 dissolution in oil and water, adsorption, residual, and mobile gas saturations). Results show that molecular diffusion can speed CO2 flux delivery to larger matrix area and thus contribute to oil recovery, and become trapped and adsorbed on minerals or organic contents.