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Abstract: CO2 injection is a significant enhanced oil recovery method in shale oil reservoirs and facilitates the mitigation of CO2 emissions. However, the phase behavior and miscibility process of light shale oil and CO2 system in shale reservoirs with widely distributed nanopores remain uncertain. Based on the thermodynamic equilibrium theory and the modified Peng-Robinson equation of state (PR-EOS), a confined fluid model considering the effect of nanoconfinement (critical property shift and adsorption) and capillarity was used to study the phase diagram and thermodynamic property of shale oil-CO2 mixtures. The validity of the fluid model in bulk and in nanopores was verified with the pressure-volume-temperature (PVT) experiments and literature data, respectively. The interfacial tension (IFT) and minimum miscible pressure (MMP) were determined by the Parachor model and IFT vanishing method (VIT), respectively. The effects of pore sizes, temperature and injected gas type and compositions on the IFT and MMP was comprehensively investigated. The result shows that the nanoconfinement effect causes the two-phase region in the phase diagram of reservoir fluids to contract and enhances the ability of CO2 and light components to enter smaller pores, thus reducing the bubble point pressure, oil density, oil viscosity and IFT of shale oil-CO2 mixtures in nanopores. The nanoconfinement effect is more pronounced in pore radius of less than 50 nm, with roughly 16% reduction in the MMP of shale oil-CO2 mixtures. Temperature has a negative effect on the IFT and MMP of shale oil-CO2 mixtures due to the decreased solubility of CO2 under high temperature. The miscibility of CO2 and shale oil is improved by propane (C3H8) and ethane (C2H6), while decreased by methane (CH4).