Abstract: The light oil from Gulong shale is widely distributed, with favorable components and temperature-pressure conditions for miscibility with CO₂. Pre-fracturing with CO₂ injection and huff-and-puff can offer significant potential to enhance oil recovery. However, there is a lack of sufficient understanding of the high-pressure phase behavior of Gulong shale oil. Based on the equation of state and two-phase equilibrium theory, through verifying the results of constant mass expansion experiment and slim tube experiment for shale oil, the paper establishes a thermodynamic oil-CO₂ two-phase equilibrium model considering the nano-confinement effect, and also a calculation method for the minimum miscibility pressure based on the two-phase equilibrium model. This paper is a case study of Gulong shale oils in Well Guye 2HC and Well Guye 9HC, and elucidates the interphase mass transfer behavior of Gulong shale oil and CO₂, which is influenced by the factors such as maturity, oil/CO₂ ratio, pressure, and nano-confinement effect. The results show that with an increase in CO₂ mole fraction, the saturation pressure of Well Guye 2HC and Well Guye 9HC shale oils gradually decreases. Under reservoir temperature and pressure conditions, both Well Guye 2HC and Well Guye 9HC shale oils can be miscible with CO₂. Under the same amount of CO₂ injection, Well Guye 2HC shale oil exhibits a higher molecular weight and viscosity with a greater drop, and a lower saturation pressure and expansion coefficient with a smaller variation than Well Guye 9HC shale oil. The multilevel contact process of CO₂ injection shows that the dissolution capacity and extraction effect of CO₂ are similar in both Well Guye 2HC and Well Guye 9HC shale oils. After sufficient contact, C₁-C₆ components in the oil phase of Well Guye 2HC and Well Guye 9HC shale oils at the far end opposite to injection gas front are all extracted into the gas phase, and the mole fractions of CO₂ in the oil phase increase to 86.63 % and 87.35 %, respectively. The presence of nano-confinement effect reduces the compositional differences between oil and gas inside nanopores, leading to a decrease in the interfacial tension and minimum miscibility pressure, which is beneficial to the mutual dissolution and miscibility between CO₂ and shale oil. The impact of the nano-confinement effect on Well Guye 2HC and Well Guye 9HC shale oils is not significantly different. When the pore radius decreases from 100 nm to 10 nm, the minimum miscibility pressure of Well Guye 2HC and Well Guye 9HC shale oils with CO₂ is decreased by 20.90 % and 21.31 %, respectively. Understanding the phase behavior of fluids in shale oil reservoirs can provide a theoretical guidance for the optimization of CO₂ injection development.