Abstract: A field test of nitrogen injection for enhanced oil recovery was conducted in the deep carbonate oil reservoir of Fuman Oilfield, Tarim Basin. In the nitrogen injection process, whether phase miscibility occurs can affect oil recovery efficiency, and understanding the variations in fluid phase states during nitrogen injection under non-equilibrium conditions in the formation is of significant importance. To clarify the changes in fluid phase state and the miscibility pressure during nitrogen injection-assisted gravity flooding in deep carbonate reservoirs, Manshen X reservoir was taken as an example. By conducting nitrogen injection phase experiments, the phase characteristics under non-equilibrium and equilibrium conditions after nitrogen injection were obtained. The discrimination diagram of variation of formation fluid type with injection volume and the diagram of variation of formation fluid saturation pressure with injection volume were established, thereby determining the miscibility pressure during nitrogen injection. The results showed that under non-equilibrium conditions, when a small amount of nitrogen was injected, it mixed with formation fluids to form a single phase. As the injection volume increased, a three-phase coexistence state appeared with a nitrogen phase at the top, a gas phase in the middle, and an oil phase at the bottom. When the nitrogen injection volume reached 0.8 PV, the nitrogen phase gradually disappeared. Under equilibrium conditions, after nitrogen injection, the fluids showed an "opalescence phenomenon" at the saturation pressure, and the saturation pressure increased with increasing injection volume. After the injection volume reached 0.2 PV, the fluids exhibited two phases under an injection pressure of 55 MPa. After the injection volume reached 0.8 PV, the fluids presented two phases under a formation pressure of 81 MPa. Based on the discrimination diagram of variation of formation fluid type with injection volume and the diagram of variation of formation fluid saturation pressure with injection volume, the minimum miscibility pressure for nitrogen injection in the Manshen X reservoir was determined to be 48 MPa. Under an injection pressure of 55 MPa, nitrogen injection-assisted gravity flooding could achieve miscibility. Under non-equilibrium conditions, a significant gravity segregation characteristic developed from top to bottom, resulting in a gradual decrease in nitrogen content and a gradual increase in hydrocarbon component content from the top to the bottom of the Manshen X reservoir after gas injection.