The pore throat structure of sand-gravel reservoirs in Xinjiang is mainly composed of intergranular pores and intragranular dissolution pores. The absolute content of clay minerals is 2.93%, which is moderate to weak water sensitivity. The reservoir heterogeneity is strong, and the remaining oil occurrence state and production mechanism after gas flooding are complex, which restricts the development effect of the reservoir. This article evaluates the distribution of core pores through high-pressure mercury injection, quantitatively characterizes the occurrence status of residual oil in gas flooding at different pore throat scales through gas flooding experiments combined with nuclear magnetic resonance (NMR) technology, and analyzes the mechanism of residual oil production under different gas flooding conditions. The recovery degrees of the three injected gases are 57.24% (CO2), 52.17% (CH4), and 47.71% (N2), respectively. CO2 is mainly driven by the mechanism of mixed phase displacement dissolution expansion, which is effective on nano scale pores and throats such as small pores. The degree of recovery is mainly contributed by micro and small pore throats with pore throat radii less than 0.5 μm, with recovery rates of 26.81% and 13.18%, respectively; CH4 is mainly driven by a mixed phase drive dissolution extraction mechanism, and its utilization is similar to CO2. The extraction degree of micro and small pore throats with pore throat radius less than 0.5 μm is 34.36% and 14.36%, respectively; The degree of utilization of N2 on small pores is extremely small, with extraction rates of 5.24% and 26.68% for medium and large pores, respectively. The extraction rate for micropores with pore throat radius less than 0.1 μm is only 3.54%.