Abstract: In order to provide clarity in the microscopic oil displacement mechanisms of tight reservoirs displaced by different gas injection media, core displacement experiments of N₂ and CO₂ injection were carried out respectively based on the principle of NMR T₂ spectrum test. The microscopic displacement mechanisms of N₂ immiscible flooding and CO₂ miscible flooding were studied at the microscopic pore scale, and the oil production from pores with different pore sizes during displacement was evaluated. The results show that final recovery percent from N₂ immiscible flooding and CO₂ miscible flooding has little difference. The N₂ flooding process can be divided into three stages: the non-breakthrough stage, the early breakthrough stage and the mid-late breakthrough stage. The recovery percent from small pores is higher than that from large pores. However, the oil recovery percent from large pores is greatly improved in CO₂ miscible flooding, while it is relatively low from small pores. The distribution of microscopic pore structure is the main cause for the difference in recovery percent between large and small pores in the process of N₂ and CO₂ flooding. The results show that the development effect of N₂ flooding in tight reservoirs is better than that of CO₂ flooding, which provides a theoretical basis for the development of Chang 6 reservoir by N₂ flooding in Ansai Oilfield .