Research on the Gangxi III area in the Dagang Oil?eld shows that there was still a signi?cant amount of oil remaining in oil reservoirs after many years of polymer ?ooding. This is a potential target for enhanced oil recovery (EOR). Surfactant–polymer (SP) ?ooding is an effective chemical EOR method for mobilizing residual oil and improving displacement ef?ciency macroscopically, but the microscopic oil displacement ef?ciency in pores of different sizes is unclear. Nuclear magnetic resonance (NMR) is an ef?cient method for quantifying oil saturation in the rock matrix and analyzing pore structures. In this paper, the threshold values of different pore sizes were established from the relationship between mercury injection curves and NMR T2 spectrums. The distribution and migration of residual oil in different ?ooding processes was evaluated by quantitatively analyzing the change of the relaxation time. The oil displaced from pores of different sizes after the water ?ood, polymer ?ood, and the SP ?ood was calculated, respectively. Experimental results indicate that (1) the residual oil in medium pores contributed the most to the incremental oil recovery for the SP ?ood, ranging from 40 % to 49 %, and small pores usually contributed\30 %; (2) the residual oil after the SP ?ood was mainly distributed in small and medium pores; the residual oil in medium pores accounted for 47.3 %–54.7 %, while that trapped in small pores was 25.7 %–42.5 %. The residual oil in small and medium pores was the main target for EOR after the SP ?ood in oil?elds.