Abstract: The microscopic oil displacement mechanism of three chemical flooding systems applied in Shengli Oilfield was studied,and its applicability in heterogeneous reservoirs was appraised.Using wettability controlling method for micro-etching model and contribution ratio analysis method of conformance efficiency to displacement efficiency,chemical flooding mechanism under different conditions and different displacement stages were studied through microscopic flooding experiment.In addition,a new method using contribution ratio analysis method of conformance efficiency to displacement efficiency to evaluate the microscopic displacement mechanism of chemical flooding system was established.Experiment results showed that oil recovery increase from polymer or polymer-surfactant flooding after water driving was less than 10% and contribution ratio of conformance efficiency to displacement efficiency was less than 1.5 in the model with permeability contrast of 1:9,which verifies that polymer and polymer-surfactant system had a weak ability to expand conformance efficiency and the main drive mechanism could not be brought into full play.Experiment results also showed that average contribution ratio of conformance efficiency to displacement efficiency was greater than 3 from heterogeneous composite system in the models with different permeability contrasts,which verified that heterogeneous composite system had a strong ability to expand conformance efficiency and the main mechanism of oil displacement was the combination of intermittent sealing and enlarging conformance efficiency by changing direction of gel particles.The results of micro-migration and displacement mode research and field test showed that heterogeneous compound flooding can greatly enhance oil displacement efficiency and sweep efficiency for heterogeneous reservoirs after polymer flooding.