Abstract: In order to find out the quantitative changes in reservoir physical properties after long-term waterflooding development of the Z oil field in the Liaodong Bay Depression, taking the sealed core of E₃d₂^L, a delta front reservoir as the research target, and comprehensively using microscope, scanning electron microscope, X diffraction, scanning gamma ray as tools, we systematically studied the permeability changes of the reservoirs of different genesis with different permeability before and after water flooding, and quantitatively correlated the variation in reservoir permeability with water output. The results show that, in the study area, clay minerals and pore throat structure are the main factors affecting the reservoir physical properties, including both the original reservoir permeability and the permeability after water injection. The total amount of clay minerals is rather different in the distributary channel sands and the mouth bar sands. And the composition of clay minerals, or the kaolinite and illite/smectite ratio depends upon the genesis of sand body. The total volume of clay minerals and the original permeability of reservoirs are negatively related. The larger the total volume of clay minerals, the lower the original permeability of reservoir. However, the pore throat size is positively correlated with the original permeability of the reservoir. The larger the mean pore throat radius, the higher the original permeability of the reservoir. With the increase in water flooding, the integral number of clay minerals decreases gradually, and the distribution of pore throat radius tends to be homogenized. As the result, the reservoir permeability will show a tendency of increase with the increase in water-out. Statistical analysis shows that the genetic type of reservoir sand bodies are critical important to the permeability and their changes in the water flooding process as well as the residual oil distribution. This study provides a new idea for the detailed description of remaining oil in the middle to late development stage of an oil field, and is effective to improve development efficiently in the high water cut stage.