Abstract: Chang 7 oil formation in Ordos Basin is distributed in the center of lake basin in a large area. The porosity ranges mainly from 4 percent to 12 percent while the permeability is less than 0.3mD. It is a typical tight sandstone reservoir. Determination of the lower limits of effective reservoir is one key technologies to divide the oil-bearing boundary of tight rock. Based on the geological data, such as logging interpretation, oil testing data and analysis of physical properties of reservoirs and in accordance with the statistical principles and super-low permeable oil reservoir fl uid fi ltration mechanism, fi ve methods are adopted to the lower limits of physical properties of the effective reservoirs in this block, such as experience statistical method, physical oil testing method, porosity-permeability intersection method, oil and gas displacement simulation method and minimum fl owing pore throat radius method. The lower limit of physical properties of effective reservoir in Chang 7 oil formation is determined on the basis of the research and comparison. The porosity and permeability are 5.7 percent and 0.0276mD.