Abstract: When one accumulates the permeability contribution values for ranges of poresizes,derived from Wall's Equation, to 99.9%, it often occures that the pore throat is still large but the corresponding oil saturation is low. This is explained with one field example from Well No. 2 District, Chepaizi, and one theoretical capillary pressure curve. It is therefor suggested that Qu's Equation, which takes into account the interfacial tension between oil and water (σ_ow) the difference between oil and water densities (△d), and the height of the oil column， be used to estimate the original oil saturation. When rnmin is calculated from rnmin = Bow/ ,ow/ tk)+4.9b*△d], the corresponding mercury saturation is the original oil saturation. This results in a more reasonable value.