Abstract: Lamadian, Saertu, and Xingshugang oil reservoirs located in placanticline structure of Daqing Oilfiledare sandstone reservoirs with anticlinal structure. The depths of oil-water interface are different in the eastern and western limbs, which have been interpreted by many geological origins, such as hydrodynamic force and terrestrial stress. Based on the physical principle of gravity differentiation of structural oil and gas reservoirs, as well as the theory of equilibrium between capillary force and buoyant force, the equation of capillary force and buoyant force is deduced, which can be used to describe the distribution law of oil-water interface. A conclusion is drawn that the depth of the oil-water interface in any position of the reservoir is determined by its pore configuration, that is, the size of the pore radius (rd) corresponding with the displacement pressure. Rd has positive correlation with reservoir properties. Due to the existence of reservoir heterogeneity (lithologic factors), the oil/water transitional zone and oil-water interface of general anticlinal structural oil and gas reservoirs are undulating, as well as the thickness tends to be conico-cylindrical.

Key words: placanticline structure in Daqing Oilfield, anticlinal-structural reservoir, oil-water interface, equation of capillary force and buoyant force