Crude oil mobility and its controlling factors in tight sand reservoirs in northern Songliao Basin, East China

Taking tight oil in Gaotaizi and Fuyu oil layers of the Upper Cretaceous Qingshankou Formation in northern Songliao Basin as an example, based on analyses of nuclear magnetic resonance and high pressure mercury injection, experiment methods of supercritical carbon dioxide displacement and extraction are firstly employed to quantify crude oil mobility in tight sand reservoirs with different lithologies and oil contents. The results show that, under the conditions of simulating the Cretaceous Qingshankou Formation in the northern Songliao Basin at a temperature of 76-89 °C and a pressure of 35-42 MPa, the lower limit of the porosity of the movable oil is4.4%, and the lower limit of the permeability is 0.015′10-3 mm2. The lower limit of the average pore throat radius is 21 nm. On this basis,a classification standard for three types of tight sand reservoirs is proposed. Type I reservoirs are characterized by the movable fluid saturation larger than 40%, the movable oil ratio(ratio of movable oil to total oil) greater than 30% and the starting pressure gradient in the range of 0.3-0.6 MPa/m; Type II reservoirs are characterized by the movable fluid saturation in the range of 10%–40%, the movable oil ratio in the range of 5%–30% and the starting pressure gradient in the range of 0.6–1.0 MPa/m; Type III reservoirs are characterized by the movable fluid saturation less than 10% in general, the movable oil ratio less than 5%, and the starting pressure gradient greater than1.0 MPa/m. The fluid mobility in tight sand reservoirs is mainly affected by diagenesis and sedimentary environment. Reservoirs with depth lower than 2000 m are dominated by type I reservoir, whereas those with greater depth are dominated by type I and II reservoirs.Reservoirs in inner delta-front facies are dominated by type I reservoir, whereas those in outer delta-front facies and shore-shallow lacustrine facies are dominated by type II and III reservoirs.