Evaluation of movable tight oil in different reservoir types in the Central Sichuan region

Tight oil is abundant in the Central Sichuan area. Understanding the differences in tight oil movability among different types of formations is important for the development of tight oil reservoirs in Central Sichuan. The movability of tight oil in the shell limestone, argillaceous shell limestone, and shell shale reservoirs in Central Sichuan was evaluated by nuclear magnetic resonance (NMR) and X-ray diffraction (XRD) analyses. The results show that small pores are more common in the argillaceous shell limestone and shell shale, whereas large pores are more evenly distributed in the shell limestone. The average pore size of the core samples is linearly related to the ratio of permeability to porosity. The displacement pressure differential has little effect on the movable oil saturation in shell limestone but has a significant effect on argillaceous shell limestone and shell shale. Movable oil in argillaceous shell limestone is derived from both large and small pores, whereas in shell limestone, it primarily comes from large pores. In low-permeability shell shale, movable oil mainly originates from small pores, whereas in medium- to high-permeability samples, it primarily originates from large pores. The XRD results indicate that a lower clay mineral content and higher calcite content are associated with a higher movable oil saturation.