The Relationship between Fractures and Tectonic Stress Field in the Extra Low-Permeability Sandstone Reservoir at the South of Western Sichuan Depression

The formation and distribution of fractures are controlled by paleotectonic stress field,and their preservative status and effects on development are dominated by the modern stress field. Since Triassic,it has experienced four tectonic movements and developed four sets of tectonic fractures in the extra low-permeability sandstone reservoir at the south of western Sichuan depression. The strikes of fractures are in the S-N,NE-SW,E-W,and NW-SE directions respectively. At the end of Triassic,under the horizontal compression tectonic stress field,for which the maximum principal stress direction was NW-SE,the fractures were well developed near the S-N faults and at the end of NE-SW faults,because of their stress concentration. At the end of Cretaceous,in the horizontal compression stress fields of the NE-SW direction,the stress was obviously lower near the NE-SW faults,thus,fractures mainly developed near the S-N faults. At the end of Neogene-Early Pleistocene,under the horizontal compression tectonic stress fields of E-W direction,stress concentrated near the NE-SW faults and fractures developed at these places,especially at the end of the NE-SE faults,the cross positions of NE-SW,and S-N faults. Therefore,fractures developed mostly near S-N faults and NE-SW faults. At the cross positions of the above two sets of faults,the degree of development of the fractures was the highest. Under the modern stress field of the NW-SE direction,the NW-SE fractures were mainly the seepage ones with tensional state,the best connectivity,the widest aperture,the highest permeability,and the minimum opening pressure.