Simulation of paleotectonic stress field of Silurian Longmaxi Formation in Wulong area of southeastern Chongqing

The development of a shale reservoir bed in the Silurian Longmaxi Formation in the Wulong area of southeast Chongqing is affected by the superposition of multiple tectonic processes, resulting in complex structural deformations in the area. Therefore, it is necessary to analyse the main controlling factors of shale gas preservation. The shale reservoir is characterised by low porosity and low permeability. Thus, the tectonic fissure is the main controlling factor of shale gas movement and accumulation. The paleotectonic stress field controls the development of tectonic fractures, increasing the reservoir storage space and the connectivity between fractures, which in turn affects the exploration and development of shale gas in the Wulong area. Therefore, the analyses of the sequence of tectonic evolution and the direction of the paleotectonic stress field, as well as the quantitative description of the tectonic stress field characteristics, have great theoretical and practical significance for shale gas exploration and exploitation in the Wulong area. 　　In this paper, the data of conjugate shear joints in Wulong area and its periphery are measured in the field, and they are divided into stages and matched. Based on the trend and superposition relationship of fold axis in the study area and its periphery, the characteristics of paleotectonic stress direction are analyzed. Combined with geochronological data and rock acoustic emission experimental results, the tectonic evolution process of Wulong area is restored, the tectonic evolution period and the corresponding stress of each period are determined, and the key tectonic deformation period and the fracture distribution characteristics of the study area are clarified. The three-dimensional geological model of the key tectonic deformation period of the Silurian Longmaxi Formation in Wulong area was established by using ANSYS finite element software.According to the distribution characteristics of rock mechanical parameters in the study area, the mechanical units are divided and the mechanical model is established.After multiple simulation corrections, the optimal boundary stress application conditions of the model are determined by analyzing the characteristics of the paleotectonic stress field. The numerical simulation of paleo-tectonic stress field is carried out to clarify the distribution characteristics of paleo-tectonic stress field in the key tectonic deformation period. 　　The results show that the Wulong area has mainly experienced two phases of structural stress in the SE-NW direction and near the EW direction since the middle Yanshan age. The tectonic stress is manifested as compressive stress. In the mid-Yanshanian period, affected by the progressive expansion of Xuefeng intracontinental orogenic movement from SE-NW direction, NE-trending faults and related folds were generated in Wulong area and its periphery, forming the main structural pattern of Wulong area, which was the main structural deformation period.These formations have become the main tectonic pattern in the Wulong area.The stress environment formed by Xuefeng intracontinental orogeny in the late middle Yanshan period has changed in the study area. The stress direction changes from the SE-NW compression environment to the near EW compression stress environment. The near SN-trending faults and related folds are formed in the area, and the structural superposition is formed with the pre-existing structural morphology. In the study area and the surrounding area, L-shaped and T-shaped fold structures are formed. However, the tectonic action in the late middle Yanshan period failed to change the main tectonic pattern of the NE direction in Wulong area. 　　The maximum horizontal principal stress in the middle Yanshan period is 112−194 MPa, and the minimum horizontal principal stress is 60.93−147.99 MPa.The high value of the maximum principal stress is distributed in the core of the western Wulong syncline. In the late middle Yanshan age, the maximum horizontal principal stress ranges from 75.67 MPa to 168.32 MPa, and the minimum horizontal principal stress ranges from 31.19 MPa to 95.56 MPa. The stress value of the western Wulong syncline is higher than that of the east syncline. There is a certain correlation between the stress distribution characteristics and the buried depth. The stress contours are distributed in a ring shape, and the maximum stress value in the central region gradually decreases outward. The stress gradient of the low-value zone is also closely related to fault intensity. The greater the fault intensity is, the greater the stress gradient of the maximum principal stress is. The fault zone area is the stress release area. Therefore, this area is a low stress area. At the end and inflection point of the fault, there is a high value area of stress, and stress concentration occurs. The stress value of the upper wall of the fault is higher than that of the lower wall of the fault. As a result, the fracture development degree of the upper wall of the fault is greater than that of the lower wall of the fault. Therefore, the favorable preservation area of shale gas is farther away from the hanging wall of the fault than the footwall of the fault.Moreover, stress concentration occurs at the end point and inflexion point of the fault, and structural fractures are relatively developed and easily connected, resulting in the escape of shale gas. Thus, the area is also unfavourable for shale gas preservation. The deformation degree of the western Wulong syncline is greater than that of the eastern Wulong syncline. The larger the deformation degree of the syncline is, the greater the curvature of the syncline core is, and the easier it is to form ‘Λ’-type fractures. These fractures are conducive to the storage and preservation of shale gas. Therefore, the shale gas favorable preservation area in the west of Wulong syncline is better than that in the east of Wulong syncline. Studying the characteristics of paleo-tectonic stress field is the basis for studying the development degree of tectonic fractures and deep fracturing effects. It is also an important factor affecting high yield.