Factors influencing the in-situ stress orientations in shales: A case study of the Wufeng-Longmaxi formations in the Jiaoshiba Area, southeastern Sichuan Basin, China

Abstract

Determining the present-day stress orientation is essential for the exploration and development of shale gas. The Longmaxi Formation is the most significant shale gas reservoir in the Jiaoshiba Area of the Sichuan Basin, China. However, little is known about the orientation of the maximum horizontal stress (SHmax) and the factors influencing it within the formation. In this study, the orientation of the SHmax in the Jiaoshiba Area was determined based on observations of drilling-induced fractures on resistivity image logs. Then, the tectonic activity, the mechanical properties of the rock, and finite element simulations were studied in detail to determine the major factors influencing the SHmax orientation in the Jiaoshiba Area. The results show that the SHmax orientation within the study area ranges from nearly EW to NEE-SWW. The vertical variations of the in-situ stress orientation are controlled by the structural strength and the mechanical properties of the rock. In the area that has experienced intense tectonic movements, the deviation in the SHmax orientation (from 30° to 80°) was caused by the differences in the mechanical properties of the rocks. The finite element simulations indicate that the lateral variations in the SHmax orientation was mainly controlled by the faults. The stress rotation near the end of the fault is 14°–21° higher than that in the middle of the fault. Furthermore, the stress variation caused by the mechanical properties of the fault zone ranges from 12° to 18°, and this deviation increases as the Young's modulus of the fault zone decreases. In the study area, the azimuth deflection (18°–46°) of the SHmax orientation influenced by the NE faults is higher than that (12°–36°) influenced by the nearly NS faults, which is the main reason for the lateral stress rotation.