Disturbance factors of current geostress field of Longmaxi Formation shale in southeastern Sichuan Basin and their geological significance for gas exploitation

For the current deep shale reservoirs in southeastern Sichuan, the variables such as geostress magnitude, orientation and structure are complex and changeable, and their changing laws are still unclear, thus severely restricting the deployment, implementation, and production efficiency of shale gas exploration and development. This study targets at Longmaxi Formation in key blocks of southeastern Sichuan Basin. Based on the core analyses and the multi-source and multi-dimensional stress responses data from wells, as well as geomechanical analyses and numerical simulations, the study identifies the key geological processes causing geostress disturbances, reveals the mechanical mechanisms and patterns of stress variations and further clarifies their impacts on shale gas enrichment and high production. Results indicate that the southeastern Sichuan Basin can be divided into five regions according to the current geostress, and the stress machanism is mainly presented as strike-slip stress regime. However, in complex marginal zones, the stress mechanism transitions from reverse faulting at shallow depths to strike-slip or normal faulting at greater depths. Folds and faults are identified as the critical external factors causing stress field deflections. Above the neutral surface of the folds at the first and second submembers of Member 1 of Longmaxi Formation, the stress orientation deflects along the fold axis, with the deflection angle controlled by the mechanical properties and deformation intensity of the rock layers, and stress magnitudes decrease. Below the neutral surface, the opposite trend from stress magnitudes and orientation is observed. Moreover, the stress orientation deflects along the fault strike as the distance from the fault decreases, with the stress magnitude decreasing and the differential stress between principal directions increasing. Pore pressure variations mainly influence the minimum horizontal principal stress, with the deflection angles reaching up to 35°. Vertically, as influenced by lithological disturbances, stress values are low for siliceous shales from Wufeng Formation to the first sub-layer of the third submember of Member 1 of Longmaxi Formation, and these weak-stress layers are favorable for fracturing. Comprehensive analysis suggests that below the neutral surface, syncline zones or areas near low-level (Grade Ⅳ or below)NE-trending faults exhibit good fracture sealing and high gas content. The tensile stress disturbance areas exhibit low stress magnitudes and small differential stresses, facilitating the formation of complex fracture networks with high fracture heights and high production rates. These areas are highlighted as priority zones for future shale gas exploration and development. The research results are expected to provide important insights and guidance for the accurate evaluation of geostress fields and optimal selection of sweet spots in deep and structurally complex shale reservoirs.