关键词: 走滑断层, 构造地貌, 反射地震剖面, 四川盆地南部, 莫尔-库仑破裂准则, 稳定性评价

Abstract:

As global energy demand continues to increase, shale gas development through large-scale hydraulic fracturing, involving subsurface fluid injection and extraction, has been increasingly practiced worldwide. In this context, identifying pre-existing geological structures and evaluating their stability in shale gas production areas are essential for ensuring safe shale gas development. This study focuses on a shale gas production block in the southern Sichuan Basin, China, where a prominent N-S trending strike-slip fault system is developed. By integrating tectonic geomorphological analysis, 3D seismic reflection interpretation, microseismic monitoring data, and mechanical analysis based on the Mohr-Coulomb failure criterion, we systematically delineate the spatial geometry and kinematic characteristics of pre-existing faults in the study area and assess the stability of the representative Fault F₂ and its secondary faults, providing a reference for subsequent shale gas development and production safety.

A joint surface-subsurface structural analysis of the fault system was conducted using high-resolution surface DEM data and seismic reflection data. The results show that the fault system consists of dozens of subparallel, nearly N-S-trending strike-slip faults with right-lateral kinematics. These faults extend to the surface in the core of an NEE-SWW-trending anticline, forming distinct linear fault valleys, and widely produce right-lateral offsets of several hundred meters, approximately 300-1 000m, in drainage systems and ridges.

Furthermore, the stability of a representative fault, F₂, and its impact on shale gas production were assessed by combining real-time microseismic monitoring data acquired during hydraulic fracturing with quantitative mechanical analysis based on the Mohr-Coulomb failure criterion. Microseismic event locations indicate that, although most events are concentrated near the main Fault F₂, they are not distributed along F₂ in the N-S direction. In addition, focal mechanism solutions suggest that these microseismic events are dominated by left-lateral strike-slip motion, opposite to the kinematic characteristics of F₂. These observations indicate that the large N-S-trending main fault, which exceeds 10km in length, is unlikely to be the seismogenic fault. Instead, NE-trending secondary left-lateral strike-slip faults derived from F₂ and located within approximately 300m of the main fault may be more susceptible to stress-perturbation-induced seismicity during shale gas development.

Mechanical analysis based on the stress field inverted from focal mechanism solutions and the Mohr-Coulomb failure criterion further supports this interpretation. The results indicate that the present-day maximum principal stress is oriented approximately N17°E. Under this stress regime, the NE-trending secondary faults are more favorably oriented for slip and require a smaller fluid-pressure perturbation to reach failure than the main N-S trending fault. Consequently, these secondary faults exhibit significantly higher instability potential, whereas the main fault remains comparatively stable owing to its less favorable orientation relative to the contemporary stress field and its larger scale, which may facilitate faster fluid-pressure diffusion.

These findings highlight that not all pre-existing faults pose high risks to shale gas development and that fault stability must be evaluated in relation to the in situ stress state. In subsequent development of this block, attention should be paid to the high-risk NE-trending secondary faults, and appropriate avoidance or mitigation measures should be adopted. The results provide support for optimizing engineering design, mitigating induced-seismicity risk, and promoting safe and efficient shale gas development.

Key words: strike-slip fault, tectonic geomorphology, seismic reflection profile, southern Sichuan Basin, Mohr-Coulomb criterion, stability evaluation