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

摘要：

在页岩气生产区开展先存构造的调查并评估其地质体稳定性, 对保障能源开发安全至关重要。文中以四川盆地南部某页岩气开采区块为对象, 综合利用构造地貌分析和地震反射剖面解译, 解析研究区内先存断层的空间结构与运动学特征; 同时, 结合页岩气开发过程中的监测资料和莫尔-库仑破裂准则, 评价该断层系统的稳定性及其对生产的影响。结果表明: 1)研究区内发育由十余条近平行展布的SN向断层组成的断层组, 其运动学特征以右旋走滑为主; 2)注水压裂同期的监测资料显示, 该断层组中的主要断层分支(F₂)现今基本稳定, 但其周围数百米长的NE向次级断层在页岩气开发过程中易受扰动; 3)基于莫尔-库仑破裂准则的力学分析证实NE向次级断层在现今局部应力场(最大主应力方向为NNE)中处于最优破裂方位, 失稳风险更高。据此, 文中认为研究区的走滑断层系统中主断层稳定性较好; 而NE向次级断层稳定性差, 在生产中应注意避让。文中的研究成果将有助于优化生产工程设计, 规避地震风险, 实现页岩气的安全高效开发。

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