Identification and Impacts of Small-Displacement Faults in Industry-induced earthquake: Insights from the Southern Sichuan Shale Gas Field

During the siting of hydraulic-fracturing (HF) wells within industrial activity areas, identifying potential seismogenic faults and effectively avoiding them is critical for mitigating induced seismicity risk. Meanwhile, characterizing the fine-scale structures of seismogenic faults provides the essential foundation for analyses of the mechanisms and rupture processes of induced earthquakes. However, multiple case studies have demonstrated that, even where seismic reflection data are available, it remains difficult to identify small-displacement seismogenic faults, particularly those dominated by strike-slip faults. Consistently, the four representative M5+ induced earthquakes in the Changning and Weiyuan shale gas blocks of the Sichuan Basin also exhibit difficulties in identifying the seismogenic faults from seismic reflection data. Moreover, the scales of faults that can be identified through seismic reflection data and related interpretation methods, and their corresponding seismogenic potential, remain to be systematically defined and quantitatively constrained. This study integrates spatiotemporal data from HF operations, seismicity data, and high-resolution 3D seismic reflection data, together with surface deformation measurements, to address the above questions.

The results show that potential seismogenic faults with moment magnitude (Mw) greater than approximately 3.3 that displace strong reflection horizons can be effectively identified using high-resolution 3D seismic reflection data. In addition, the associated structures of small-displacement strike-slip faults facilitate their recognition in seismic reflection profiles. A common feature of the seismogenic fault systems of the four representative earthquakes is that small-displacement subsidiary faults (including strike-slip faults) intersect the fracturing wells within the reservoir interval, forming downward migration pathways for fracturing fluids and thereby activating the underlying thrust or strike-slip seismogenic faults. More importantly, such small-displacement faults are widely developed within the fractured intervals of the Sichuan Basin shale gas fields, yet their identification remains challenging. As a result, numerous horizontal wells intersect these faults, constituting a key reason for the frequent occurrence of induced seismicity in these areas. The most effective approach to recognizing these faults is to trace multiple strong reflection horizons to construct structural maps. By applying multi-azimuth illumination and vertical stretching, fault traces can be visualized more clearly, in combination with various types of seismic reflection attribute volumes.

Beyond the Sichuan Basin, injection-induced earthquakes in most shale gas fields worldwide are also closely associated with small-displacement faults, particularly strike-slip faults. The failure to avoid such faults during the siting of HF wells is also likely a major reason for the frequent occurrence of induced seismicity in these areas. The small-displacement fault identification techniques presented in this study facilitate a more precise delineation of seismogenic fault system structure. More importantly, during well site selection, from the perspective of fault identification and avoidance based on 3D seismic reflection data, this study provides theoretical support and practical strategies for preventing induced earthquakes with a magnitude (Mw) greater than approximately 3.3. These findings also offer significant implications for the prevention of induced seismicity caused by fluid/gas injection in a broader range of applications.