Three-Dimensional Fault Slip Risk Analysis in a Shale Gas Development Area: A Case Study of the Luxian Shale Gas Field, Sichuan Basin, China

The occurrence of induced moderate to strong earthquakes is generally believed to be the reactivation of pre-existing faults of a certain size due to stress disturbances caused by the industrial activities. Therefore, pre-existing faults in the crust are already subjected to the background tectonic stress field, and different orientations of faults experience different stress state. Therefore, evaluating the risk of faults slipping with different orientations under the tectonic stress field is the basis for guiding industrial construction design to reduce induced seismic risk.

The Luxian County shale gas field is one of the four shale gas development areas in southern Sichuan, China. In this study, we conduct a fault slip risk analysis based on the distribution of the three-dimensional (3D) faults in Luxian County shale gas field . Luxian County is situated in the southwestern part of the Huayingshan fold belt, nestled between the steep Gufoshan anticline and the Luoguanshan anticline, with a broad and gentle Fuji syncline in between. The region exhibits extensive development of faults and fractures. Since the extensive implementation of hydraulic fracturing related to shale gas extraction in this region in 2019, there has been a noticeable increase in seismic activity.

In this study, we interpret the 3D fault planes in Luxian County based on seismic reflection profiles. Based on the collection of in-situ stress and formation pressure data, we establish the background tectonic stress field in the Luxian shale gas field and calculate the critical pore pressure increment required for the slipping of 3D fault planes under the background tectonic stress field, as well as the fault slip tendency. Furthermore, we construct a 3D fully coupled poroelastic finite element model to calculate the static Coulomb stress perturbation that injection operations might cause on faults. Considering the uncertainty of stress field and fault orientations, based on the Monte Carlo method, the potential of fault reactivation is calculated. Our research  provides a mechanical basis for the seismic hazard analysis of the Luxian shale gas development area, serving as an excellent example for conducting seismic hazard analysis in shale gas development fields.