Tectonic and geological setting of the earthquake hazards in the Changning shale gas development zone, Sichuan Basin, SW China

Abstract

Hydraulic fracturing is a key technology in shale gas extraction, whether hydraulic fracturing induces earthquakes has become a hot topic in the public and the focus of scholars’ research. The urgency of shale gas mining and the catastrophic nature of earthquakes highlight the urgent need to study this issue. The Changning anticline at the southern margin of the Sichuan Basin is a key area for shale gas exploitation. Taking this as an example, this paper applies the velocity model of the study area to reposition the M5.7 magnitude earthquake on December 16, 2018 and the M5.3 magnitude earthquake on January 03, 2019 and their aftershock sequence in this area. Using shale gas exploration drilling and reflection seismic data to carry out structural analysis, and recovering the tectonic geological setting of earthquake occurrence by restoring the formation process of the Changning anticline, to further explore the seismic mechanism. Our results show that the Changning anticline is a large basement fault-bend fold, and the displacement of the fault forming the anticline is 18 km, and the Changning anticline absorbs 33% of the fault slip. The Silurian Longmaxi Formation of the Changning anticline experienced larger-parallel shearing along underlying basement faults, forming a micro-fracture system. The footwall ramp of the basement fault is reactivated at present, earthquakes in this area mostly occur along the footwall ramp of the basement fault and above and below it. The anticlinal and synclinal hinge zones are also the earthquake concentration areas, but the earthquake magnitude decreases upwards along the kink-band, and small earthquakes below M2.0 occur in the Silurian Longmaxi Formation. So far, the earthquake in the Changning anticline mainly occurred in the southern limb of the anticline, which is a natural earthquake formed along the footwall ramp of the basement fault. The earthquakes in the Changning area are possible related to the geo-tectonic setting for the southeast outward compression of the Qinghai-Tibet Plateau at present, the moderate or large-scale earthquakes in the southwest Sichuan Basin are mainly due to the reactivation during late Quaternary of the earlier formed faults. It is suggested to carry out scientific monitoring of seismic activities in shale gas development zones.