Crustal electrical structure and seismicity of the Weiyuan shale gas block in Sichuan basin, southwest China

Abstract

Hydraulic fracturing, a significant contributor to seismic activity within and around operational fields, has been extensively employed in shale gas production. Magnetotelluric Sounding (MT) as an effective geophysical tool for identifying high-conductivity fluid-filled and/or molten regions. In this study, we deploy a dense grid of rectangular MT sites to investigate the three-dimensional (3-D) geoelectrical resistivity structure beneath the Weiyuan shale gas block (WSGB) and subsequently examine the causes of seismic activity. The resistivity data, obtained through 3-D inversion accounting for topography using ModEM, reveals a shallow low-resistivity layer (< 10 Ω-m) within the WSGB, ranging from approximately 2 to 5 km in depth. This layer exhibits multiple isolated areas with very low resistivity (< 5 Ω-m), indicative of fluid-filled zones associated with hydraulic fracturing or shale gas-bearing formations. In the northwestern WSGB, the Weiyuan anticline presents a high-resistivity dome extending possibly to depths beyond 20 km, without extending beyond the northern boundary of our study area. Conversely, the sedimentary zone in the southeastern WSGB displays a low-resistivity feature, with an extremely low-resistivity center (< 1 Ω-m). Since a consistent high resistivity zone exists beneath each fault and its top depth is less than 5 km, so faults might not extend downward below 5 km. Earthquakes with magnitudes (ML) of 3.0 or higher predominantly occur close to the faults, when considering industrial production data, we found a noteworthy correlation between earthquakes with ML < 3.0 and annual shale gas production within the WSGB. Tectonic faulting is not the leading cause for ML < 3.0 earthquakes but likely the primary contributor to seismic events with ML ≥ 3.0.