Crustal structure in the Weiyuan shale gas field, China, and its tectonic implications

We applied joint inversion of multifrequency receiver functions and surface wave dispersion to image crustal S-wave velocities and discontinuities beneath 29 temporary seismic stations with similar to 5 km station spacing in the Weiyuan shale gas field. The results show similarities and differences in the crust between the anticline and syncline areas and a widespread low-velocity layer in the synclinal sedimentary rocks. Similar crustal S-wave velocities and high velocities in the middle-lower crust and a thickness of similar to 12 km in the upper crust are observed in both the anticline and syncline areas. The sedimentary thickness changes mainly from 3 km to 4 km, and the average crustal thickness is similar to 42.4 km in the anticline area, while sedimentary thickness varies mainly from 4 km to 5 km and the average crustal thickness is similar to 41.0 km in the syncline area. These results suggest that the Weiyuan anticline should be a thin-skinned structure and that the study region maintains a typical cratonic structure in the crust. The observed 1-2 km-thick low-velocity layer at depths of 3-4 km in the syncline area may be related to Ordovician and Silurian shale gas strata with high porosity and low density. The observed characteristics of alternating high-/low-velocity layers and long-term hydraulic fracturing and water injection may have induced the occurrence of M-L > 4.0 earthquakes in the Weiyuan shale gas field since 2018. Our study suggests that dense temporary seismic data can not only delineate small-scale structural characteristics but also provide valuable references for oil-gas exploration and earthquake mitigation.