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

Abstract

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 ~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 ~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 ~42.4 km in the anticline area, while sedimentary thickness varies mainly from 4 km to 5 km and the average crustal thickness is ~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 ML > 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.