Title: Improved Crustal Structure, Seismicity, Stress and Pore Pressure Characterization of Changning-Xingwen Shale Gas and Salt Mine Areas, Sichuan Basin
Abstract: In the Changning-Xingwen region of Sichuan Basin, China, which experiences decades-long injection of freshwater for commercial salt mining and hydraulic fracturing for shale gas production, an Ms6.0 earthquake occurred in June 2019 and four Ms>5 aftershocks, as well as 2018 Xingwen Ms5.7 and 2019 Gongxian Ms5.3 earthquakes in the Xingwen shale gas block. Using data from local and regional seismic stations, we determine precise locations, accurate focal mechanisms for these earthquakes, high-resolution velocity models, and excess pore fluid pressures of the source regions. Our results reveal two distinct seismogenic zones corresponding to the salt mine and shale gas regions, with most induced seismic events forming widespread lineaments that sometimes extend to the basement and remarkably align with fault and fracture trends interpreted on reflection seismic and outcrops, respectively. Our 3-D crustal velocity models show that the top of the crystalline Sichuan basement is characterized by a 6.5 km/s Vpcontour, which is new for earthquake tomographic studies in the region. Additionally, most of the small magnitude earthquakes within the Xingwen shale gas block are associated with relatively high Vp/Vs values (1.77-1.87), indicating a fluid-influenced inducing mechanism. The 2018 Xingwen Ms5.7 and 2019 Gongxian Ms5.3 earthquakes are located in low Vp/Vs zones, suggesting they could be structurally controlled. In comparison, the Changning Ms6.0 earthquake is extremely shallow (~1.6 km below the mean sea level) and is associated with low Vs and high Vp/Vs values and excess fluid pressures, indicating a fluid-induced earthquake. The Changning mainshock is likely the shallowest M>6.0 induced earthquake, and it is triggered by the combined effect of pore pressure increase from the diffusion of injected fluids, differential formation subsidence from salt caverns, and highly fractured slip-prone rocks enriched in quartz and silica content. Following Coulomb stress transfer from the mainshock, the sequence ruptured in a cascading manner involving pre-existing oblique faults of varying dips. Combined with outcrop analysis, these pieces of seismic evidence help constrain the mechanisms of induced seismicity beneath the salt mine and shale gas regions.
Biography: Dr. Uzonna Anyiam’s focus as a geophysicist is in the study of earthquakes triggered by onshore and offshore human activities such as hydraulic fracturing for shale gas development, CO2 sequestration, enhanced oil recovery, dissolution mining of salt, and geothermal resource development using double difference tomography for improved subsurface imaging. His integrated basin analysis research employs well data, core data and 3D reflection seismic data in conducting attribute-assisted seismic interpretation, reservoir characterization and prospect evaluation.
He joined the faculty at Hope College in mid-2022. During his graduate study he taught geology and seismic interpretation classes, including teaching hands-on skills in use of industry software, as he does with students at Hope. He was involved in university-based research sponsored by oil companies that explored oil and gas fields to identify potential drilling sites in the Gulf of Mexico and off the coast of West Africa, as well as induced seismicity research sponsored by the National Natural Science Foundation of China in Sichuan Basin. His early research as a college graduate in his native Nigeria included a hydrogeophysical project that resulted in clean water supplies for improved sanitation for more than eight communities.
To join virtually: Zoom
Contact: anyiam@hope.edu
Website: https://hope.edu/directory/people/anyiam-uzonna/index.html
Recording: Zoom Recording (will be available within a week after the seminar)