Southern Texas has a history of active oil and gas production, hydraulic fracturing (HF), wastewater disposal, and seismicity, with some areas of pervasive faulting. Previous studies of this region have attributed cases of seismicity to increases in production of oil/water and a few cases to wastewater disposal. With the advancements in horizontal drilling and HF, the Upper Cretaceous Eagle Ford shale has become a focus of shale-gas development since 2008, and we sought to investigate how HF may have contributed to a recent seismicity rate that is 75 times higher than background levels. We used times and locations of HF from the FracFocus catalog and enhanced the TexNet catalog of earthquakes from January 2017 to December 2018 by employing single-station template matching back to March 2014 when continuous recording began. We identified over 2000 earthquakes (84%) that were closely related in time and space to 200 HF wells mostly in Karnes, Atascosa, and Gonzales counties, representing the first documented cases of HF-induced seismicity in Texas. Much of the HF-induced seismicity occurred in a 15 x 15 km area just north of Karnes City where 33% of the HF wells correlated with seismicity. We found that "zipper" wells stimulating synchronously with another nearby lateral were more than twice as likely to produce seismicity than asynchronous wells. The magnitude of HF-induced seismicity and low b-value support the idea that these events represent slip on pre-existing faults and not the process of fracturing the target formation. Furthermore, TexNet fault plane solutions for HF-induced earthquakes are in agreement with the orientation of SHmax and previously mapped faults. A Mw 4.0 earthquake on May 1, 2018 would be the largest reported HF-induced earthquake in the US, occurring ~10 km from a Mw 4.8 earthquake in 2011 attributed to fluid extraction. This represents the most critical findings in our study: faults in this area are capable of producing felt and potentially damaging earthquakes due to ongoing operational activities.
AAPG Datapages/Search and Discovery Article #90373 © 2019 AAPG Eastern Section Meeting, Energy from the Heartland, Columbus, Ohio, October 12-16, 2019