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AAPG Southwest Section Annual Convention

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Lessons from Fort Worth Basin Induced Earthquake Sequences

Abstract

The Fort Worth basin in north Texas has experienced over 30 magnitude (M) 3.0+ earthquakes and one M4.0 since 2008. Events have been linked to disposal of produced wastewater into the Ellenburger formation, which appears to be in hydrogeologic contact with faults located in the crystalline basement. The presentation provides an overview of earthquake catalogs, data availability, and current research conclusions regarding induced earthquakes in the Fort Worth basin. Seismicity rates have decreased significantly since peak injection volumes in 2014 but earthquakes continue to occur into 2019. Earthquakes have occurred on northeast (NE)-southwest (SW) trending basement faults and to a more limited extent, within the overlying Ellenburger dolomitic limestone formation. The seismogenic faults do not appear unique relative to other similarly oriented faults in the system imaged using 2D and 3D seismic reflection data. Focal mechanisms indicate primarily normal faulting with maximum horizontal stress striking 20-30° NE. The faults appear optimally or near-optimally oriented for failure within the modern stress regime, suggesting that small perturbations in stress via poro-elastic or pore fluid pressure changes could trigger failure. Stress drop estimates are consistent with intraplate earthquakes in natural tectonic settings. The spatio-temporal history of earthquakes relative to wastewater injection data varies. The 2015 M4.0 Venus earthquake was the largest but not the first earthquake on the Venus fault. At the DFW airport, where the causative well was shut-in within a few months of the initial earthquakes, events migrated nearly 6 km away from the injector over 5 years. The Azle sequence began abruptly, while the Irving-Dallas seismicity rate gradually increased leading up to the M3.6 doublet of 2015. There are no nearby (<10 km) injection operations to the Irving-Dallas sequence while Azle-linked wells operated for years prior to the onset of earthquakes. The cumulative history of wastewater injection and fluid production within the Ellenburger, and the evolution of pressure in the injection formation and units in hydrogeological contact, appear to be key drivers of the seismogenic evolution of the basin.