REVISITING THE PRAGUE, OKLAHOMA, EARTHQUAKES, NATURAL,TRIGGERED, OR INDUCED: NO SIMPLE ANSWER

Oklahoma’s largest recorded earthquake (Mw 5.6) occurred on 11-05-2011 near the town of Prague along a previously unidentified splay off of the Wilzetta fault zone. Within the realm of public debate and media coverage the “Prague Earthquake” features as an example of the dangers posed by seismicity induced by high volume water disposal in a fault sealed compartment, a model initially proposed by Keranan, et al. in 2013. Subsequent studies, such as Walsh and Zoback (2015), suggest a more complex phenomena. The petroleum geology of the Wilzetta field shows that the principal reservoir, the Hunton Formation, is trapped in 3-way upthown fault closure. The field has a water-drive reservoir and a well-defined spill point at a structural saddle near the terminus of its bounding fault. An alternate structure contour map with a crossing sealing fault yields a permissible, albeit awkward, solution with not more than 100ft of throw across the putative sealing fault. This throw is not sufficient to offset and seal either the Hunton or the main injection reservoir, the Arbuckle dolomite (> 500ft of 6% porosity over an 1800ft interval). 3D seismic data do not support the presence of a fault sufficiently large to seal the Arbuckle. Thus, there is no evidence to support the presence of a fully fault sealed compartment. This leads to the question of treating the system as a much larger compartment in which changes in deviatoric stress make the system sensitive to “enhanced remote triggering” (van der Elst et al. 2013). Another problem a simple model for the Prague earthquakes is that for the last 3 years seismic activity below Mw 3.0 has ceased even though injection rates and volume are 50% high than those in the 6 years prior to the Prague seismic events. The fact that seismic activity has diminished may reflect the long-term relaxation of the system and could point to a fundamental difference between intra-plate and plate boundary seismicity. Further forensics are needed and issues clearly stated in the public domain.