Distinguishing the Causal Factors of Induced Seismicity in the Delaware Basin: Hydraulic Fracturing or Wastewater Disposal?

Identifying the causal factors behind elevated seismicity rates in areas with multiple ongoing oil and gas activities has always been a challenging issue that is lacking a homogeneous response, hindering regulatory actions. A good example is the Delaware basin, inWest Texas and NewMexico, where the seismicity rates have been gradually increasing since 2009. To study the area, we expand the framework presented in Grigoratos et al. (2020a) to daily hydraulic fracturing (HF) operations and pressure-driven modeling of wastewater disposal (SWD). Our analysis hindcasts seismicity rates of magnitude (M) = 1.5 events after 2017 on a 5 km grid using either HF or SWD data as input and compares them against the null hypothesis of solely tectonic loading. In the end, each block is assigned a p-value, indicating the statistical confidence of its causal link with either HF, shallow SWD or deep SWD injection operations. Our results indicate that 68% of the earthquakes occurred in blocks with p = 0.05 (95% confidence interval) for at least one examined causal factor and are thus triggered by oil and gas activities. This percentage increases to 81% for p = 0.10 (90% confidence interval). Overall, more events than previously identified are associated with HF, especially within Reeves County. That said, shallow SWD is linked to many more earthquakes, likely reactivating parallel faults of significant length. We should note the seismicity around Pecos city seems to be affected by more than one injection activity, with higher frequency operational data and better depth resolution needed to fully decouple causality between HF and shallow SWD. Within Culberson County, and around the epicenter of the 2020 Mentone Mw 5.0 earthquake, the earthquakes occur predominantly within the basement, with the identified triggering mechanism being SWD from deep wells.