Does prior depletion affect the occurrence of earthquakes induced by hydraulic fracturing in the Delaware Basin of Texas and New Mexico?

There is substantial evidence that hydraulic fracturing induces seismicity in the Delaware Basin of west Texas and southeast New Mexico. Since the TexNet Seismic Monitoring Program was deployed in this area in January 2017, numerous small earthquakes have been documented that are correlative, in space and time, with hydraulic fracturing operations (principally in the Wolfcamp formation) in the southern part of the basin. In marked contrast, the northern part of the basin has experienced essentially no induced earthquakes during this same period, despite intensive hydraulic fracturing and wastewater disposal also occurring in that part of the basin. The objective of this study was to better understand the mechanisms responsible for triggering the seismicity and to identify ways that might limit the occurrence of earthquakes in the future. Earthquake epicenters and focal plane mechanisms indicate triggered slip on NW-striking normal faults, consistent with the stress field in the southern Delaware Basin. The concentration of seismicity in the southern part of the basin does not appear to be associated with disposal of produced water, which predominantly targets the relatively shallow Delaware Mountain group in this area. Rather, the hydraulic fracturing-induced earthquakes are localized where there has not been prior production of the Bone Springs formation, the unit immediately above the Wolfcamp. In other words, in the southern part of the Delaware basin where the induced earthquakes are occurring, there has been no prior depletion of the Bone Springs formation. In the northern part of the basin where there has been no recent seismicity, the Bone Springs has been depleted. Two mechanisms appear to explain this observation. First, it is likely that the poroelastic reduction in stress magnitudes resulting from depletion within the Bone Springs formation makes fault slip less likely. Second, the poroelastic reduction of the least principal stress in the Bone Springs would tend to cause hydraulic fractures to propagate upward from the Wolfcamp, away from more deep-seated faults. This is consistent with the great majority of earthquakes occurring at or below the level of the Wolfcamp, although there remains considerable uncertainty in earthquake depths in this area.