A Hydro-Mechanical Investigation of the First M 4 + Seismicity Sequence in the Midland Basin, Texas

The M 4.2 2020 December Stanton, TX seismicity sequence marks the first of several M 4 + seismicity sequences in the Midland Basin. In this study, we investigate its causal link to the surrounding field-scale, multi-decadal, multi-zone saltwater disposal (SWD) by 180 + SWD wells through integrated data analysis and high-end hydro-geomechanical numerical modeling. We test six scenarios by permutating three fault upper extents and two fault-zone structures and interpret their plausibility based on their performance in reproducing seismicity onset and distribution as characterized by an aggregated earthquake catalog. We do so via Coulomb faulting analysis at inferred seismogenic depths complemented by rate-and-state friction-based seismological modeling on the event-hosting fault. Our analysis suggests that the basement-rooted faults, despite being poorly seen in the reflection seismic data, likely extend into the deep disposal interval but not the two shallower disposal intervals and contain impermeable fault gouge. The work also strongly suggests that the seismicity sequence was induced mainly by intermediate and deep SWD and negligibly by shallow SWD through an over three-decadal stressing process involving 10 + wells up to 60 km laterally away from the event epicenters. The joint stressing by a cluster of wells to the Northwest and a near-fault well produced two Coulomb stress lobes that explain the bifurcating seismicity near the top of the basement. Meanwhile, the mainshock was likely preceded by a half-year-long nucleation period, over which the joint stressing also drove the development of a seismicity funnel zone that explains the remaining seismicity in the basement.