Example of well-imaged fault networks that connect deep injection zones to intra-basement seismogenic faults: Fault slip sensitivity analysis

Abstract The Permian Basin has experienced >9900 local magnitude (ML) 2.0+ earthquakes since 2017 due to the reinjection of produced water from approximately 35,000 horizontal wells into strata above (shallow-salt water disposal [SWD]) and below (deep-SWD) shale production intervals. Curtailment of deep-SWD within regulated seismic response areas (SRAs) has resulted in declined monthly rates since 2021; however, in the Northern Culberson Reeves SRA (NCR SRA), seismicity rates have remained high despite curtailment of deep-SWD, and this has caused ongoing concern. In the NCR SRA, there are three distinct levels of faulting identified within an approximately 1500 km2 3D seismic reflection data set: (1) intrabasement (IB), (2) basement-rooted (BR), and (3) shallow, strata-bound (SSB). IB faults are low-to-moderate-angle (~20°–45°) Proterozoic thrust faults, which are truncated by the Great Unconformity. BR faults are moderate-to-high-angle reverse and subvertical strike-slip faults (~50°–90°), which offset the Great Unconformity and overlying Paleozoic strata including deep-SWD injection strata. SSB faults are steeply dipping (~60°–80°), elongate, narrow graben, which deform upper-Permian age units, including shallow-SWD injection strata. A regional stress model, which reflects a normal-faulting regime, was applied, and fault-slip sensitivity was assessed using the Coulomb shear failure model. Under these conditions, most SSB faults are optimally oriented for slip, BR faults range from stable to critical, and IB faults are stable, requiring a significant pore-pressure increase to reach criticality. In the NCR SRA, significant earthquakes (ML 4.0+) are located exclusively within the basement. These events are spatially linked to BR fault segments, which sole into IB imbricate faults and extend into deep-SWD injection strata, providing direct hydraulic communication between faults and SWD injection intervals.