From 1983 to 2022, >9 billion bbl of wastewater were injected into Ordovician to Pennsylvanian rocks of the Midland Basin and surrounding regions; between 2021 and 2023, in the same region, >30 local magnitude 3.0+ seismic events occurred each year. Evaluation of the injection fluid diffusion processes leading to fault rupture and seismogenic release requires a thorough analysis of the coupled injection reservoir and seal properties. In this study, we integrate regional subsurface mapping (>8000 wells) and petrophysical analyses (∼400 wells) with published geologic interpretations to characterize the stratigraphic architecture of injection reservoirs and seals and the distribution of lithofacies and matrix porosity; these results provide an opportunity to assess reservoir quality and pore volume for wastewater injection.

In the Midland Basin, we identify three deep injection intervals bracketed by regionally extensive unconformities. The highly dolomitized, karsted, and laterally discontinuous Ellenburger Group rocks are characterized by <5% matrix porosity and alternate with siliciclastic and shale rocks representing intraformational seals. Proximity of the Ellenburger Group to basement-rooted faults poses a challenge to its otherwise high storage capacity for wastewater disposal. Multiepisodic subaerial exposure diagenetically altered the highly compartmentalized Montoya to Thirtyone Formation carbonate and chert succession, limiting its <5% porosity reservoirs to the northcentral Midland Basin and Central Basin platform. Widespread Pennsylvanian siliciclastic and carbonate rocks with up to 10% matrix porosity reflect the greatest pore volume and highest reservoir heterogeneity due to coeval tectonics and icehouse deposition. The clay-rich Woodford Formation and siliciclastic-carbonate-rich Wolfcamp formation serve as cap rocks confining saltwater disposal in the Midland Basin.