Tales from the deep: a new national-scale analysis of ultra-overpressure in sedimentary basins

Ultra-overpressure, defined here as pore pressure approaching fracture pressure (>90%) that results in extremely low effective stress, is observed in many of the world’s most prolific oil and gas reservoirs. In the Western Bossier shale gas play, ultra-overpressure has been linked to porosity preservation, and reservoir pressures that yield wells producing five to ten times more gas than typical shale plays. To identify other potential ultra-overpressured systems, this study analyzes approximately 700,000 mud-weight measurements from sedimentary basins across the contiguous United States. Using mud-weights as a proxy for pore pressure is the most practical approach for identifying regional pressure trends since drill stem tests or other direct measures of pressure are not abundantly available. Fracture gradients were calculated and a cutoff of >90% of the fracture gradient was applied to identify ultra-overpressured intervals and delineate regional pressure fairways. Continuous fairways of ultra-overpressured values within individual formations were identified in the Gulf Coast, Anadarko, Permian, Uinta, Great Valley, and Greater Green River basins. These fairways are further evaluated using finite-element numerical basin modeling, well logs, and sedimentation-rate estimates to determine the dominant mechanisms responsible for ultra-overpressure. Results indicate that ultra-overpressure arises from a number of processes and that time since pressure generation, lithology, organic matter richness, degree of maturation, stress regime, and fracture connectivity all contribute to the preservation of ultra-overpressure. Results also indicate that sedimentation-driven ultra-overpressure is characterized by shallow onset depths (~6000–10,000 ft) and relatively high preserved porosity. In maturity-driven systems, ultra-overpressure commonly develops at deeper burial (>10,000 ft) after mechanical compaction and cementation are advanced, promoting long-term pressure retention. These results highlight the value of integrated analyses that combine pressure indicators, geologic history, interpretation of lithology and petrophysical logs, and numerical basin modeling to identify the mechanisms and spatial distribution of ultra-overpressure.