Abstract: Possible Role of a Pressure Seal in Overpressure Development and Maintenance in the Eastern Delaware Basin, West Texas
WILLIAMS, DAPHNE, D., and LEE, MING-KUO
The Delaware Basin is a tectonically stable, mature basin that has undergone insignificant sedimentation during the past 250 million years yet sustains fluid pressures exceeding hydrostatic within the Mississippian, Pennsylvanian, and Permian shales. Although the occurrence of overpressure is common in young actively subsiding basins undergoing rapid sedimentation and compaction, it is rare in old, mature basins. If disequilibrium compaction is the major cause of overpressure development, an effective low-permeability rock layer, or a pressure seal, capable of confining anomalous pressures over geologic time is required. We hypothesize that Permian-age evaporites in the basin may be capable of hydraulic sealing of overpressure developed in the underlying shales.
A preliminary one-dimensional numerical model was used to evaluate how permeability and thickness of a "pressure seal" may affect the dissipation of overpressure with time. A simple three-layer system consisting of a shale, a pressure seal, and a sandstone unit, was used. Results show that the time over which pressure is confined in shales is roughly proportional to the thickness of the overlying seal and inversely proportional to the permeability of the seal. Results also indicate that a seal with permeability less than 10{-10} darcy, at least two orders of magnitude lower than normal shale, is capable of maintaining overpressures for over 250 million years, as in the eastern Delaware Basin. Both one- and two-dimensional simulations will be used to further examine whether Permian salt can act as an effective seal for maintaining overpressure in the basin shales. Modeling results will lead to a better understanding of the hydrologic evolution of the basin and the subsurface conditions influencing overpressure development.
AAPG Search and Discovery Article #90940©1997 AAPG Foundation Grants-in-Aid