Paleohydrology of the Delaware basin, western Texas: Overpressure development, hydrocarbon migration, and ore genesis

This study integrates quantitative modeling techniques with field observati ons to establish a paleohydrologic framework of the Delaware basin, western Texas. The reconstructed paleohydrologic models allow for a better underst anding of the development and maintenance of anomalous overpressures, hydro carbon generation and migration, and ore genesis in the basin. Results of n umerical modeling show that disequilibrium compaction and oil generation mi ght generate excess fluid pressures during the Late Permian in response to the rapid deposition of evaporite beds. The preservation of this overpressu re to the present, however, requires the presence of an extremely low-perme ability (<10(-11) d) top seal. Most shaly sediments, with permeability rang ing from 10(-4) to 10(-8) d, thus may be too permeable, by several orders o f magnitude, to preserve overpressure for more than 250 m.y. The predicted present-day gas window is located within the overpressure zone, suggesting that the volume increase associated with the oil-to-gas conversion may be a ttributed to present overpressures. The native sulfur deposits Likely forme d in a fluid mixing zone resulting from the Laramide uplift of the western basin during the Tertiary. In our model, meteoric water recharged along the basin's uplifted western margin and discharged basinward. Hydrocarbons mig rated landward by pressure gradients and buoyancy and discharged upward alo ng faults in the western basin, where they mixed with meteoric water. Many oil and mineral reservoirs may have formed in the take place. In the Culber son sulfur ore district, for example, fluids including hydrocarbons and met eoric water migrated upward through faults from underlying carrier beds, in to the Permian Salado limestone. There, the mixture of fluid drives biochem ical reactions that precipitate native sulfur.