Abstract

This study integrates quantitative modeling techniques with field observations to establish a paleohydrologic framework of the Delaware basin, western Texas. The reconstructed paleohydrologic models allow for a better understanding of the development and maintenance of anomalous overpressures, hydro carbon generation and migration, and ore genesis in the basin. Results of numerical modeling show that disequilibrium compaction and oil generation might generate excess fluid pressures during the Late Permian in response to the rapid deposition of evaporite beds. The preservation of this overpressure to the present, however, requires the presence of an extremely low-permeability (<10-11 d) top seal. Most shaly sediments, with permeability ranging from 10-4 to 10-8 d, thus may be too permeable, by several orders of 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 attributed to present overpressures. The native sulfur deposits likely formed 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 migrated landward by pressure gradients and buoyancy and discharged upward along faults in the western basin, where they mixed with meteoric water. Many oil and mineral reservoirs may have formed in the fluid mixing zone, where extensive chemical reactions take place. In the Culberson sulfur ore district, for example, fluids including hydrocarbons and meteoric water migrated upward through faults from underlying carrier beds, into the Permian Salado limestone. There, the mixture of fluid drives biochemical reactions that precipitate native sulfur.

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