THE HYDRODYNAMICS AND RELATED BIOGEOCHEMICAL PROCESSES IN THE PERMIAN BASIN, WESTERN TEXAS

The Permian Basin in western Texas and southeastern New Mexico contains oil reservoirs, native-sulfur deposits, large caverns (e.g., Carlsbad Caverns), and biogenic calcite along its western margins. These seemingly unrelated geologic phenomena are all products of the groundwater processes operating within one dynamic hydrologic environment. The combination of sediment compaction and hydrocarbon generation created zones of overpressure in the eastern Delaware basin (DB). The overpressure drove the migration of deep-basin brines and oil eastward into the Central Basin Platform (CBP) and westward into the overlying Permian evaporites. The biochemical interaction between migrating fluids, gypsum, and sulfate reducing bacteria (SRB) resulted in the formation of biogenic calcite and H2S. Subsequent oxidation of H2S by infiltrating meteoric water might have resulted in the formation of native sulfur and sulfuric acid. Sulfuric acid is responsible for the formation of Carlsbad-type caves within the Permian Capitan Reef Complex. We used a multi-disciplinary approach to investigate the hydrologic and biochemical processes that link the oil reservoirs, native sulfur, and caves. Geophysical log analyses showed that overpressured zones are characterized by low resistivity, high conductivity, low seismic velocity, and low bulk density. We hypothesized that episodic dewatering of fluids from the overpressured zones was responsible for oil migration and ore genesis in the basin. GC-MS analyses revealed the geochemical similarity of oil samples taken from CBP reservoirs and source beds within the DB. Hydrologic simulations show that oil and gas generation and sediment compaction all contributed to the formation and preservation of the modern overpressures. Although there has been significant gas production in the CBP, our model indicated that the source beds in CBP are not deep enough to enter the gas window. The results of modeling and GC-MS analyses support the long-distance migration of hydrocarbons from the DB to CBP. Migrating hydrocarbons stimulated the growth of SRB and the genesis of native sulfur; SRB were successfully grown from groundwater taken from native sulfur deposits. A geochemical model was developed to show the kinetic growth of SRB and the formation of native sulfur and biogenic calcite.