REGIONAL FLUID FLOW, PETROLEUM DESTRUCTION, AND BACTERIOGENIC ALTERATION OF EVAPORITES: THE PERMIAN DELAWARE BASIN EXAMPLE, WEST TEXAS, U.S.A

Alteration of sulfate-rich evaporitic strata to regionally extensive assemblages of calcite and sulfur is common in sedimentary basins of many ages; similar alteration products are present in salt dome cap rocks. The general mechanism for sulfur formation involves reduction of aqueous sulfate by sulfate-reducing bacteria that utilize hydrocarbons as their energy source. Hydrogen sulfide and carbon dioxide are produced, resulting in the formation of calcite alteration bodies within sulfate strata that locally host elemental sulfur concentrations.

Extensive calcite+sulfur alteration zones within Permian evaporitic strata of the Delaware Basin provide a major example of the results of these processes. Alteration calcites have a wide d¹³C range with values as light as -48 ‰. d³⁴S values for elemental sulfur ranges from -9 to 11‰, whereas associated barite and celestite range from 30 to 74 ‰, in marked contrast to Permian evaporite sulfates with d³⁴S values of 10 to 12 ‰. Sr isotopic values for anhydrite within the alteration zone exhibit typical late Permian seawater values (~0.7068). Values for barite and calcite are higher (~0.7079), suggesting the involvement of radiogenic Sr-bearing fluids, likely from Permian siliciclastic reservoirs. Fluid inclusions typically are single-phase, suggesting formation at <70°C, which is compatible with temperature constraints for sulfate-reducing bacteria. Gaseous hydrocarbons are present in some inclusions. Freezing temperatures show a substantial range with values as low as -6.5°C, suggesting the presence of a ~10 wt % brine during alteration.

This integrated study suggests that calcite+sulfur alteration zones formed along faults where upward-migrating basinal formation waters bearing Ba, Sr, and petroleum mixed with shallow ambient ground water in which sulfate-reducing bacteria were active. Incomplete oxidation of hydrogen sulfide, probably by oxygenated ground water, formed concentrations of elemental sulfur. Oxidation of hydrogen sulfide to sulfate produced the isotopically heavy barite. If anaerobic conditions are required for bacterial sulfate reduction, then temporal and spatial variation in oxygen abundance is suggested, related to prolonged regional subsurface fluid flow.