KYLE, J. RICHARD¹, JENNIFER A. RODELL¹, and HUAN-ZHANG LU^2
1Department of Geological Sciences, University of Texas at Austin, Austin, TX 78712
²Science de la Terre, Universite du Quebec A Chicoutimi, Chicoutimi, Quebec, Canada GASH 2B1

Abstract: Bacteriogenic Calcite-Sulfur Alteration Of Permian Evaporites, Delaware Basin, West Texas

Numerous calcite+sulfur+/-barite+/-celestite alteration zones occur within upper Permian carbonate-evaporite strata of the Delaware Basin, including one of the world's largest elemental sulfur concentrations. The general mechanism for sulfur formation involves reduction of aqueous sulfate derived from upper Permian gypsum by the sulfate-reducing bacteria, Desulfovibrio desulfuricans, utilizing hydrocarbons as their energy source. Hydrogen sulfide and carbon dioxide are' produced, resulting in the formation of structurally controlled calcite alteration zones within evaporitic strata. Oxidation of hydrogen sulfide, probably by oxygenated groundwater or possibly by sulfur-oxidizing bacteria, formed elemental sulfur.

Alteration calcites have d¹³C values of -47.5 to -8.3 ^o/oo and d¹⁸0 values from -9.3 to -3.8 ^o/oo (n=43), indicating involvement of hydrocarbons in calcite precipitation. Four distinct calcite types display signatures that appear to be controlled not by type, but by location within the alteration zone. Sr isotopic values for anhydrite within and outside of the alteration zone exhibit typical late Permian seawater values (~0.7068). Sr values of barite and calcite are higher (0,7079), indicating involvement of radiogenic Sr-bearing fluids. Fluid inclusions within calcites typically are single-phase, suggesting formation at T<70°C. Tm for inclusion fluids show a considerable range with values as low as -6.5°C for celestite, suggesting the presence of ~10 wt % brine.

These data suggest that the calcite+sulfur alteration zones formed along faults in zones where upward-mig rating basinal formation waters mixed with shallow ambient groundwaters in which sulfate-reducing bacteria were active.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas