Part of the Permian Capitan Reef Complex is exposed in the Guadalupe Mountains of southeastern New Mexico and western Texas (Fig. 1). The reef complex includes the Capitan Limestone and the carbonate backreef beds of the Artesia Group that comprise a lithosome called the Capitan aquifer (Hiss, 1976). This lithosome contains well-developed solution openings that range from microscopic to voids the size of Carlsbad Cavern. This solution porosity was once thought to be caused by weak carbonic acid in the phreatic zone within the Capitan aquifer (Bretz, 1949). During the last 15 years, however, workers have obtained evidence indicating that sulfuric acid may be a major cause of carbonate dissolution (Egemeier, 1973; Jagnow, 1977; Palmer et al., 1977; Maslyn, 1979; Davis, 1980; Kirkland, 1982; and Hill, 1987). Sulfuric acid is generated when oxygen (O₂) is introduced into solutions containing dissolved hydrogen sulfide (H₂S) gas (Hill, 1987).

Hydrogen sulfide is common in subsurface formations in southeastern New Mexico (Bjorklund and Motts, 1959; Hinds and Cunningham, 1970, pp. 4 and 7). In southeastern New Mexico and elsewhere along the subsurface trend of the Capitan reef, H₂S is present in accumulations of oil and gas and in associated saline water (Schram, 1956a, p. 103, and 1956b, p. 307; Wilson, 1956, p. 179; and Roswell Geological Society Symposium Committee, 1956a, p. 181, and 1956b, p. 291).