Abstract Caves exist in the Capitan Formation in the Guadalupe, Apache, and Glass Mountains, southeastern New Mexico and West Texas. Four episodes of karsting have been identified in the Guadalupe Mountains: (1) Stage 1 fissure karst (Late Permian), (2) Stage 2 spongework karst (Mesozoic), (3) Stage 3 thermal karst (Miocene), and (4) Stage 4 sulfuric acid karst (late Miocene to present). The last sulfuric acid episode was responsible for the large cave passages and for the distinctive deposits within these caves (gypsum blocks; native sulfur; the pH-indicator minerals endeliite, alunite, and natroaiunite; and the uranium minerals tyuyamunite and metatyuyamunite). The hydrogen sulfide responsible for forming sulfuric acid in the Capitan reef originated from hydrocarbons in the Delaware Basin. Stage 4 caves in the Glass Mountains are also of sulfuric acid origin, showing that the basin probably degassed hydrogen sulfide around its entire margin. Caves also exist in the subsurface Capitan on the eastern and northern sides of the Delaware Basin. San Simon Swale/Sink on the east and breccia pipes/domes on the north may stope down to the cavernous Capitan. The same sulfuric acid mechanism that formed caves in the exposed Capitan may have also been partly responsible for caves in the subsurface Capitan.

Abstract “H 2 S-related porosity” refers to porosity created in a H 2 S system where dissolution can be produced by the mixing of waters of different H 2 S content or by the oxidation of H 2 S. “Sulfuric acid oil-field karst” refers to a specific kind of H 2 S-related porosity where carbonate reservoirs of cavernous size have been dissolved by a sulfuric acid mechanism. In a H 2 S system, porosity can be produced entirely in the deep subsurface and does not have to represent a paleokarst surface or dissolution in the shallow-phreatic or vadose zones. H 2 S-related porosity is characterized by the large volume of hydrocarbons it can host, by extensive fracture permeability interconnected with “sponge-work” cavities or caves of tens to hundreds of meters in extent, by porosity related to structural and/or stratigraphic traps, and by the presence of high uranium and/or iron. Possible examples of H 2 S-generated porosity systems are the Lisburne field, Prudhoe Bay, Alaska, and some of the extremely productive fields of the Middle East.

Abstract This paper summarizes an ongoing research project on the Delaware Basin studying interrelationships between oil-gas, sulfur deposits, Mississippi Valley-type (MVT) ores, and cave genesis. Such an integrated approach was taken by Hill (1987), who suggested that basinal H 2 S - generated by oil-anhydrite reactions at the Castile-Bell Canyon contact - was the ultimate source for the sulfuric-acid speleogenesis of Guadalupe caves and for MVT ores in carbonate rock along the margins of the basin.