Abstract 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 (O 2 ) is introduced into solutions containing dissolved hydrogen sulfide (H 2 S) 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, H 2 S 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).

Abstract The Capitan depositional system was studied in the subsurface using seismic and well data from the northeastern Delaware basin. Seismic data of the Capitan depositional system show characteristics that include (1) a massive prograding reef/slope, (2) back-reef/shelf reflectors that dip and diverge basinward before disappearing into the massive reef, and (3) layered bottomset beds that thicken basinward by the addition of younger reflectors. A wireline log cross-section of nearby wells illustrates the stratigraphy in more detail than the seismic line. Basinward-dipping shelf strata are interbedded sandstones and carbonates that diverge and pass basinward into massive carbonate of the reef. Correlative markers within the massive reef are difficult to find. Slope carbonate beds thin and basinal siliciclastics thicken toward the basin. Bottomset beds in the basin consist of interbedded sandstones/siltstones and low-porosity carbonates. This subsurface stratigraphy is very similar to outcrop stratigraphy described in the Guadalupe Mountains. Lithologic differences between outcrops and their subsurface equivalents are due largely to variations in dolomitization and evaporite dissolution on outcrops. Distribution of porosity in the Capitan depositional system is closely related to depositional facies. Back-reef sandstones and some shelf carbonates adjacent to the reef have good porosity and moderate permeability, but porosity and permeability in those strata generally decrease landward. The subsurface Capitan reef has moderate porosity and high permeability and is a regional aquifer. Carbonate beds in the basin are generally not porous, but some basinal sandstones filling elongate channels have good porosity and moderate permeability. Hydrocarbons are not present in the Capitan reef because it does not occur in a setting that allows structural or stratigraphic closure and/or isolation from active meteoric aquifers. Many oil fields (10-400 million barrels recoverable) occur in back-reef equivalents of the Capitan reef, primarily the Seven Rivers and Yates formations, on the Northwestern Shelf and western edge of the Central Basin Platform. Those reservoirs are generally in stratigraphic or combination stratigraphic-structural traps, where porous and permeable sandstones pass up-dip into impermeable sandstones/siltstones, carbonates, and/or evaporites. Oil also occurs in channelized basinal sandstones equivalent to the reef, but the basinal fields have <5-30 million barrels of oil recoverable, and hence are generally smaller than those of the back-reef.

Abstract The Late Permian (Guadalupian) mixed carbonate/siliciclastic sequences of the Delaware Basin, one of the long-lived subbasins of the Permian Basin, are well known both for their classic outcrop exposures revealed by basin and range structuring in the Guadalupe Mountains and for their prolific hydrocarbon production. A large number of stratigraphic and sedimentologic studies have established the Capitan reef and associated facies as a model for the understanding of carbonate facies in a shelf margin setting and of reciprocal sedimentation relations between a shelf and basin. Early studies focused on biostratigraphy, lithostratigraphy, and early concepts of reciprocal sedimentation. Focus shifted in the 1970s and 1980s to analysis of depositional facies and processes and on the relatively new understanding of early diagenesis of reef margins. More recently, the outcrops have been analyzed from a cyclostratigraphy and sequence stratigraphy perspective.

Abstract The Late Permian (Guadalupian) mixed carbonate/siliciclastic sequences of the Delaware Basin, one of the long-lived subbasins of the Permian Basin, are well knovvn both for their classic outcrop exposures revealed by basin and range structuring in the Guadalupe Mountains and for their prolific hydrocarbon production (Figure 18 ). A large number of stratigraphic and sedimentologic studies have established the Capitan reef and associated facies as a model for the understanding of carbonate facies in a shelf margin setting and of reciprocal sedimentation relations between a shelf and basin. Early studies focused on biostratigraphy, lithostratigraphy, and early concepts of reciprocal sedimentation. Focus shifted in the 1970’s and 1980’s to analysis of depositional facies and processes and on the relatively new understanding of early diagenesis of reef margins. More recently, the outcrops have been analyzed from a cyclostratigraphy and sequence stratigraphy perspective. It will be interesting to see the impact of a refined stratigraphic fi-amework on the direction of future facies, paleo-ecological, and diagenetic studies. Providing the initial interest in the area were the superb field studies and subsequent detailed reporting of the geology of the southern Guadalupe Mountains by King (1942, 1948 ). The book by Newell et al. (1953) on the Capitan did much to further enhance the outcrops as research models for sedimentary geologists. The critical treatment of the Capitan sedimentology by Dunham in the late 1950’s and 1960’s, culminating in his detailed 1972 guidebook, stimulated interest and added new understanding. Dunham’s work, plus the