Outcrop studies are increasingly being used to supply realistic data on interwell conditions in complex subsurface reservoirs. The Grayburg Formation of the Permian Basin, with its prolific but heterogeneous reservoirs and its extensive exposures in the Guadalupe Mountains, provides an ideal test of this methodology.

The outcrop area in the northern Guadalupe Mountains is less than 60 mi west of producing reservoirs of the Northwest Shelf and Central Basin Platform. This mixed siliciclastic/carbonate unit is 280 ft thick above the terminal San Andres shelf margin and thickens seaward where it is made up of (1) an undetermined number of lowstand parasequences (minimum 400 ft thick), (2) 4 transgressive parasequences (80 ft thick), and (3) 17 highstand parasequences (300 ft thick). Average parasequence thickness is 20 ft. A continuous 10-mi dip section exhibits inner ramp lagoon (4-mi dip width), sandstone/tepee ramp-crest (2-mi dip width), ooid-shoal outer ramp-crest (1-mi dip width), and fusulinid-peloid outer-ramp (3-mi dip width) facies tracts. Maximum lateral variability of facies occurs in upward-shallowing parasequences of the ramp-crest and outer ramp-crest tracts; here porous ooid shoals show local topographic relief of up to 15 ft, with intercalated sandstones occurring as channel-form lenses filling shoal topography.

The Grayburg subsurface section is represented by nine continuous cores in the ARCO North Foster field, Central Basin Platform, in Ector County, Texas. The thickness of the Grayburg at the San Andres shelf margin is 270 ft. The basal Grayburg consists of 20 ft of coarse-grained grainstone followed by 60 ft of fusulinid wackestone, the latter representing maximum flooding of the Grayburg sequence. The overlying highstand section consists of 19 upward-coarsening cycles from 4 to 20 ft thick. Cycle bases are defined by silty mudstone to siltstone.

Three facies tracts have been identified: (1) tidal-flat birdseye wackestone-packstone of the ramp crest, (2) coated-grain grainstone and coarse-grained grainstone of the outer ramp-crest, and (3) diagenetically altered fine-grained grainstone of the outer ramp. Ramp-crest and outer ramp-crest facies occur on the western side of the field in north-south-oriented bands approximately 1 mi wide, but the western limit of these facies has not been delineated. The outer-ramp facies tract occurs as a northwest-trending, 2-mi-wide band on the east side of the field. To the east, the outer-ramp facies grades into the fusulinid facies.

The closely comparable dimensions of parasequences and facies tracts between the outcrop and subsurface suggests that data on facies dimensions within parasequences attained from mapping of continuous outcrops will be useful for stochastic modeling of interwell heterogeneity in the North Foster/Johnson leases.