The Delaware Basin is a pear-shaped, NNW-SSE elongate, intracratonic basin, about 250 km long and 150 km wide at its bulging northwestern end. The Capitan Reef Complex forms a narrow carbonate belt along the rim of the Delaware Basin for some 600-700 km, but outcrops occur only along about one-sixth of its rim. Most of these are in the Guadalupe Mountains where they fringe the Northwestern Shelf, but some occur in the Apache Mountains and fringe the Diablo Plateau, and also in the Glass Mountains where they fringe the Southern Shelf (index map of Fig. 1).

The most extensive and laterally continuous exposures are in the Guadalupe Mountains, and most subsurface data is from the area east and southeast of the Guadalupes. By far the most sedimentologic research has been of outcrops of the Guadalupe Mountains. Here excellent exposures, tectonic simplicity, and the large scale of the dramatic shelf-to-basin transitions have, justifiably, made the Permian of this area, and especially the Capitan Reef and its associated facies, a world-famous sedimentologic model.

But the extensive research over more than half a century on the Guadalupe outcrops (and on the petroleum-bearing area adjacent to them) has created two dangerous impressions: (1) that the Capitan Reef and its associated facies are well understood, and (2) that the "model" derived from study of the Guadalupe Mountains outcrops can be safely extrapolated along the remainder of the Capitan trend rimming the Delaware Basin. Despite major progress in recent decades as concerns "what is where" and "how did it form," much remains to be learned to arrive at a satisfactory sedimentologic understanding of the Capitan Reef and its associated facies. Moreover, the available surface and subsurface evidence indicates that considerable depositional and diagenetic lateral variability exists along the Capitan trend, and application of sedimentologic principles indicates that considerable lateral variability should be expected.