Most porosity development in carbonate reservoirs can be attributed, partly or totally, to diagenesis. This is especially true in Permian reservoirs of the Permian Basin, nearly all of which are dolomitized. Linking this diagenesis and associated porosity to unconformities representing major falls in sea level, however, is commonly problematic when using typical reservoir data sets. New subsurface data from the San Andres Formation, a major oil-producing reservoir in Texas and New Mexico, coupled with relationships previously defined from equivalent outcrops, illustrate both the style and expression of such porosity development.

Although the San Andres is characterized by an upward-shallowing succession of outer- to inner-ramp carbonate lithofacies in most platform settings, previous studies have demonstrated that the section is broken by a major unconformity that represents a sea-level fall of at least 100 m and a hiatus of .5 to 1 m.y.

Both the evidence and the result of this sea-level fall are clearly expressed in the San Andres reservoir at Fuhrman-Mascho field. Although cryptic in some core sections, the hiatus is indicated by an abrupt shift from outer-ramp deposits, composed of open-marine fusulinid wackestones and packstones, to exposed tidal-flat deposits. Porosity is developed in solution-enhanced vertical burrows below the unconformity. Although highly heterogeneous, the porosity in this burrowed zone reaches 15 m in thickness and extends for many kilometers. The widespread porosity development at this unconformity is a potential new target for both field exploitation and regional exploration.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah