Stoudt, Emily L.¹ 
(1) University of Texas Permian Basin, Midland, TX

ABSTRACT: Core Description Reveals Patterns of Reservoir Compartmentalization in the Leonardian/Guadalupian (Permian) San Andres Formation, Vacuum Field, Lea County, New Mexico

Carbonate reservoir rocks are typically deposited in shallow marine environments, where variations in depositional setting produce tight and porous lithologies in lateral and vertical proximity. Additionally, carbonate minerals are strongly susceptible to diagenetic alteration, further enhancing porosity and permeability variations. The Leonardian/Guadalupian (Permian) San Andres Formation in Vacuum field, Lea County, New Mexico, is a good example of a carbonate reservoir that has been compartmentalized by depositional and diagenetic variations. Dolomitized oolitic/peloidal and fusulinid/peloidal packstones and grainstones constitute the productive reservoir facies. Interbedded within these porous units are zones that are clearly “tight”, as indicated by both log character and production tests. Early geologic models of the San Andres formation in Vacuum field were primarily generated from petrophysical data, with no core examination to clarify the nature of the “tight” zones. These models resulted in lithostratigraphic correlations that crossed time lines and flow units. 
Examination of 3,000 feet of core accurately reveals reservoir lithologies, and the origin of nonporous intervals. Tight zones are anhydritic or quartzose dolomudstones to mud-rich dolopackstones and dolomitic sandstones. Textures, fabrics and grain types indicate that these intervals formed as either: (1) tidal flat (peritidal) deposits or (2) exposure (karst) overprinting of a variety of depositional facies. Karst features include collapse breccias, sinkholes, caves and vertical fractures plugged with quartz sand or anhydrite cement. 
The “rock based”, updated reservoir model for Vacuum field includes: 1) recognition of localized tight tidal flat cycles in separate flow units in the youngest San Andres, 2) identification of by-passed pay in shelf margin, porous, strike-parallel dolopackstones, and 3) recognition of the presence of tight karst intervals in deeper flow units that compartmentalize the more continuous San Andres pay interval.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.