Pervasively dolomitized carbonates of the Guadalupian San Andres Formation at Vacuum Field, NM compose an overall shallowing-upward sequence of lithofacies. Represented are five major depositional environments that occurred along the shelf edge and restricted inner platform of the Northwest shelf. Depositional facies include subtidal open marine fusilinid packstones and wackestones, shoal complex skeletal grainstones and peloidal skeletal packstones, restricted lagoonal peloidal wackestones and mudstones, restricted peritidal ooid skeletal packstones and grainstones, and supratidal cryptalgal mudstones and siltstones. Sedimentologic variations control the gross distribution of porosity in the reservoir; skeletal packstones and grainstones have the highest porosities, whereas wackestones and mudstones remain relatively non-porous. Depositional facies distribution does not appear to be the major controlling factor in porosity development.

Dolomitization of the Vacuum San Andres occurred during three episodes: (1) penecontemporaneous dolomitization of peritidal and supratidal facies associated with storm recharge and tidal pumping of hypersaline seawater, (2) later dolomitization and dolomite neomorphism by gravity-driven reflux of hypersaline waters, and (3) late dolomite overgrowth and pore-lining cementation. Stable carbon and oxygen isotope values of San Andres penecontemporaneous dolomites are significantly enriched relative to late Guadalupian marine calcite cements. Dolomite neomorphism is indicated by red-luminescent replacement and overgrowth precipitation. Red-luminescent dolomite is characterized by high Mn and Fe contents. The most depleted oxygen isotope values correspond to those samples containing significant proportions of paragenetically late dolomite cement. Siliciclastic-rich zones, such as the Lovington Sand, likewise show strongly depleted isotopic values; origins of these depleted compositions are equivocal.

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