Abstract
Since its discovery in 1929, Vacuum field, Lea County, New Mexico has produced 361 million bbl of oil and 298 bcf of gas from the Leonardian–Guadalupian (Permian) San Andres Formation. Despite this impressive performance, 65–70% of the estimated original oil in place remains in the ground. Complex variations in the porosity and permeability of reservoir lithologies are the principal causes for incomplete hydrocarbon recovery. Dolomitized oolitic-peloidal and fusulinid-peloidal packstones and grainstones constitute the productive reservoir facies. Interbedded in these porous units are tight, anhydritic or quartzose dolomudstones to mud-rich dolopackstones and dolomitic sandstones. Cores recovered from the nonporous intervals reveal textures, fabrics, and grain types indicative of (1) deposition in tidal-flat (peritidal) environments or (2) diagenetic modification, including collapse breccias, sinkholes, caves, and vertical fractures plugged with quartz sand or anhydrite cement, suggestive of exposure (karst) overprinting. Early geologic models of the Vacuum field were primarily generated from petrophysical data, resulting in lithostratigraphic correlations that crossed time lines and flow units. Application of a chronostratigraphic framework for the San Andres based on outcrop studies results in an updated reservoir model for Vacuum field. Improvements include (1) recognition of localized, tight, tidal-flat cycles in separate horizons in the youngest San Andres high-frequency sequence (HFS) instead of correlating all nonporous, high gamma-ray intervals as a single, regional quartzose sandstone (Lovington sand), (2) identification of bypassed pay in porous, strike-parallel dolopackstones that are stratigraphically equivalent to, but downdip from, the tidal flats, and (3) recognition of the presence of tight karst intervals in older HFSs that compartmentalize the most continuous San Andres pay interval. Deeper, uncored San Andres lithologies can be more accurately characterized by comparing petrophysical information with facies attributes from older San Andres outcrops. An untested reservoir may occur in basal San Andres skeletal dolopackstones that are sealed by dolomudstones.
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