Reservoir Characterization in the San Andres Formation of Vacuum Field, Lea County, New Mexico – “Rock Work” Improves the Model
Emily L. Stoudt1, Gregory D. Hinterlong2
1University of Texas Permian Basin, Midland, Texas
2ChevronTexaco, Midland, Texas
Since its discovery in 1929, Vacuum field, Lea County, New Mexico has produced approximately 375 million barrels of oil and 300 billion cubic feet of gas from the Leonardian/Guadalupian (Permian) San Andres Formation. Despite this impressive performance, 65 to 70% of the estimated original-oil-in-place remains in the ground. Complex variations in the porosity and permeability of reservoir lithologies are the principle causes for incomplete hydrocarbon recovery. Dolomitized oolitic/peloidal and fusulinid/peloidal packstones and grainstones constitute the productive reservoir facies. Interbedded within 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 suggestive of exposure (karst) overprinting. These diagenetic features include collapse breccias, sinkholes, caves and vertical fractures plugged with quartz sand or anhydrite cement.
Early geologic models of Vacuum field were primarily generated from petrophysical data, resulting in lithostratigraphic correlations that crossed time lines and flow units. Detailed examination of 3,000 feet of core more accurately revealed reservoir lithologies, permitted an understanding of facies distribution, and allowed correlation of lithologic variations within the same time-stratigraphic intervals. Application of the resulting chronostratigraphic framework resulted 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, 2) identification of by-passed 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 high frequency sequences that compartmentalize the most continuous San Andres pay interval.
Figure 1. Present-day structural cross-section showing log-based lithostratigraphic correlation of the San Andres reservoir interval in Vacuum field. The reservoir was originally subdivided into 4 major lithologic units: 1) the porous "lower San Andres" carbonates (LSA), 2) the Lovington sandstone (LVSS), 3) the porous "upper San Andres" carbonates (USA), and 4) one or more other sandstones in the upper San andres (SASS). The "lower San Andres" was recognized as being the best and most continuous reservoir.
Figure 2. Present-day structural cross-section showing chronostratigraphic correlation of the same San Andres interval shown in Figure 1. Note that chronostratigraphic boundaries cross-cut the lithostratigraphic boundaries.