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 petro-physical 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.

Abstract This chapter describes an integrated approach to reservoir characterization and three-dimensional (3-D) geologic modeling of the San Andres Formation at Vacuum field, New Mexico, United States. We present techniques to identify significant heterogeneities within a carbonate reservoir using stratigraphic, petrophysical, and 3-D multicomponent seismic data. This integrated approach provides a detailed static description of reservoir heterogeneity and improved delineation of the reservoir framework in terms of flow units. We use a petrophysics-based method to identify hydraulic flow units within a sequence-stratigraphic framework. Flow units are characterized within high-frequency carbonate sequences through analysis of the vertical variation of flow (kh) and storage capacity (ϕh) and pore-throat radius (R35) associated with successions of subtidal, intertidal, and supratidal rocks. Pore-throat radii from cored wells are used to modify the empirically derived Winland equation to estimate values of pore-throat radius in non-cored wells. Flow profiles, constructed from log porosities and neural-network permeabilities, are correlated and used to build a 3-D geologic-model framework. Characterization of both matrix and fracture properties within a reservoir is possible using 3-D multicomponent seismic data and wire-line logs. Compressional- and shear-wave amplitude attributes together provide more accurate porosity estimates than those determined from compressional-wave data alone. Shear-wave anisotropy measurements provide information about inferred fracture density and orientation that can be used to modify permeability models to account for regions with open fractures. Because of this study, reservoir-simulation models that incorporate modified permeability distributions more accurately account for unexpected early CO 2 -breakthrough times observed in the field. In addition, flow-simulation results indicate that the need to upscale the geologic model was significantly reduced or eliminated by describing flow units using the combined sequence-stratigraphic- and petrophysics-based method.

ABSTRACT In carbonate-ramp reservoirs, stacking of rock-fabric facies within a high-frequency, sequence stratigraphic framework provides the most accurate framework for displaying the distribution of petrophysical rock properties of porosity, permeability, relative permeability, and capillarity. Rock-fabric facies are defined on the basis of grain and crystal size and sorting, interparticle porosity, separate-vug porosity, and the presence or absence of touching vugs. Outcrop geostatistical studies of the Algerita Escarpment suggest little spatial correlation of permeability within rock-fabric facies, and petrophysical properties can be averaged at rock-fabric-facies scale. An outcrop reservoir model has been constructed by mapping rock-fabric facies and using average petrophysical values for each rock-fabric facies. Experimental waterflood simulations show that performance depends upon the stacking of the rock-fabric facies, the dense layers, and the location of production and injection wells. In the subsurface, high-frequency cycles can be observed in cores and calibrated with wireline log response. Grain and crystal size and sorting and separate-vug porosity can be determined from gamma-ray, porosity, acoustic, and resistivity logs. Permeability profiles can be calculated using rock-fabric-specific transforms between interparticle porosity and permeability. A reservoir model of part of the Seminole San Andres Field was constructed using these methods. Three-dimensional waterflood simulations using this model result in a more realistic display of remaining oil saturation than the traditional layered model and show the importance of thin, dense mud layers in controlling vertical migration.

Abstract San Andres Formation (Permian) outcrops in Last Chance Canyon are interpreted to contain two large-scale, fourth-order depositional sequences, upper San Andres sequence 3 and upper San Andres sequence 4. Embedded within each of these sequences are numerous higher frequency sequences. Reciprocal siliciclastic/carbonate sedimentation patterns enhance the recognition of cyclicity within both large-scale sequences and within many of the higher frequency sequences. Upper San Andres sequence 4 contains at least 12 smaller scale sequences that occur in landward-stepping, vertically stacked, stratigraphically rising seaward-stepping, and stratigraphically falling seaward-stepping geometric patterns. In outer-ramp/slope environments, landward-stepping, high-frequency sequences of the lowstand to transgressive systems tracts record a progressive decrease in sediment accumulation rate, depositional energy, and siliciclastic content. This reflects a long-term transition from siliciclasticdominated slope sedimentation to increasingly carbonate-dominated slope sedimentation. Thick outer-shelf carbonate strata, highly aggradational to mounded fusulinid-rich shelf-margin deposits, and pronounced stratigraphic rise of the fusulinid facies tract all reflect long-term increases in accommodation. Concurrent with the highly aggradational and moderately progradational mode of outer-shelf to slope carbonates, siliciclastic shelf-feeder systems are inferred to have stepped progressively landward. These contrasting reflections of increasing accommodation during lowstand to transgressive systems tract deposition accentuate the dissimilar nature of carbonate versus siliciclastic sediment supply. The most carbonate-rich and bioherm-bearing interval of the entire sequence overlies a distinctive maximum flooding surface capping the transgressive systems tract. Ensuing sea-ward-stepping high-frequency sequences of the middle to late highstand systems tract show pronounced progradational offlap and record a progressive increase in the volume of siliciclastics accumulated in the basin with a concomitant decrease in the volume of siliciclastics accumulated on the outer shelf. A karsted toplap surface represents a subaerial unconformity and sequence boundary capping upper San Andres sequence 4. Peritidal cycles within the overlying Grayburg Formation exhibit a significant seaward shift in coastal onlap across this sequence boundary. Depositional topography and time-significant surfaces within upper San Andres sequence 4 were traced across multiple geomorphic environments that extend from shelf to basin. As a result, the evolution of seismic-scale stratal geometries, depositional topography, facies associations, and volumetric proportions of carbonate and siliciclastic strata can be related with a high degree of temporal precision. By calibrating these changes to position within a stacking pattern of high-frequency sequences, we observe ordered, predictable changes in the volumetric partitioning of siliciclastics, grain-supported carbonates, and mud-supported carbonates. This volumetric partitioning is accompanied by differentiation of carbonate facies that accumulated in similar paleobathymetric settings, but in different positions of the large-scale stacking pattern. For example, siliciclastic-free, mud-supported, bioherm-prone slope carbonates attain their greatest seaward extent during the latest transgressive to earliest highstand systems tract period of long-term maximum shelfal accommodation. By contrast, grain-rich carbonate and mixed carbonate-siliciclastic facies predominate in lower-slope settings during periods of reduced shelfal accommodation such as the lowstand, early transgressive, and middle-late highstand systems tracts. If facies models are to become predictive at the scale of reservoirs, they must be calibrated to the systematically changing sediment supply and accommodation conditions that appear to drive volumetric partitioning and facies differentiation.

Abstract Depositional porosity and permeability distribution in Guadalupian-age grainstone-dominated parasequences of the central Guadalupe Mountains have been moderately to strongly modified by diagenesis. Early diagenetic features in the ramp crest and inner shelf crest can be related to inferred paleohydrology that developed during high-frequency (parasequence-scale) sea-level lowstands. In the San Andres example, leaching of carbonate grains and resulting precipitation of intergranular cement in a meteoric lens beneath an emergent bar crest produced a well-defined, highly porous but relatively low-permeability moldic zone in the thickest part of a grain-dominated parasequence. In the Grayburg example, preferential preservation of intergranular porosity and minor leaching along a paleo-water table produced a thin (less than 30 cm) stratiform zone of higher than average permeability. A zone of preferentially preserved intergranular porosity developed within an inferred meteoric lens beneath an emergent bar crest. Diagenetically influenced lateral and vertical changes in permeability distribution are predicted to be typical of intermittently exposed ramp crest and inner shelf crest environments. These examples demonstrate the efficacy of parasequence-scale mapping of depositional and diagenetic facies for improving prediction of permeability distribution in analogous strata.