Effects of Facies and Diagenesis on Reservoir Heterogeneity: Emma San Andres Field, West Texas

Pervasively dolomitized, anhydritic carbonates of the upper San Andres Formation in the Emma field of West Texas constitute an upward-shallowing sequence of lithofacies representing four major depositional environments. Open Platform fusulinid packstone/wackestone and burrowed wackestone form the base of the sequence. These deposits, which accumulated in a shallow-water, normal marine setting, contain moldic and intercrystalline porosity and constitute the lower of two major porosity intervals in the reservoir section. Overlying deposits consist of interbedded skeletal grainstone (Shoal) and burrowed mudstone and skeletal wackestone (Restricted Inner Platform). Skeletal grainstone, which accumulated in migrating sand shoals in a shallow-water, inner-platform setting, contains well-developed intergranular porosity and forms the upper, more permeable reservoir interval in the field. Associated, much less porous mudstone and wackestone were deposited between shoal areas in relatively quiet water. The top of the San Andres is characterized by supratidal pisolite grainstone, cryptalgal mudstone, and interbedded fine-grained siliciclastics that contain fenestral fabric, tepees, and desiccation structures indicative of deposition under conditions of frequent subaerial exposure.Lithification of the Emma San Andres reservoir resulted primarily from early, pervasive dolomitization. Two major episodes of replacement dolomitization occurred. Red-luminescent, matrix-replacive dolomite appears only in a siliciclastic-rich interval in the middle of the reservoir section. Petrographic and stable isotope data suggest that this earliest dolomite (Dolomite 1) formed contemporaneously with deposition from seawater having near-normal salinity during late San Andres time (middle Guadalupian). Alternatively, formation of this dolomite may have been associated with subaerial exposure during a major regional fall in relative sea level. Pervasive, brown-luminescent Dolomite 2 constitutes the bulk (> 90 percent) of the dolomite in the sequence. …Dolomite 2 formed from seawater brines evaporated to near gypsum saturation. Trace element and strontium isotope data imply that dolomitizing fluids had a downward component of flow through the section. Strontium isotopes also constrain the timing of this diagenetic event as late Guadalupian in age and indicate that the dolomitizing fluids were derived from late Guadalupian seawater. Trace element and strontium isotope compositions of dolomite reflect varying degrees of intraformational rock/water interaction between late Guadalupian seawater-derived brines and siliciclastic beds within the San AndresPetrographic relationships indicate that most anhydrite was emplaced after dolomitization of the section. Strontium and sulfur isotope data are consistent with this interpretation and indicate that sulfates precipitated from fluids having an origin similar to that of most dolomite. Minor leaching of sulfate locally increased reservoir porosity. Early lithification by stable replacement dolomite resulted in preservation and retention of most primary porosity. Thus, porosity distribution in the reservoir is controlled mainly by variations in original depositional texture and fabric. The upper, skeletal grainstone (Shoal) porosity interval contains significant reservoir porosity well off the axis of the field structure. Comparison of production data with facies maps suggests that 85 percent of the oil recovered to date from the reservoir has come from the upper porosity interval. Calculations indicate, however, that as much as 8 MMbbl of unrecovered mobile oil remains in this zone. The lower, fusulinid packstone/wackestone (Open Platform) porosity interval may contain an additional 7 MMbbl of recoverable mobile oil