DEPOSITIONAL ENVIRONMENTS AND DIAGENESIS OF THE EASTERN PORTION OF THE HORSESHOE ATOLL, WEST TEXAS

The sedimentary record of the eastern portion of the Horseshoe Atoll preserves a thick bank-to-basin transitional sequence deposited during Strawn through Wolfcamp time. Dip-oriented facies cross-sections through the Sacroc Unit of the Kelly-Snyder Field indicate a facies tract that represents the following sequence of depositional environments. Updip fenestral dolomicrite was deposited on a tidal flat. Currents concentrated pellets, small intraclasts, and foraminifera to form well-sorted grainstone bodies in tidal creeks. Downdip of the tidal-flat deposits a broad subtidal zone rich in phylloid algae. Discontinuous wave-fronting sponge-algal-bryozoan mounds grew in waters as much as sixty feet deep along the mid-section of the Kelly-Snyder Field. Finger-size calcareous sponges were characteristic of the sponge-algal-bryozoan mounds. Oolite shoals flanked and prograded updip portions of sponge-algal-bryozoan mounds in the northeastern part of the Kelly-Snyder Field. Clasts and debris derived from sponge-algal-bryozoan mounds accumulated as submarine debris-flow deposits basinward of the sponge and algal buildups and were interbedded with basin-margin shaly micrites.;Subaerial exposure and concomitant meteoric diagenesis created excellent secondary porosity. Porosity in the Horseshoe Atoll is dominantly secondary in origin and is facies-controlled. Most reservoir porosity was created by meteoric leaching of allochems in grainstone facies or leaching of fossils, particularly phylloid algae.;An abundance of porous zones on the structure repeatedly affected by subaerial exposure is evidence of a diagenetic overprint of facies control of porosity. Diagenetic features attributed to vadose diagenesis include solution fissures, microbrecciation, pedogenic-like textures, pendulous calcite cement, meniscus cement, and fine equant calcite cement. Neomorphism, leaching, and precipitation of medium-crystalline to coarse-crystalline calcite cement were the most striking products of meteoric diagenesis, particularly that originating in the phreatic zone.;Late-stage diagenesis resulted in continued precipitation of coarse-equant calcite cement, syntaxial calcite overgrowths, fractures, and stylolitization. A late-leaching phase associated with maturation and migration of organic material is present but volumetrically insignificant. Oxidation of late-stage sphalerite by meteoric waterflood waters may be responsible for rare occurrences of native sulfur in the reservoir. (Abstract shortened with permission of author.);Precipitation of bladed Mg-calcite and micrite cement and internal sedimentation causes reduction of primary porosity in marine environment. Early cementation prevented compaction of numerous grainstone facies and was the first step in creating the giant Kelly-Snyder reservoir.