Phylloid Algal and Sponge-Bryozoan Mound-To-Basin Transition: A Late Paleozoic Facies Tract from the Kelly-Snyder Field, West Texas
Richard A. Schatzinger, 1983. "Phylloid Algal and Sponge-Bryozoan Mound-To-Basin Transition: A Late Paleozoic Facies Tract from the Kelly-Snyder Field, West Texas", Carbonate Buildups-A Core Workshop, Paul M. Harris
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The Sacroc Unit of the Kelly-Snyder field, located on the eastern portion of the Pennsylvanian Horseshoe Atoll, northern Midland Basin, has produced over a billion barrels of hydrocarbons since 1948. Cross sections based on core descriptions and supplemented by electric log correlations allow reconstruction of the ancient facies tract across the middle of the field.
Updip dolomitic fenestral lime mudstone is interpreted as a tidal-flat deposit. Pellets, small intraclasts, and forams were concentrated by currents to form well-sorted grainstone bodies in tidal creeks. A wide phylloid algal zone interfingered with other proximal-shelf deposits just downdip of these tidal flat deposits. Phylloid algal mounds contained abundant Eugonophyllum, palaeotextulariid forams, Apterrinella, Bradyina, Globivalvulina, Tetraxis, Tubertina and unidentified tubular forams. Local binding and encrustation by blue-green algae and small forams was common in the phylloid algal mounds. Luxuriant algal mounds grew behind discontinuous, wave-fronting sponge-algal-bryozoan mounds. These latter mounds grew into waters as much as sixty feet deep along the mid-section of the Kelly-Snyder field. Finger-sized calcareous sponges were characteristic of the sponge-algal-bryozoan mounds but did not directly bind or stabilize the substrate. Binding by blue-green algae, fenestrate and massive bryozoans, and Tubiphytes was identified in thin sections by cathodoluminescence. Oolite shoals flanked and possibly overlapped shoaling portions of sponge-algal-bryozoan mounds in the northeastern part of the field. Sponge-algal-bryozoan mound-derived debris and ciasts accumulated as submarine debris flows basinward of the buildups and were interbedded with, basin-margin shaly lime muds.
Subaerial exposure and concomitant meteoric diagenesis left excellent secondary porosity. Oomoldic porosity locally exceeds 20%. Leaching of algal thaiIi and skeletal grains provided permeabilities of 10 to 25 md or more.