Reservoir Modeling of an Icehouse Giant: Horseshoe Atoll Complex, West Texas, U.S.A.

Kerans, Charles, F. Jerry Lucia, Hongliu Zeng, Fred Wang, Bureau of Economic Geology, Jackson School of Geosciences, University of Texas at Austin, Austin, TX

    The Horseshoe Atoll is an isolated carbonate platform with a cumulative production of 2.7 Bbbl from stratigraphically complex icehouse Late Carboniferous carbonate reservoir facies. The high OOIP, substantial bypassing of original pay caused by high degree of het­erogeneity, and the contained, isolated atoll setting, makes these reservoirs ideal targets for tertiary CO2 flooding.

In order to predict sweep efficiency and plan well patterns a detailed understanding of the reservoir architecture and permeability system was sought. The northern third of the SACROC and the entire Cogdell units operated by Kinder Morgan and Oxy Permian respec­tively were studied using cores, core analysis, wireline logs, and 3D seismic data. Integration of above data supplemented by an inversion-guided porosity model, proved essential in interpreting a robust sequence framework and quantifying this in terms of porosity and permeability.

Three Canyon-age sequences form the lower 50-70% of these units, formed principally by stratiform skeletal (Canyon 1 and 2 sequences) or oolitic (Canyon 3 sequence) cycles with tight flooding surfaces. The top-Canyon 3 sequence boundary was subaerially exposed to yield enhanced oomolidc porosity and karstic touching-vug porosity. Collapse and ero­sion of the margins of the platforms was common, creating up to 300 ft of local relief between wells. Drowning of the karst-modified Canyon platform tops in latest Carboniferous (Virgilian or Stephanian) was followed by deposition of steep-sided deep-water crinoidal mud mounds capping the platforms in a pinnacle style. Extreme heterogeneity in these units is best resolved by seismic mapping and inversion.