Core studies in the CO₂ pilot area of Shell's Denver unit in the Wasson San Andres field of west Texas revealed two basin end-member rock types: pelletal packstones, which exhibit high porosity and permeability due to the effective inter-pellet pore fabric; and moldic wackestones, which have lower porosity and significantly lower permeability, due to the disconnected fabric of the moldic pores. The majority of the rock section consists of a mixture of these end-member rock types due to cyclical variations in the degree of organic burrowing, which created the pellet packstones by reworking the original wackestone lithology. The high quality pellet packstones are dominant in rocks with greater than 15% porosity. The geologic model of the pilot area thus consists f numerous correlative high porosity zones composed dominantly of packstones, interbedded with poorer quality moldic wackestones.

An examination of cores from representative wells throughout the Denver unit also documented the occurrence of pelletal packstones, dominantly in rocks with greater than 15% porosity. By using digitized sonic logs from 688 wells in the Denver unit and the LOGPAK program, isopach mapping of packstone thickness in each of the correlative field-wide San Andres subzones was accomplished. Recognition of the detailed zonation of rock types from the pilot area in wells throughout the major part of the Denver unit permitted expansion of the pilot area model to the larger Denver unit model.

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