Permian (Guadalupian) Reservoir Facies, Mc Elroy Field, West Texas
McElroy Field, situated on the eastern margin of the Central Basin Platform, produces from Upper Permian (Guadalupian) dolomites of the Grayburg Formation. Six lithofacies were recognized in the Grayburg during detailed core and wireline log studies. Stratigraphic relations indicate that prior to deposition of the Grayburg Formation a regression exposed the underlying carbonate platform. During subsequent transgression, open shelf deposits of the Grayburg formed and were overlain by shallow shelf deposits. The capping regressive sequence comprises shallow shelf-to-intertidal sediments of the upper Grayburg and supratidal sediments of the Queen Formation that prograded from west to east across the field.
Approximately 75 percent of the reservoir intervals in the field lie within dolomites of shallow shelf and shallow shelf-to-intertidal origin that contain secondary intercrystalline porosity. Carbonate sands form an important component in these lithofacies. Shallow shelf deposits are homogeneous, unstratified, bioturbated dolowackestones and dolopackstones. Vertical continuity within this zone is good, despite localized nonporous zones due to sulphate cementation. Shallow shelf-to-intertidal deposits are stratified and show abrupt lithological variation vertically and horizontally. Accompanying this variation are changes in porosity and permeability. Dolograinstones and dolopackstones comprising this facies are porous along the crest of the structure and nonporous on the flanks. Numerous laterally continuous barriers to vertical permeability are formed where porosities are cemented with sulphates, making correlation of pay zones and prediction of continuity between wells difficult.
Figures & Tables
Carbonate Sands-A Core Workshop
Carbonate sands, both skeletal and non-skeletal, have been studied by geologists as intensely as carbonate buildups. The underlying reason for the studies is the importance of those sands as significant hydrocarbon reservoirs. This core workshop is intended to provide a “hands on” look at the subsurface geologic record of carbonate sands with emphasis on lithofacies, stratigraphy of the sands and surrounding deposits, geometry of the sand deposits, diagenesis and porosity evolution, and wireline log data.