Abstract

Dolomites were studied in two oil fields in west Texas (North Riley and South Cowden) to evaluate factors affecting the distribution of porosity in dolomitized carbonate platforms. North Riley field is at the northern margin of the Central Basin platform, with its main reservoir in dolomites of the Leonardian (Permian) Clear Fork Formation. Dolomites in the Grayburg Formation (Guadalupian, Permian) were studied at South Cowden field on the east-central margin of the Central Basin platform. Replacive dolomitization in both fields is interpreted to have occurred in refluxing evaporatively concentrated seawater shortly after deposition. That interpretation is supported by stratigraphic, petrographic, and geochemical data and is similar to several previous studies of Permian dolomites in the vicinity. Porosity and permeability are lower in platform-interior dolomites and higher in dolomites near the platform margin in both fields, but porosity and permeability are not strictly facies dependent. The distribution of porosity and permeability is interpreted to be largely the result of location within the dolomitizing system. Saline dolomitizing brines generated by evaporative concentration of seawater in restricted lagoons and tidal flats percolated downward and flowed basinward (refluxed) through the sediments due to greater brine densities. Refluxing brines, supersaturated with respect to dolomite, precipitated large quantities of dolomite in proximal parts of the refluxing system, lowering dolomite saturation in waters flowing into distal portions of the reflux system. Lower dolomite saturations resulted in less dolomite precipitation and more porosity and permeability in distal parts of the system. After the original limestone was dolomitized, refluxing brines continued to circulate, precipitating more dolomite and decreasing porosity further, especially in platform-interior dolomites near where the refluxing brines were generated. Many platform-interior wells are structurally higher than wells near the platform margin, yet structurally lower wells near the platform margin are more porous and permeable, and have produced three to ten times more oil than the platform-interior wells.

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