Petrographic analysis of samples from 8 Minnelusa cores from Campbell County, Wyoming, and Powder River County, Montana, reveals that high-permeability reservoirs (up to 3,200 md) are the result of extensive dissolution of early precipitated gypsum or anhydrite cement. The Minnelusa reservoirs are in eolian sandstones (dune and interdune facies) that are very fine to coarse-grained, moderately to bimodally sorted quartz-arenites, subarkoses, and sublitharenites. Dune and interdune sandstones exhibit differences in detrital mineralogy that are the result of postdepositional dissolution of labile grains.
The most common cements in the sandstone are anhydrite (0-30%), quartz overgrowths (0-10%), dolomite (0-10%), kaolinite (< 5%), and illite (< 1%). Most cementation occurred during the pre-Jurassic when the sandstones were buried less than 1,500 ft. The porosity network within the sandstone is a combination of primary and secondary porosity created by the dissolution of anhydrite cement. Burial history curves suggest that anhydrite dissolution occurred during the Late Jurassic to Early Cretaceous, when the top of the sandstones was still near the surface. During this time, 3 periods of uplift and erosion occurred in which meteoric waters undersaturated in calcium sulfate may have flowed through the sandstones. The distribution of the reservoirs is probably controlled by the regional structure during the periods of flushing.
Dune sandstones are the most productive facies in the high-permeability reservoirs. Porosity in the dune facies averages 21% compared with an average of 9% in the interdune facies. This difference is the result of both lower depositional porosity and greater quartz and dolomite cementation in the interdune sandstones. Porosity loss due to mechanical compaction is similar for both facies.