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Upper Minnelusa reservoir sandstones of Wolfcampian (Permian) age show a linear reduction of porosity with depth. However, porosity is not a simple function of compaction, but is related also to composition and maximum depth of burial.
Wolfcampian sandstones are fine-grained orthoquartzites that have calcite (and dolomite), anhydrite, and silica cements. From grain-packing studies in thin section, 3 stages of compaction are evident. The first stage is characterized by a linear decrease of effective porosity to 26% at 6,000 ft of maximum depth of burial, during which quartz grains and calcite clasts were compacted. The second stage is characterized by decrease in effective porosity to 10% or less by change of calcite clasts to calcite cement. The third stage of compaction is characterized by a linear decrease of intergranular porosity to 26% at 16,000 ft maximum burial during which quartz grains were compacted and calcite and anhydrite cements were dissolved and replaced by secondary silica. Intergranular porosity in ludes both voids and cement, as distinct from total and effective porosity.
It appears that permeability is greatly reduced at less than 10% effective porosity at a maximum burial of 12,000 ft and present drilling depths of 10,000 ft. Therefore, this depth may represent economic limits for Minnelusa exploration.
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