Diagenetic Controls on Reservoir Properties of Low-Permeablity Sandstone, Frontier Formation, Moxa Arch, Southwest Wyoming

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The Upper Cretaceous Frontier Formation, a low-permeability gas reservoir along the Moxa Arch in southwest Wyoming, comprises marine and nonmarine facies deposited in a fluvial-deltaic depositional system. The Second Frontier interval is present along the entire Moxa Arch and contains the most prolific Frontier gas reservoirs. Clean sandstone in the Second Frontier commonly occurs in marine upper-shoreface facies and fluvial channel-fill facies.

According to petrographic examination of 199 thin sections, Frontier sandstones are fine- to medium-grained litharenites and sublitharenites having an average composition of 64 percent quartz, 6 percent feldspar, and 30 percent rock fragments. Clean sandstones contain an average of 1.6 percent primary intergranular porosity and 4.4 percent secondary porosity, which formed by dissolution of feldspar, chert, and mudstone clasts. Microporosity, estimated as the difference between porosimeter and thin-section porosity, averages 6 percent. Calcite, quartz, mixed-layer illite-smectite, and illite are the most abundant cements. Authigenic mixed-layer clays consist of about 80 percent illite layers, suggesting that clays may be only moderately sensitive to fresh water. On the basis of petrographic evidence, the relative order of occurrence of the major events in the diagenetic history of Frontier sandstones were (1) mechanical compaction by grain rearrangement and deformation of ductile grains, (2) formation of illite and mixed-layer illite-smectite rims, (3) precipitation of quartz overgrowths, (4) precipitation of calcite cement, (5) generation of secondary porosity by dissolution of calcite cement and detrital feldspar, chert, and mudstone, and (6) chemical compaction by intergranular pressure solution and stylolitization.

Low permeability in Frontier sandstones is caused by (1) loss of porosity due to compaction, (2) occlusion of pores by cements, particularly calcite and quartz, and (3) lining of primary pores by fibrous illite. Unstressed permeability to air averages 0.21 md in 56 upper-shoreface sandstones (porosity = 15 percent), 0.14 md in 121 fluvial channel-fill sandstones (porosity = 10 percent), and 0.08 md in 279 lower-shoreface sandstones (porosity = 12 percent).


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