The Upper Cretaceous Frontier Formation, a low-permeability gas reservoir in southwestern Wyoming, comprises marine and nonmarine fades deposited in a fluvial-deltaic depositional system. Reservoir quality of Frontier sandstones is determined in part by detrital clay content, which is related to original depositional environment, and in part by subsequent diagenetic modifications during burial. In this study, regional thickness and continuity of Frontier sandstones were mapped along the Moxa Arch from the La Barge Platform to the Utah border, using more than 700 well logs and cores from 10 wells. Many of the Frontier sandstones (First, Third, and Fourth Frontier intervals) are discontinuous and occur only at the northern end of the Moxa Arch where the formation is up to 1,500 ft (460 m) thick. The Second Frontier interval, however, is present along the entire Moxa Arch and contains the most prolific Frontier gas reservoirs. Clean sandstone is variably developed within the Second Frontier and commonly lies in laterally continuous marine shoreface fades and discontinuous fluvial channel-fill fades. Clean sandstone geometries are inherited from the depositional environment. Unstressed permeability to air averages 0.83 md in 47 upper-shoreface sandstones, 0.12 md in 232 lower-shoref ace sandstones, and 0.39 md in 120 fluvial channel-fill sandstones.

According to pétrographie examination of 151 thin sections, Frontier sandstones are fine- to medium-grained litharenites and sublitharenites having an average composition of 65% quartz, 4% feldspar, and 31% rock fragments. Clean sandstones contain an average of 1.8% primary intergranular porosity and 4.6% secondary porosity formed by dissolution of feldspar, chert, and mudstone clasts. Microporosity, estimated as the difference between porosimeter and thin-section porosity, averages 6%. Calcite, quartz, mixed-layer illite-smectite, illite, chlorite, and kaolinite are the most abundant cements. Authigenic mixed-layer clays consist of about 80% illite layers, suggesting that clays may be only moderately sensitive to fresh water. Low permeability in Frontier sandstones is caused by (1) loss of porosity due to compaction, (2) occlusion of pores by cements, particularly caldte and quartz, and (3) lining of primary pores by fibrous illite-smectite.

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