The Lower Cretaceous Muddy Sandstone forms a stratigraphic trap at Bell Creek field, Powder River County, Montana. The thin, lenticular sandstone has an average thickness of only 20 ft, but porosity values range up to 33 percent and permeability values up to 13,500 md. The sandstone is fine grained to very fine grained. Quartz content ranges from 50 to 99 percent and averages 86 percent. Minor constituents are detrital feldspar, 1.7 percent; matrix, 6.0 percent; rock fragments, 2.2 percent; mica, 1.7 percent; and calcite cement, 1.8 percent. Average grain size of quartz is 0.16 mm, and sorting is good. Generally, grain size increases upward, but in most sandstone beds the size decreases in the uppermost 5 ft. The basal few feet is silty and shaly, laminated, burrowed, and apparently gradational with underlying beds. Bedding Changes upward from indistinct and massive to laminated and thi with a few large- and small-scale cross-laminations. Some siltstone and shale are interbedded within and lateral to the sandstone body.

The Muddy Sandstone at Bell Creek appears to be a barrier bar on the basis of its mineral composition, vertical textural change, bedding, and gross morphology. The updip permeability barrier may be formed by lagoonal siltstone and shale that are adjacent to the bar on the southeast.

A plot of quartz grain size versus quartz content shows that grain size increases with increasing quartz percentage, as might be expected. The quartz parameters segregate according to environment, however, and size and percentage increase from lower shoreface and lagoonal, through middle shoreface, to beach and upper shoreface. These relations are an aid to identification of environments and will be useful in exploration.

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