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Stratigraphy and Lithofacies of the Almond Formation, Washakie and Great Divide Basins, Wyoming
The Almond Formation in the Washakie and Great Divide Basins varies from 250 to more than 500 ft in thickness. Both its upper contact with the Lewis Shale and its lower contact with the Ericson Sandstone stratigraphically rise to the west. Variations in formation thickness and in lithofacies are interpreted as being due, at least in part, to syndepositional movements along basement block faults.
Commonly, the Almond is informally divided into upper and main Almond. Upper Almond refers to the dominantly shoreface sandstone tongues that interfinger with the Lewis, and main refers to the underlying section of interbedded marine and nonmarine sandstones, siltstones, shales, and coals.
In the eastern parts of the basins, the Almond can be subdivided into three genetic units: (1) an upper unit, Unit 1, including a highly productive barrier-bar facies and a coal-bearing facies deposited dominantly in back-barrier environments; (2) a middle unit, Unit 2, consisting of several dominantly marine, coarsening-upward shale-to-sandstone sequences that contain little to no coal; and (3) a lower, fluvially dominated, coal-bearing unit, Unit 3. The upper unit is transgressive in origin and is generally correlative with the lower Almond of the Rock Springs uplift. The middle unit is bounded above and below by unconformities, pinches out to the west, and appears to correlate with a marine shale in the lowermost Almond on the Rawlins uplift. The lowermost unit may correlate to the east with the Pine Ridge Sandstone, but the extent of its disposition to the west is unclear.
The presence in the main Almond of numerous coals as well as reservoir-type sandstones, some of which are depositionally similar to the upper Almond sands, suggests that the main Almond may have significant gas potential.
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