3-D Reservoir and Stochastic Fracture Network Modeling for Enhanced Oil Recovery, Circle Ridge Phosphoria/Tensleep Reservoir, Wind River Reservation, Arapaho and Shoshone Tribes, Wyoming Page: 29 of 238
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and purple. The Crow Mountain is 64 m thick of purple, very fne-grained sandstone,
and gray to red siltstone (Anderson and O'Connell, 1993. The main depositional
environment for the Crow Mountain Member was probably a tidal flat complex (Picard,
1978). The upper Chugwater member, the Popo Agie, is dark red to purple on the base,
and weathers to a dark rust color or ochre color. The upper portion is a bright tan
siltstone with sharp brittle edges. It sounds like glass when walked upon. The difference
in color is probably from different amounts of iron oxides present. The Popo Agie
Member represents dominantly fluvial conditions, whereas the ochre-colored unit was
deposited in a lacustrine environment on a deltaic plain (Picard, 1978). At Circle Ridge,
the Popo Agie Member measures 52 m in thickness (Anderson and O'Connell, 1993).
1.1.2.6 Nugget Sandstone
Unconformably overlying the Popo Agie Member is the Lower Jurassic Nugget
Sandstone. This formation is only present along the northwest corner of Circle Ridge and
can be up to 15 m thick when present. The base of the Nugget is a fine- to very fine-
grained, tan to buff sandstone with interbedded red siltstone beds. Above the basal unit is
a red sandstone of the same texture. Large- and small-scale cross-bedding structures
were found that might indicate eolian deposition. Much controversy exists today
concerning the discontinuous surface exposure of the Jurassic Nugget Formation in the
Wind River Basin. Picard (1978) has suggested that regional and local tectonism may
have contributed to the observed thickness variations.
1.1.2.7 Gypsum Springs Formation
The lower and upper contacts of the Gypsum Springs are unconformities (Anderson and
O'Connell, 1993). The basal portion of the Gypsum Springs Formation at Circle Ridge is
a 29 m thick massive layer of gypsum (Anderson and O'Connell, 1993). The upper
portion contains a pink to red gypsiferous siltstone with interbedded shale measuring 32
m thick (Anderson and O'Connell, 1993). The gypsum weathers to a dark, puffy texture
similar to heads of broccoli. The abundance of gypsum with the interbedded siltstone
and shale beds would represent deposits that accumulated on the continental platform
with intervals of isolation from the main sea. Kinsman (1969) interpreted the
depositional environment as a semi-restricted basin with sabka and tidal mudflat
conditions.
1.1.2.8 Sundance Formation
The Sundance Formation and overlying Morrison Formation are the two Upper Jurassic
units at Circle Ridge. A distinctive strip of grass is seen at the contact between the
Sundance Formation and the Gypsum Springs Formation. At Circle Ridge, a series of
shale, sandstone, and limestone beds compose the 96 m thick Sundance Formation
(Anderson and O'Connell, 1993). Colors range from gray to light brown to green. The
fine-grained sands and shale can be highly glauconitic with sedimentary characteristics
that include thin cross-beds, ripples, and planar beds. An abundance of Gryphea was
found loosely atop the soils of the Sundance and cemented within beds. The combinationDE-FG26-OOBC 15190
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La Pointe, Paul; Hermanson, Jan; Parney, Robert; Eiben, Thorsten; Dunleavy, Mike; Steele, Ken et al. 3-D Reservoir and Stochastic Fracture Network Modeling for Enhanced Oil Recovery, Circle Ridge Phosphoria/Tensleep Reservoir, Wind River Reservation, Arapaho and Shoshone Tribes, Wyoming, report, November 18, 2002; United States. (https://digital.library.unt.edu/ark:/67531/metadc738026/m1/29/: accessed March 28, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.