|
<br />CRAPTERTHREE
<br />AffectE;d Environment ,
<br />and siltstones of the Uinta Formation. These tongues, which are associated with lacustrine
<br />deposition, include (in stratigraphically descending order) the Coughs Creek, Black Sulfur
<br />(Black Sulphur), Thirteerunile Creek, Dry Fork, and Yellow Creek Tongues (Duncan et al. 1974,
<br />O'Sullivan 1975).
<br />Pazachute Creek Member The Parachute Creek Member is the uppermost unit of the Green
<br />River Formation. The Pazachute Creek Member contains virtually all of the oil shale, nahcolite,
<br />and dawsonite resources in the Piceance Creek Basin. The Parachute Creek Member exceeds
<br />2,000 feet in thickness in the depositional center of the basin. At the Piceance Site, the Parachute
<br />Creek Member is about 1,500 feet thick. The upper part of the Pazachute Creek ]Member is
<br />transitional with the Uinta Formation.
<br />The Parachute Creek Member is composed of mazlstone and lean to rich oil shale, some of which
<br />contains nahcolite, halite, and nahcolitic halite. These sedimentary rocks were predominantly
<br />chemically precipitated, which resulted in indistinct boundaries between the different rock types.
<br />The mineral composition of the Parachute Creek oil shale and mazlstones is similar, except that
<br />oil shale has a higher kerogen content. The general composition of these two rock types includes
<br />quartz, calcite, dolomite, analcime, potassium feldspaz, aalite, and minor concentrations of illite.
<br />Dawsonite maybe disseminated throughout certain stratigraphic horizons.
<br />The Parachute Creek Member is generally considered to comprise three distinct :tones referred to
<br />(in stratigraphically descending order) as the Mahogany, Leached, and Saline Zones.
<br />• Mahogany Zone -The Mahogany Zone is a rich oil shale interval that behaves as a leaky
<br />semi-confining layer. The Mahogany Zone is about 180 feet thick and is located between two
<br />thin layers of lean oil shale known as the A-Groove (above) and the B-Groove (below).
<br />Leached Zone -The stratigraphic interval between the Mahogany Zone and the Saline Zone
<br />is referred to as the Leached Zone, a badly degraded oil shale zone. Some documents refer to
<br />this interval as "the upper leached portion of the saline facies of the Saline Zone of the
<br />Pazachute Creek Member." The Leached Zone is usually discussed sepazately because all
<br />soluble saline minerals such as halite and nahcolite have been removed by thy: percolation of
<br />groundwater through this zone. The leached shale is badly fractured and contains numerous
<br />interstratified horizons of breccia, rubble, wgs, collapse intervals, and cavities.
<br />Saline Zone -The Saline Zone has significant concentrations of nahcolite, dawsonite, and
<br />halite. The Saline Zone is separated from the Leached Zone by swell-defined contact known
<br />as the Dissolution Surface. The depth to the top of the Dissolution Surface ranges from about
<br />1,400 to 1,800 feet bgs at the project site.
<br />Stratigraphic analyses of the Saline Zone have shown that various oil shale zones, along with
<br />nahcolite, dawsonite, and halite horizons, aze laterally continuous over long distances. One of
<br />the most widespread beds of microcrystalline nahcolite is the Love Bed, which wtderlies a
<br />minimum area of b0 squaze miles and ranges in thickness from 6.2 to 11.8 feet. Plahcolite in this
<br />layer averages about 60 percent by weight.
<br />Gazden Gulch Member, Douglas Creek Member, and Anvil Points Members The: Douglas Creek
<br />and Garden Gulch Members contain sandstone, siltstone, marlstone, algal limestone, and some
<br />lean, clay-rich oil shale. Clay-rich oil shale is abundant in the Garden Gulch Mernber. The
<br />3-4 Geology
<br />
|