--> --> Abstract: Geology and Shale-Oil Resources of White River-Gray Hills Area, Piceance Creek Basin, Northwestern Colorado, by George N. Pipiringos; #90969 (1977).

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Abstract: Geology and Shale-Oil Resources of White River-Gray Hills Area, Piceance Creek Basin, Northwestern Colorado

George N. Pipiringos

The White River-Gray Hills area lies in the northern part of the Piceance Creek basin in northwestern Colorado. The perennial White River flows west and northwest through the area. South of the White River, a conspicuous ridge stands as much as 1,500 ft (475 m) above the valley floor. North of the river, Colorow Mountain trends northeast and joins the northwest-trending Gray Hills in the eastern part of the area to form a T-shaped highland rising from 1,000 to 1,400 ft (335 to 427 m) above the surrounding lowlands. Exposed sedimentary rocks aggregate about 11,000 ft (3,353 m) in thickness.

The upper 3,600 ft (1,097 m) of the Williams Fork Formation of the Late Cretaceous Mesaverde Group is the oldest formation exposed. It consists of sandstone, siltstone, clay shale, carbonaceous shale, and coal. The Williams Fork in the White Rock quadrangle includes equivalents of the Lewis Shale, and the Lance Formation of nearby areas.

The Paleocene Fort Union Formation overlies the Williams Fork and comprises thick fluvial sandstone beds intercalated with a few thin beds of paludal shale, siltstone, and coal.

The Wasatch Formation overlies the Fort Union and consists of mainly fluvial, variegated and drab, sand/claystone, siltstone, and minor amounts of sandstone. The upper part of the Wasatch is Eocene and the lower part is Paleocene in age.

The overlying lacustrine Green River Formation, in the southwestern corner of the area, has an aggregate thickness of about 2,700 ft (640 m) and consists in ascending order of the Anvil Points, Garden Gulch, and Parachute Creek Members. The Anvil Points is predominantly sandstone; the Garden Gulch is predominantly kerogen-rich clay shale. The lower part of the Anvil Points Member may be of early Eocene age, but the rest of the formation is of middle Eocene age. The Parachute Creek Member consists almost entirely of kerogen-rich marlstone or barren marlstone.

The overlying middle Eocene Uinta Formation mainly is composed of tuffaceous, brown to yellowish-gray sandstone of dominantly lacustrine origin. The formation attains a maximum thickness of about 1,000 ft (305 m) near the southwest corner of the area, the top being the present surface of erosion.

A southwest-trending syncline underlies Colorow Mountain and divides the Midland Basin anticline on the north from the White River dome on the south. Both these anticlines are separated from the Danforth Hills anticline on the east by a northwest-trending syncline underlying the Gray Hills. The sinuous surface axis of the White River dome plunges northwestward and northeastward from a crest near the west-central part of the area. Maximum structural relief within the area is about 11,000 ft (3,353 m).

The southern and northern parts of the area are separated by a graben which trends about N55°W. It is bounded by steeply dipping normal faults having as much as 500 ft (152 m) of vertical displacement. A fault on the north side of the graben may account for some of the closure on the south flank of the White River dome.

The shale-oil resources, all in the Parachute Creek Member, are calculated only for zones averaging at least 20 gal of shale oil per ton and are contained within a stratigraphic sequence that averages about 420 ft (128 m) thick. In-place shale-oil resources in the Mahogany zone near the top of the Parachute Creek Member amount to about 2,614,000,000 bbl and those in the next underlying, relatively rich oil-shale zone (R-6) amount to about 1,923,000,000 bbl of shale oil.

AAPG Search and Discovery Article #90969©1977 AAPG-SEPM Rocky Mountain Sections Meeting, Denver, Colorado