OIL SHALE 



499 



and other saline minerals, as well as thin zones of 

 uraniferous phosphate, but the oil-shale beds in this 

 sequence are so thin that they rarely constitute a 

 resource where the trona and halite predominate. In 

 the Washakie basin the richest oil shale is in the 

 Laney Member of the Green River Formation (Roeh- 

 ler, 1969). The Wilkins Peak Member here is free of 

 saline minerals and has been replaced in part by 

 fluvial beds of the Wasatch Formation. The Tipton 

 Shale Member is widespread, but available data indi- 

 cate that its oil shale is lean. 



In the Great Divide and Sand Wash basins, the 

 Green River Formation is similar to that of the 

 Washakie basin, but the oil shales are generally low 

 grade. In the Fossil basin the Green River Forma- 

 tion was presumably deposited from a small lake 

 separate from the main Green River lakes. The oil- 

 shale sequence is thin and mostly low grade. 



The oil shale of the Green River Formation is a 

 dense, tough marlstone that is generally well lami- 

 nated and ranges in color from tan to dark brown or 

 olive-brown to brownish black. According to Bradley 

 (1970, p. 985), the organic fraction is derived from 

 "microscopic algae, and other microorganisms, that 

 grew and accumulated in the central parts of large, 

 shallow lakes that existed under a sub-tropical cli- 

 mate," and from "wind-blown, or water borne, pol- 

 lens and waxy spores." The inorganic fraction con- 

 sists predominantly of dolomite and calcite with 

 varying amounts of quartz, feldspar, analcime, clay, 

 and pyrite. The mineralogy and the variation in min- 

 eral composition of the oil shale in the Piceance 

 Creek basin, Colorado, are described in detail by 

 Brobst and Tucker (1973). 



The sites of deposition of the oil shale were very 

 flat. The lakes were large in area and the relatively 

 shallow water ranged from fresh to highly alkaline. 

 The surrounding highlands contributed clastic sedi- 

 ment to the margins of the lake, but during most of 

 their history little or none of the clastic material was 

 carried into the central part of the lakes. Conditions 

 were such that algae grew abundantly on the surface 

 of the lake, and their remains were preserved in the 

 bottom sediments. 



The basins sank slowly, and at irregular rates, 

 causing the centers of deposition of oil shale to shift 

 during Eocene time. In general, the areas were tec- 

 tonically quiescent during deposition and for millions 

 of years after the sediments were buried. Subse- 

 quently, uplift caused local tilting and erosion of 

 these beds. 



MARINE BLACK SHALE DEPOSITS 



Black shale deposits of marine origin which under- 



lie large areas in most of the United States consist 

 of two main types. The platform-type marine black 

 shales were formed in shallow seas on continental 

 platforms where circulation of water near the sea 

 floor was restricted. Characteristically, the platform 

 shales may be several hundred feet thick, but the oil- 

 shale zones are thin and persistent, ranging in thick- 

 ness from a few feet to a few tens of feet (Duncan, 

 1967, p. 661) . Although few of them have a high oil 

 yield, they underlie such large areas throughout the 

 world that their resources of shale oil are larger than 

 that of any other type of deposit. Geosynclinal ma- 

 rine shales were formed in subsiding geosynclinal 

 basins, and are associated with limestone, phosphate 

 rock, and chert in assemblages much thicker than the 

 platform tj^e. Some deposits of this type, such as 

 Miocene shales in California, yield significant 

 amounts of oil. 



The organic matter, which gives marine black 

 shales their characteristic color, contains relatively 

 little hydrogen, and therefore only a small percent 

 of the organic matter converts to oil. According to 

 Swanson (1960, p. 28), the organic matter is of two 

 types: sapropelic (derived from algae, spore, pollen, 

 etc.) and humic (derived from cellulose, lignin, and 

 analogous woody parts) . The inorganic fraction pre- 

 dominantly consists of quartz and clay minerals, al- 

 though some shales contain carbonate. Many shales 

 also contain small amounts of uranium, vanadium, 

 and other metals, and phosphate. 



A well-known example of the platform-type shale 

 is the marine black shale of Late Devonian and Early 

 Mississippian age that underlies more than 250,000 

 square miles in Eastern and Central United States. 

 It is known principally as the Chattanooga Shale in 

 the Central United States, but the names New Al- 

 bany, Ohio, Antrim, Mountain Glen, and Woodford 

 Shales refer to approximate stratigraphic equiva- 

 lents elsewhere of the Chattanooga Shale. The black 

 shale generally ranges from to 100 feet in thick- 

 ness, its average oil yield is about 5 gallons per ton 

 of shale, and its average uranium content is 0.003 

 percent (Swanson, 1960, p. 7) . 



SHALES ASSOCIATED WITH COAL BEDS 



Carbonaceous shales associated with coal beds are 

 of two types: thin persistent beds deposited in a 

 shallow marine environment and beds deposited in a 

 swampy, nonmarine environment that are limited in 

 extent and exhibit considerable lateral variation in 

 organic content. In the Appalachian coal province, 

 the Pennsylvanian rocks contain mostly nonmarine 

 carbonaceous shales that have been reported to yield 

 from 1 to 100 gallons of oil per ton, but most yield 



