Oil shale is a widespread sedimentary rock. It was formed in non-marine basins, some of which covered several hundred thousand square miles and others of which must be measured in acres. The rate of deposition of inorganic matter in oil-shale basins was slow and of extremely fine-grained material, whereas the growth of organic matter was rapid. A foot of oil shale usually represents several thousand years of deposition. Because winter and summer deposition differed, the oil shales may be considered varves and the annual bands can be counted. The inorganic matter, which is the major constituent of oil shales, may be clay minerals, volcanic ash, feldspars, silica, calcium or magnesium carbonates. The organic matter, which is solid and insoluble in organic solvents, is unrecognizable but is often attributed to algae and spore cases. Many oil shales contain recognizable organic matter such as fronds, stems, leaves, fish, shellfish and reptiles. Microscopically, the organic matter appears in micronsized blebs which are yellow, orange or reddish in color, though black organic matter usually is present. Most oil shales are black, therefore, though many rich oil shales are from light gray-brown to dark gray-brown in color, and the rich Mahogany Ledge of Colorado receives its name from its color and appearance. Polished, oil shale makes attractive jewelry. Oil shale usually is a strong, nonporous, impermeable rock of low specific gravity in which the bedding planes are tightly cemented (Fig. 1). However, some oil shales (usually those with a high moisture content) are weak and break readily along bedding planes. The major parts of most oil- shale deposits are flat-lying or gently dipping and appear to be free from serious dislocations such as faults (Fig. 2). Many oil searchers believe that oil shale is a source rock for petroleum, and hopes for oil discoveries in the Piceance, Uintah and Irati basins are encouraged by the existence of thousands of billions of tons of oil shale in those basins. Oil shale, however, contains no oil. To obtain oil from oil shale, the organic matter must be decomposed by heat in an oxygen-free atmosphere (the reaction begins at about 750F and is complete at 1,000F). The decomposition yields three products:
a high-carbon residue which remains with the rock;
noncondensable gas that is chiefly hydrogen and methane; and
a condensable gas that is condensed to shale oil.
The shale oil recovery may represent up to 70 per cent of the fuel value of the original organic matter (Fig. 3). Oil shales of commercial interest yield 5 to 15 weight per cent shale oil. Shale oil may be considered a protein or carbohydrate, in contrast to petroleum which is a hydrocarbon. The molecules of shale oil are thought to contain not only carbon and hydrogen, but also oxygen, nitrogen and sulfur. A high percentage of the molecules are unsaturated. The organic matter in the oil shale or the shale oil, itself, often is referred to as kerogen (which means wax-forming) because much wax can be crystallized from shale oils.
Some of the oil-shale deposits, such as the Chattanooga black shales of the United States and the Irati oil shale of southern Brazil, underlie more than 100,000 sq miles of area. Others, like the Swedish Kvarntorp deposit, the Spanish Puertollano deposit and the German Messel deposit, occur in basins only a few square miles in area.
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