Combustible Shales

(oil shales), minerals of the group of solid caustobioliths which, with dry distillation, yield a large amount of a petroleum-like resin.

Combustible shales consist predominantly of mineral substances (calcites, dolomite, hydromicas, montmorillonite, kaolinite. feldspars, quartz, and pyrite) and some organic matter (kerogen). The latter constitutes 10–30 percent of the mass of the rock, and in the highest quality shales it reaches 50–70 percent. The organic part is biologically and geochemi-cally transformed matter from the simplest algae, in which the cellular structure may either be preserved (thallomoalgi-nite) or lost (colloalginite). The modified residues of higher plants may be present in admixture in the organic part (vitri-nite. fusinite and lipoidinite).

Combustible shales are classified as sapropelitic and humitosapropelitic according to the ratio between their algal and humic components. The first group is distinguished from the second by a higher hydrogen content (8–10 percent) and a lower content of humic acids (0.5 percent) in the organic portion. Sapropelitic combustible shales show a higher resin yield (20–30 percent) and have a greater combustion heat (to 14.6–16.7 megajoules per kg [3,500–4,000 kilocalories per kg]). These values are lower for humitosapropelitic combustible shales with an equivalent content of mineral impurities. The range of values for the combustible shales extracted and used in world practice is very broad.

Combustible shales may be marine, lagoon, or lacustrine sapropelites by origin; by composition they may be argillaceous, carbonaceous, or siliceous. Alternating with other paragenetically related sedimentary rocks, combustible shales form shale-bearing formations tens and hundreds of meters thick, which spread over areas as great as several thousand square kilometers.

Combustible shales are present in the deposits of all the geological systems of the Paleozoic, Mesozoic. and Cenozoic eras. In a structural-tectonic sense the deposits are primarily of the platform type; they are found in geosynclinal areas somewhat less often.

In 1970, 24.3 million tons of combustible shale were extracted in the USSR. Commercial extraction abroad is carried on in the People’s Republic of China. Spain, the Federal Republic of Germany, and Austria. According to rough calculations, total world reserves of combustible shale with between 10 and 65 percent organic matter come to 1,500 billion tons; this includes (in billions of tons) 370 in Africa. 500 in Asia, 90 in Australia, 120 in Europe, 220 in North America, and 180 in South America.

Total geological reserves of combustible shale in the USSR are estimated at 195.1 billion tons, including 16.6 billion tons of profitable reserves (1968). Chief deposits in the USSR are located in the European part: the Estonian and Leningrad beds (Ordovician), the Byelorussian (Devonian), the Pechora-Vychegda (Jurassic and Devonian), the Volga (Jurassic), and the Boltyshka in the Ukraine (Paleogene). In Middle Asia the Baisun and other Paleogene deposits are notable; in Kazakhstan there is the Kenderlyk (Carboniferous and Permian) deposit; and in Eastern Siberia there are the Olenek (Cambrian) and Transbaikal (early Mesozoic) deposits. The United States has major deposits of Paleogene shales in the Green River formation of Colorado, Utah, and Wyoming; in addition there are shales of various ages in Alaska and California. Other deposits of combustible shales are found in Canada, China. Brazil, Great Britain, Sweden, the Federal Republic of Germany. Spain. Austria. Yugoslavia, Egypt, Mali, and Somalia.

Combustible shales are industrially important as raw material for fuel and chemical energy. They are widely used for power engineering purposes (fuel, gas, fuel oil). Various chemical products are obtained from shale oil. including phenols, plasticizers. tanning agents, glues. Ichthyol. and preparations for combating soil erosion and weeds. Building materials, such as cement, are obtained from the waste products (ash). The large shale-processing combines and electric power stations in the Estonian SSR and the Leningrad and Kuibyshev oblasts are the centers of combustible shale consumption. In the United States combustible shales are viewed primarily as a potential raw material from which to obtain petroleum and gas by underground gasification, using the energy of nuclear explosions to create an enormous subsurface cavity filled with broken shale rock in order to make the gasification process easier. According to the data of the seventh World Energy Conference (1968), world resources of combustible shale oil are at least as great as the petroleum and gas reserves. For some developing countries that are poor in energy resources, such as Mali and Somalia, the problem of using combustible shales is very urgent.