Oil shale is a sedimentary rock containing solid and combustible organic matter (kerogen). Oil shale can be burned as any other fossil fuel or it can be heated without oxygen to extract shale oil.
Oil shale (variery Kukersite) from Estonia is very rich in fossils (bryozoans, trilobites, brachiopods). The light-colored and very well-preserved fossils on this rock sample are bryozoans. The rock is of Ordovician age.
The reserves of oil shale are enormous (nearly 1000 times more energy is stored in oil shales than in coal deposits2) but so far it has found little use because better alternatives like coal, crude oil, and natural gas are available. The main problem with oil shale is purity. Most oil shales contain much more mineral content than organic matter and usually unpleasant environmental problems are associated with its use.
“Oil shale”, although used as a lithologic term, is actually an economic term referring to the rock’s ability to yield oil (shale oil). Such rocks often carry other names like black shale, bituminous shale, carbonaceous shale, torbanite, tasmanite, kukersite, kerogen shale, etc2.
Oil shale is a lacustrine (related to lakes) or marine sedimentary rock. It is a shale (lithified mud) that contains organic matter. Many sedimentary rocks contain organic matter but in oil shale it forms a significant part of the rock. So significant that oil shale is known as a “rock that burns”. The organic matter is of algal or bacterial origin. So it is different from the organic matter which makes up coal (plant remains in most cases, instead of microorganisms). Organic matter accumulates on the bottom of sea or lake only if the conditions are strongly reducing (without free oxygen). Otherwise it would decompose before burial. Such reducing conditions exist in strongly stratified water bodys. Black Sea is a good modern example of a sea with anoxic conditions in deep water. Present day oil shale formation is taking place in Australian saline lakes where algae accumulates to form a sapropelic peat known as coorongite3.
Oil shales are usually either black or various shades of brown. Oil shales typically contain silicate minerals (quartz, clay minerals) and pyrite. Some oil shales are closely associated with carbonate rocks.
Kukersite layers in Estonia occur between limestone layers. Some oil shales (black shales) contain appreciable amount of metals. The precipitation of metallic compounds (especially sulfide minerals) is enhanced by reducing conditions. Dictyonema shale (alum shale) described below is a good example of such oil shale. Uranium, vanadium, antimony, molybdenum, silver, gold, nickel, cadmium, selenium, and zinc are common metals in black shales2.
Oil shale is a source rock of crude oil and natural gas. These more volatile hydrocarbons migrate upward from oil shale deposits when it gets buried deep enough for the temperature to rise so much that kerogen starts to decompose. So oil shale is a rock that has not yet matured enough to yield crude oil. Tar or oil sand may be confused with oil shale but they are actually different things. Tar sand is a sandstone that contains natural asphalt (sticky hydrocarbons, heavy fraction of crude oil) from which the lighter fraction of crude oil have escaped. Tar sands, unlike oil shales, are not the original source rocks of crude oil.
Most of the following examples are from Estonia — country where oil shale has been mined for almost 100 years. There are two varieties of oil shale in Estonia — kukersite and dictyonema argillite. Both were formed during the Ordovician period. Kukersite has been mined from 1919 and it is by far the most extensively mined type of oil shale in the world. Kukersite is used as a fossil fuel in power plants and as an industrial source of shale oil. Dictyonema argillite (alum shale) is poorer in organic matter but has been mined because of metals content.
Another type of oil shale from Estonia which is currently not mined. This rock type is known as dictyonema argillite, alum shale, graptolite argillite, etc. It is a potentially promising mineral resource but not because of its organic content which is pretty low. Economically more interesting aspect is the metal content of the rock. It contains vanadium, molybdenum, uranium, rhenium, and many others. It was mined from 1946-1952 as a uranium ore and some sources claim that there is a possibility that the uranium extracted from this rock was used to make the first Soviet nuclear bomb1.
This fact is difficult to confirm but the production started very shortly after Soviets invaded Estonia and Soviet Union at that time had no other sources of uranium although they were desperate to find it. Everything associated with uranium production was of course top secret and no Estonians were allowed to work at the plant built for uranium production. It was also absolutely forbidden to use the word “uranium” in official documents. I find it amusing that they invented code A-9 for uranium and then used it in the following chemical formula: (A-9)3O8. It does not take a genius to figure out what is behind this A-9.
Local shale proved to be inferior source of uranium and in 1952 it was replaced with ores imported from Eastern Europe. This industrial plant is still in operation and nowadays it is one of the few places out of China where rare earth metals are produced.
Pictures of oil shale
An outcrop of dictyonema shale in Estonia. The reserves of this infamous rock type in Estonia are huge but it has no uses at the moment.
Oil shale quarry in NE Estonia. Oil shale variety kukersite (on the first picture above) is mined from this quarry.
Bryozoan fossil in kukersite. The width of the sample is 5 cm.
Large excavator at work in Estonia in an open-pit quarry of kukersite.
Cross section of a drill core of kukersite with a diameter of 9 cm.
Estonia has no crude oil but oil (shale oil) can be extracted from oil shale (kukersite) by heating it without oxygen. This process is known as retorting. The residue of retorting is this black stuff known as semi-coke.
Pebbles of dictyonema shale on the coast.
Shungite is a metamorphic rock consisting of carbon. It is believed by some scientists that shungite is a metamorphosed oil shale. The width of the sample from Karelia is 7 cm.
1. Maalmann, I. Historical Survey of Nuclear Non-proliferation in Estonia. (PDF in Estonian).
2. Smith, John W. & Jensen, Howard, B. (2007). Oil shale. In: McGraw Hill Encyclopedia of Science & Technology, 10th Edition. McGraw-Hill. Volume 12. 330-335.
3. Robb, L. (2005). Introduction to Ore-Forming Processes. Blackwell Science Ltd.