shale

(shāl)

[Probably from Middle English, shell, from Old English scealu.]

View more Science videos

For more information on shale, visit Britannica.com.

A class of fine-grained clastic sedimentary rocks with a mean grain size of less than 0.0625 mm (0.0025 in.), including siltstone, mudstone, and claystone. One-half to two-thirds of all sedimentary rocks are shales. See also Sedimentary rocks.

Shale is deposited as mud, that is, small particles of silt and clay. The particles are deposited when fluid turbulence caused by currents or waves is no longer adequate to counteract the force of gravity, or if the water evaporates or infiltrates into the ground. Clay particles often form larger aggregates which settle from suspension more rapidly than individual particles. Silt particles and clay aggregates are often deposited as thin layers less than 10 mm (0.4 in.) thick called laminae. See also Depositional systems and environments.

Mineralogically, most shales are made up of clay minerals, silt-sized quartz and feldspar grains, carbonate cements, accessory minerals such as pyrite and apatite, and amorphous material such as volcanic glass, iron and aluminum oxides, silica, and organic matter. The most common clay minerals in shales are smectite, illite, kaolinite, and chlorite. The type of clay particles deposited is dependent on the mineralogy, climate, and tectonics of the source area. See also Clay minerals; Chlorite; Illite; Kaolinite.

Shales are usually classified or described according to the amount of silt, the presence and type of lamination, mineralogy, chemical composition, and color. Variations in these properties are related to the type of environment in which the shale was deposited and to postdepositional changes caused by diagenesis and compaction. See also Diagenesis.

The small size of pores in shale relative to those in sandstone causes shale permeability to be much lower than sand permeability. Although fracturing due to compaction stresses or to tectonic movements can create deviations from this general trend, shales often form permeability barriers to fluid movement; this has important bearing on the occurrence of subsurface water and hydrocarbons. Ground-water aquifers are commonly confined by an underlying low-permeability shale bed or aquiclude, which prevents further downward movement of the water. Hydrocarbon reservoirs are often capped by low-permeability shale which forms an effective seal to prevent hydrocarbons from escaping. See also Aquifer.

A fine-grained sedimentary rock formed when layers of clay are compressed by the weight of overlying rocks. Shales have a layered structure and are easily split along the bedding planes.

Argillaceous sedimentary rock derived from clays or silts; typically thinly laminated and weak along planes; may be undesirable as a concrete aggregate.

A type of sedimentary rock found in the earth's crust composed chiefly of a combination of silt and clay. A great deal of the earth's usable fossil fuels are found in shale formations. There are many different types of shale, such as oil shale and bituminous shale, which are important sources of various grades of crude oil.

Investopedia Says:

For many years, the oil that was trapped in shale formations could not be extracted profitably. But modern technology now permits this type of extraction on a large scale, which has opened up huge deposits of previously unreachable reserves for oil companies. As oil prices rise, it can become cost effective for companies to retreive hydrocarbons from oil shale. Shale also has commercial value in the ceramics industry.

Related Links:
Before jumping into this hot sector, learn how these companies make their money. Oil And Gas Industry Primer
Find out how to stay on top of data reports that could cause volatility in these markets. Become An Oil And Gas Futures Detective
How a company accounts for its expenses affects how its net income and cash flow numbers are reported. Accounting For Differences In Oil And Gas Accounting
Hedge against rising energy prices and diversify your portfolio with these funds. ETFs Provide Easy Access To Energy Commodities
Gas prices are influenced by more than supply and demand. Find out what determines the price you pay at the pump. What Determines Gas Prices?
Find out how this commodity's fluctuating price affects more than just how much you pay at the pump. How Does Crude Oil Affect Gas Prices?
This organization's decisions can influence oil prices, but there is a limit to its power. Meet OPEC, Manager Of Oil Wealth

A sedimentary rock formed from layers of clay.

• Materials, Formations, and Substances - shale: consolidation of clay, mud, or silt into finely stratified rock structure of unaltered minerals
• Sedimentary rocks

For other uses, see Shale (disambiguation).

Shale

Sedimentary Rock
Shale
Composition
Clay minerals and quartz

Shale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments (silt-sized particles) of other minerals, especially quartz and calcite. The ratio of clay to other minerals is variable.^[1] Shale is characterized by breaks along thin laminae or parallel layering or bedding less than one centimeter in thickness, called fissility.^[1] Mudstones, on the other hand, are similar in composition but do not show the fissility.

Contents 1 Texture 2 Composition and color 3 Formation 4 See also 5 References

Texture

Shale typically exhibits varying degrees of fissility breaking into thin layers, often splintery and usually parallel to the otherwise indistinguishable bedding plane because of parallel orientation of clay mineral flakes.^[1] Non-fissile rocks of similar composition but made of particles smaller than 0.06 mm are described as mudstones (1/3 to 2/3 silt particles) or claystone (less than 1/3 silt). Rocks with similar particle sizes but with less clay (greater than 2/3 silt) and therefore grittier are siltstones.^[1] Shale is the most common sedimentary rock.^[2]

Sample of drill cuttings of shale while drilling an oil well in Louisiana. Sand grain = 2 mm. in dia.

Composition and color

Shales are typically composed of variable amounts of clay minerals and quartz grains and the typical color is gray. Addition of variable amounts of minor constituents alters the color of the rock. Black shale results from the presence of greater than one percent carbonaceous material and indicates a reducing environment.^[1] Black shale can also be referred to as black metal.^[3] Red, brown and green colors are indicative of ferric oxide (hematite - reds), iron hydroxide (goethite - browns and limonite - yellow), or micaceous minerals (chlorite, biotite and illite - greens).^[1]

Clays are the major constituent of shales and other mudrocks. The clay minerals represented are largely kaolinite, montmorillonite and illite. Clay minerals of Late Tertiary mudstones are expandable smectites whereas in older rocks especially in mid to early Paleozoic shales illites predominate. The transformation of smectite to illite produces silica, sodium, calcium, magnesium, iron and water. These released elements form authigenic quartz, chert, calcite, dolomite, ankerite, hematite and albite, all trace to minor (except quartz) minerals found in shales and other mudrocks.^[1]

Shales and mudrocks contain roughly 95 percent of the organic matter in all sedimentary rocks. However, this amounts to less than one percent by mass in an average shale. Black shales which form in anoxic conditions contain reduced free carbon along with ferrous iron (Fe²⁺) and sulfur (S^2-). Pyrite and amorphous iron sulfide along with carbon produce the black and purple coloration .^[1]

Formation

Limey shale overlaid by limestone, Cumberland Plateau, Tennessee

The process in the rock cycle which forms shale is compaction. The fine particles that compose shale can remain suspended in water long after the larger and denser particles of sand have deposited. Shales are typically deposited in very slow moving water and are often found in lakes and lagoonal deposits, in river deltas, on floodplains and offshore from beach sands. They can also be deposited on the continental shelf, in relatively deep, quiet water. This process could have taken millions of years to complete.

'Black shales' are dark, as a result of being especially rich in unoxidized carbon. Common in some Paleozoic and Mesozoic strata, black shales were deposited in anoxic, reducing environments, such as in stagnant water columns. Some black shales contain abundant heavy metals such as molybdenum, uranium, vanadium, and zinc.^[4][5][6] The enriched values are of controversial origin, having been alternatively attributed to input from hydrothermal fluids during or after sedimentation or to slow accumulation from sea water over long periods of sedimentation.^[5][7][8]

Splitting shale with a large knife to reveal fossils

Fossils, animal tracks/burrows and even raindrop impact craters are sometimes preserved on shale bedding surfaces. Shales may also contain concretions consisting of pyrite, apatite, or various carbonate minerals.

Shales that are subject to heat and pressure of metamorphism alter into a hard, fissile, metamorphic rock known as slate. With continued increase in metamorphic grade the sequence is phyllite, then schist and finally to gneiss.

Weathering shale at a road cut in southeastern Kentucky

See also

• Bituminous shale
• Oil shale
• Burgess shale
• Barnett Shale
• Bearpaw Shale
• Shale gas
• Wianamatta shale
• Wheeler Shale
• Slate

References

1. ^ ^{a b c d e f g h} Blatt, Harvey and Robert J. Tracy, 1996, Petrology: Igneous, Sedimentary and Metamorphic, 2nd ed., Freeman, pp. 281 - 292 ISBN 0-7167-2438-3
2. ^ "Rocks: Materials of the Lithosphere - Summary". http://wps.prenhall.com/esm_lutgens_foundations_3/0,6540,354318-,00.html. Retrieved 2007-07-31. 
3. ^ Herbert, Bucksch (1996). Dictionary geotechnical engineering: English German. Springer. p. 61. ISBN 978-3540581642. http://books.google.no/books?id=te9FMHjOR4oC&pg=PA61&lpg=PA61&dq=%22black+metal%22+%22black+gold%22+%22black+silver+ore%22&source=bl&ots=j2tbrjIj5y&sig=3Ki5iaISVKZf4ImMzwYfYW7LpSE&hl=nn&ei=XidQTI6sNN2XOIz6ib4B&sa=X&oi=book_result&ct=result&resnum=1&ved=0CBQQ6AEwAA#v=onepage&q=%22black%20metal%22&f=false. Retrieved July 28, 2010. 
4. ^ R. Zangerl and E.S. Richardson,1963, The paleoecologic history of two Pennsylvanian shales, Fieldiana Memoirs v 4, Field Museum of Natural History, Chicago, 352 p.
5. ^ ^{a b} J.D. Vine and E.B. Tourtelot, 1970, Geochemistry of black shale deposits - A summary report: Economic Geology 65 pp. 253-273
6. ^ R.M. Coveney, 1979, Zinc concentrations in mid-continent Pennsylvanian black shales of Missouri and Kansas, Economic Geology 74 pp. 131-140.
7. ^ R.M. Coveney, 2003, Metalliferous Paleozoic black shales and associated strata: in D.R. Lenz ed., Geochemistry of Sediments and Sedimentary Rocks, Geotext 4, Geological Association of Canada pp. 135-144
8. ^ H.D. Holland, 1979, Metals in black shales - A reassessment, Economic Geology 70 pp. 1676-1680

This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)

Dansk (Danish)
n. - skiferler, skiffer

Nederlands (Dutch)
(klei)schalie

Français (French)
n. - schiste, shale

Deutsch (German)
n. - Schiefer

Ελληνική (Greek)
n. - (γεωλ.) αργιλικός σχιστόλιθος

Italiano (Italian)
scisto

Português (Portuguese)
n. - xisto (m)

Русский (Russian)
глинистый сланец

Español (Spanish)
n. - esquisto, pizarra

Svenska (Swedish)
n. - skifferlera, skiffer

中文（简体）(Chinese (Simplified))
页岩, 泥板岩

中文（繁體）(Chinese (Traditional))
n. - 葉岩, 泥板岩

한국어 (Korean)
n. - 혈암, 셰일, 이판암

日本語 (Japanese)
n. - 頁岩, 泥板岩

العربيه (Arabic)
‏(الاسم) ألزيت ألحجري‏

עברית (Hebrew)
n. - ‮צפחה, אבן פצלתית‬

If you are unable to view some languages clearly, click here.

To select your translation preferences click here.