View source for Shale

{{short description|Fine-grained, clastic sedimentary rock}}
{{Use American English|date=January 2022}}
{{Other uses}}
{{Infobox rock
|name=Shale
|type=Sedimentary
|image=Shale formation, Marble Canyon, Kootenay National Park, British Columbia, 2025-07-11.jpg
|caption=Shale
|composition=[[Clay minerals]] and [[quartz]]
}}'''Shale''' is a fine-grained, [[clastic]] [[sedimentary rock]] formed from [[mud]] that is a mix of flakes of [[clay mineral]]s (hydrous aluminium [[phyllosilicates]], e.g., [[Kaolinite|kaolin]], [[aluminium|Al]]<sub>2</sub>[[Silicon|Si]]<sub>2</sub>[[Oxygen|O]]<sub>5</sub>([[hydroxide|OH]])<sub>4</sub>) and tiny fragments ([[silt]]-sized particles) of other minerals, especially [[quartz]] and [[calcite]].<ref name="Blatt2">{{cite book |last1=Blatt |first1=Harvey |first2=Robert J. |last2=Tracy |date=1996 |title=Petrology: Igneous, Sedimentary and Metamorphic |edition=2nd |publisher=Freeman |pages=281–292 |isbn=0-7167-2438-3}}</ref>  Shale is characterized by its tendency to split into thin layers ([[Lamination (geology)|laminae]]) less than one centimeter in thickness. This property is called ''[[Fissility (geology)|fissility]]''.<ref name="Blatt2" /> Shale is the most common sedimentary rock.<ref name="RocksMaterials2">{{cite web |title=Rocks: Materials of the Lithosphere – Summary |url=http://wps.prenhall.com/esm_lutgens_foundations_3/0,6540,354318-,00.html |archive-url=https://web.archive.org/web/20141224031431/http://wps.prenhall.com/esm_lutgens_foundations_3/0,6540,354318-,00.html |archive-date=2014-12-24 |access-date=2007-07-31 |publisher=prenhall.com |url-status=dead }}</ref>

The term ''shale'' is sometimes applied more broadly, as essentially a synonym for [[mudrock]], rather than in the narrower sense of clay-rich fissile mudrock.<ref>{{cite book |last1=Boggs |first1=Sam |title=Principles of sedimentology and stratigraphy |date=2006 |publisher=[[Prentice Hall|Pearson Prentice Hall]] |isbn=0131547283 |edition=4th |location=[[Upper Saddle River, New Jersey|Upper Saddle River, N.J.]] |page=139}}</ref>

==Texture==
Shale typically exhibits varying degrees of [[Fissility (geology)|fissility]]. Because of the parallel orientation of clay mineral flakes in shale, it breaks into thin layers, often splintery and usually parallel to the otherwise indistinguishable [[bedding (geology)|bedding planes]].<ref name="Blatt">Blatt, Harvey and Robert J. Tracy (1996) ''Petrology: Igneous, Sedimentary and Metamorphic'', 2nd ed., Freeman, pp. 281–292 {{ISBN|0-7167-2438-3}}</ref> Non-fissile [[Rock (geology)|rocks]] of similar composition and particle size (less than 0.0625&nbsp;mm) are described as [[mudstone]]s (1/3 to 2/3 silt particles) or [[claystones]] (less than 1/3 silt). Rocks with similar particle sizes but with less clay (greater than 2/3 silt) and therefore grittier are [[siltstone]]s.<ref name=Blatt/><ref name=RocksMaterials>{{cite web
|url=http://wps.prenhall.com/esm_lutgens_foundations_3/0,6540,354318-,00.html
|title=Rocks: Materials of the Lithosphere – Summary
|publisher=prenhall.com
|access-date=2007-07-31
|archive-url=https://web.archive.org/web/20141224031431/http://wps.prenhall.com/esm_lutgens_foundations_3/0,6540,354318-,00.html
|archive-date=2014-12-24
|url-status=dead
}}</ref>
[[File:Drill cuttings - Annotated - 2004.jpg|thumb|right|Sample of [[drill cuttings]] of shale while drilling an [[oil well]] in [[Louisiana]], United States. Sand grain = 2 mm in diameter]]

==Composition and color==

[[File:Mudrock Colors.jpg|thumb|Color chart for shale based on [[oxidation state]] and organic carbon content]]
Shales are typically [[grey]] in color and are composed of clay minerals and quartz grains. The addition of variable amounts of minor constituents alters the color of the rock. Red, brown and green colors are indicative of [[ferric oxide]] ([[hematite]] – reds), [[iron hydroxide]] ([[goethite]] – browns and [[limonite]] – yellow), or [[micaceous]] minerals ([[Chlorite group|chlorite]], [[biotite]] and [[illite]] – greens).<ref name=Blatt/> The color shifts from reddish to greenish as [[iron]] in the [[Redox|oxidized]] ([[ferric]]) state is converted to iron in the reduced ([[ferrous]]) state.<ref>{{cite book |last1=Potter |first1=Paul Edwin |title=Sedimentology of shale : study guide and reference source |last2=Maynard |first2=J. Barry |last3=Pryor |first3=Wayne A. |date=1980 |publisher=[[Springer Science+Business Media|Springer-Verlag]] |isbn=0387904301 |location=[[New York (state)|New York]] |pages=54–56}}</ref> Black shale results from the presence of greater than one percent [[carbonaceous]] material and indicates a reducing environment.<ref name=Blatt/> Pale blue to blue-green shales typically are rich in [[carbonate mineral]]s.{{sfn|Potter|Maynard|Pryor|1980|p=56}}

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 (period)|Tertiary]] mudstones are expandable [[smectite]]s, 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 (mineral)|dolomite]], [[ankerite]], hematite and [[albite]], all trace to minor (except quartz) minerals found in shales and other mudrocks.<ref name=Blatt/> A typical shale is composed of about 58% clay minerals, 28% quartz, 6% [[feldspar]], 5% carbonate minerals, and 2% [[iron oxide]]s.<ref name=":0">{{Cite journal |last1=Ferriday |first1=Tim |last2=Montenari |first2=Michael |date=2016 |title=Chemostratigraphy and Chemofacies of Source Rock Analogues: A High-Resolution Analysis of Black Shale Successions from the Lower Silurian Formigoso Formation (Cantabrian Mountains, NW Spain) |url=https://www.sciencedirect.com/science/article/abs/pii/S2468517816300053 |journal=Stratigraphy & Timescales |volume=1 |pages=123–255 |doi=10.1016/bs.sats.2016.10.004 |via=[[Elsevier Science Direct]]|url-access=subscription }}</ref> Most of the quartz is [[detrital]] (part of the original sediments that formed the shale) rather than authigenic (crystallized within the shale after deposition).{{sfn|Potter|Maynard|Pryor|1980|pp=47-49}}

Shales and other 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 waters|anoxic]] conditions, contain reduced free [[carbon]] along with [[ferrous iron]] (Fe<sup>2+</sup>) and [[sulfur]] (S<sup>2−</sup>). [[Amorphous solid|Amorphous]] [[iron sulfide]], along with carbon, produce the black coloration.<ref name=Blatt/> Because amorphous iron sulfide gradually converts to [[pyrite]], which is not an important pigment, young shales may be quite dark from their iron sulfide content, in spite of a modest carbon content (less than 1%), while a black color in an ancient shale indicates a high carbon content.{{sfn|Potter|Maynard|Pryor|1980|p=56}}

Most shales are marine in origin,{{sfn|Potter|Maynard|Pryor|1980|p=72}} and the [[groundwater]] in shale formations is often highly [[Saline water|saline]]. There is evidence that shale acts as a semipermeable medium, allowing water to pass through while retaining dissolved salts.{{sfn|Potter|Maynard|Pryor|1980|p=59}}<ref>{{cite journal|last1=Berry |first1=F.A. |year=1960 |title=Geologic field evidence suggesting membrane properties of shales |journal=AAPG Bulletin |volume=44 |number=6 |pages=953–954 |url=http://archives.datapages.com/data/bulletns/1957-60/data/pg/0044/0006/0950/0953b.htm |access-date=13 April 2021}}</ref>

== Formation ==
The fine particles that compose shale can remain suspended in water long after the larger particles of sand have been deposited. As a result, shales are typically deposited in very slow moving water and are often found in lakes and [[lagoon]]al deposits, in [[river delta]]s, on [[floodplain]]s and offshore below the [[wave base]].{{sfn|Blatt|Tracy|1996|p=219}} Thick deposits of shale are found near ancient [[continental margin]]s{{sfn|Blatt|Tracy|1996|p=219}} and [[foreland basin]]s.<ref>{{cite book |last1=Fillmore |first1=Robert |title=Geological evolution of the Colorado Plateau of eastern Utah and western Colorado, including the San Juan River, Natural Bridges, Canyonlands, Arches, and the Book Cliffs |date=2010 |publisher=University of Utah Press |location=Salt Lake City |isbn=9781607810049 |page=222–223, 236–241}}</ref> Some of the most widespread shale formations were deposited by [[epicontinental sea]]s. Black shales<ref name=":0" /> are common in [[Cretaceous]] strata on the margins of the [[Atlantic Ocean]], where they were deposited in [[fault (geology)|fault]]-bounded silled basins associated with the opening of the Atlantic during the breakup of [[Pangaea]]. These basins were anoxic, in part because of restricted circulation in the narrow Atlantic, and in part because the very warm Cretaceous seas lacked the circulation of cold bottom water that oxygenates the deep oceans today.{{sfn|Blatt|Tracy|1996|pp=287-292}}

Most [[clay]] must be deposited as aggregates and floccules, since the settling rate of individual clay particles is extremely slow.{{sfn|Potter|Maynard|Pryor|1980|p=8}} [[Flocculation]] is very rapid once the clay encounters highly saline sea water.<ref>{{cite journal |last1=McCave |first1=I.N. |title=Vertical flux of particles in the ocean |journal=Deep Sea Research and Oceanographic Abstracts |date=1975 |volume=22 |issue=7 |pages=491–502 |doi=10.1016/0011-7471(75)90022-4|bibcode=1975DSRA...22..491M }}</ref> Whereas individual clay particles are less than 4 microns in size, the clumps of clay particles produced by flocculation vary in size from a few tens of microns to over 700 microns in diameter. The floccules start out water-rich, but much of the water is expelled from the floccules as the clay minerals bind more tightly together over time (a process called [[Syneresis (chemistry)|syneresis]]).{{sfn|Potter|Maynard|Pryor|1980|p=9}} Clay pelletization by organisms that [[Filter feeder|filter feed]] is important where flocculation is inhibited. Filter feeders produce an estimated 12 metric tons of clay pellets per square kilometer per year along the [[Gulf Coast of the United States|U.S. Gulf Coast]].{{sfn|Potter|Maynard|Pryor|1980|p=10}}

As sediments continue to accumulate, the older, more deeply buried sediments begin to undergo [[diagenesis]]. This mostly consists of [[compaction (geology)|compaction]] and [[lithification]] of the clay and silt particles.{{sfn|Blatt|Tracy|1996|pp=265-280}}{{sfn|Boggs|2006|pp=147-154}} Early stages of diagenesis, described as ''eogenesis'', take place at shallow depths (a few tens of meters) and are characterized by [[bioturbation]] and mineralogical changes in the sediments, with only slight compaction.<ref name="choquette-pray-1970">{{cite journal |last1=Choquette |first1=P.W. |last2=Pray |first2=L.C. |title=Geologic Nomenclature and Classification of Porosity in Sedimentary Carbonates |journal=AAPG Bulletin |date=1970 |volume=54 |doi=10.1306/5D25C98B-16C1-11D7-8645000102C1865D |bibcode=1970BAAPG..541865DP }}</ref> [[Pyrite]] may be formed in anoxic mud at this stage of diagenesis.<ref name=":0" />{{sfn|Boggs|2006|p=148}} 

Deeper burial is accompanied by ''mesogenesis'', during which most of the compaction and lithification takes place. As the sediments come under increasing pressure from overlying sediments, sediment grains move into more compact arrangements, ductile grains (such as [[clay mineral]] grains) are deformed, and [[pore space]] is reduced.<ref>{{Cite journal |last1=Richardson |first1=Ethan J. |last2=Montenari |first2=Michael |date=2020 |title=Assessing shale gas reservoir potential using multi-scaled SEM pore network characterizations and quantifications: The Ciñera-Matallana pull-apart basin, NW Spain |url=https://www.sciencedirect.com/science/article/abs/pii/S2468517820300010 |journal=Stratigraphy & Timescales |volume=5 |pages=677–755 |doi=10.1016/bs.sats.2020.07.001 |isbn=9780128209912 |s2cid=229217907 |via=Elsevier Science Direct|url-access=subscription }}</ref> In addition to this physical compaction, chemical compaction may take place via [[pressure solution]]. Points of contact between grains are under the greatest strain, and the strained mineral is more [[soluble]] than the rest of the grain. As a result, the contact points are dissolved away, allowing the grains to come into closer contact.{{sfn|Boggs|2006|pp=147-154}} 

It is during compaction that shale develops its fissility, likely through mechanical compaction of the original open framework of clay particles. The particles become strongly oriented into parallel layers that give the shale its distinctive fabric.<ref>{{cite journal |last1=Lash |first1=G. G. |last2=Blood |first2=D. R. |title=Origin of Shale Fabric by Mechanical Compaction of Flocculated Clay: Evidence from the Upper Devonian Rhinestreet Shale, Western New York, U.S.A. |journal=Journal of Sedimentary Research |date=1 January 2004 |volume=74 |issue=1 |pages=110–116 |doi=10.1306/060103740110|bibcode=2004JSedR..74..110L }}</ref> Fissility likely develops early in the compaction process, at relatively shallow depth, since fissility does not seem to vary with depth in thick formations.<ref>{{cite journal |last1=Sintubin |first1=Manuel |title=Clay fabrics in relation to the burial history of shales |journal=Sedimentology |date=1994 |volume=41 |issue=6 |pages=1161–1169 |doi=10.1111/j.1365-3091.1994.tb01447.x|bibcode=1994Sedim..41.1161S }}</ref> Kaolinite flakes have less tendency to align in parallel layers than other clays, so kaolinite-rich clay is more likely to form nonfissile mudstone than shale. On the other hand, black shales often have very pronounced fissility (''paper shales'') due to binding of [[hydrocarbon]] molecules to the faces of the clay particles, which weakens the binding between particles.<ref>{{cite book |last1=Blatt |first1=Harvey |last2=Middleton |first2=Gerard |last3=Murray |first3=Raymond |title=Origin of sedimentary rocks |date=1980 |publisher=Prentice-Hall |location=Englewood Cliffs, N.J. |isbn=0136427103 |edition=2d |pages=398–400}}</ref>

Lithification follows closely on compaction, as increased temperatures at depth hasten deposition of [[cement]] that binds the grains together. Pressure solution contributes to cementing, as the mineral dissolved from strained contact points is redeposited in the unstrained pore spaces. The clay minerals may be altered as well. For example, [[smectite]] is altered to [[illite]] at temperatures of about {{convert|55 to 200|C||sigfig=2}}, releasing water in the process.<ref name=":0" /> Other alteration reactions include the alteration of smectite to [[chlorite]] and of [[kaolinite]] to illite at temperatures between {{convert|120 and 150|C||sigfig=2}}.<ref name=":0" /> Because of these reactions, illite composes 80% of [[Precambrian]] shales, versus about 25% of young shales.{{sfn|Boggs|2006|pp=142, 145-154}}

Unroofing of buried shale is accompanied by ''telogenesis'', the third and final stage of diagenesis.<ref name="choquette-pray-1970"/> As erosion reduces the depth of burial, renewed exposure to [[meteoric water]] produces additional changes to the shale, such as dissolution of some of the cement to produce [[secondary porosity]]. Pyrite may be oxidized to produce [[gypsum]].{{sfn|Boggs|2006|pp=147-154}}

{{anchor|Black shale}}
'''''Black shales''''' are dark, as a result of being especially rich in [[oxidation|unoxidized]] [[carbon]]. Common in some Paleozoic and [[Mesozoic]] [[stratum|strata]], black shales were deposited in [[Hypoxia (environmental)|anoxic]], reducing environments, such as in stagnant water columns.<ref name=":0" /> Some black shales contain abundant heavy metals such as [[molybdenum]], [[uranium]], [[vanadium]], and [[zinc]].<ref name=":0" /><ref>R. Zangerl and E. S. Richardson (1963) ''The paleoecologic history of two Pennsylvanian shales'', Fieldiana Memoirs v. 4, Field Museum of Natural History, Chicago</ref><ref name="E.B. Tourtelot 1970, pp. 253-273">{{cite journal|author=J.D. Vine and E.B. Tourtelot|year= 1970|title= Geochemistry of black shale deposits – A summary report|journal= Economic Geology |volume=65|pages=253–273|doi=10.2113/gsecongeo.65.3.253|issue=3|bibcode= 1970EcGeo..65..253V}}</ref><ref>{{cite journal|author=R.M. Coveney|year= 1979|title= Zinc concentrations in mid-continent Pennsylvanian black shales of Missouri and Kansas|journal= Economic Geology |volume=74 |pages=131–140|doi=10.2113/gsecongeo.74.1.131}}</ref> 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.<ref name="E.B. Tourtelot 1970, pp. 253-273"/><ref>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</ref><ref>{{cite journal|author=H.D. Holland|year= 1979|title= Metals in black shales – A reassessment|journal= Economic Geology |volume=70 |pages=1676–1680|doi=10.2113/gsecongeo.74.7.1676|issue=7|bibcode= 1979EcGeo..74.1676H}}</ref>
<gallery>
File:Shale in Potokgraben.jpg|Shale in Potokgraben, the [[Karawanks]], Austria
File:MesselShaleSplitting.JPG|Splitting shale ([[Messel Formation|Messel oil shale]]) with a large knife to reveal fossils
File:Shale 8040.jpg|[[Weathering]] shale at a road cut in southeastern [[Kentucky]]
</gallery>
[[Fossil]]s, animal [[Fossil track|tracks]] or [[Burrow fossil|burrows]] and even [[raindrop impressions]] are sometimes preserved on shale bedding surfaces. Shales may also contain [[concretion]]s consisting of pyrite, [[apatite]], or various carbonate minerals.{{sfn|Potter|Maynard|Pryor|1980|pp=22-23}}

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 [[gneiss]].{{sfn|Potter|Maynard|Pryor|1980|p=14}}

==As hydrocarbon source rock==
Shale is the most common [[source rock]] for hydrocarbons ([[natural gas]] and [[petroleum]]).<ref name=":0" /> The lack of coarse sediments in most shale beds reflects the absence of strong currents in the waters of the depositional basin. These might have oxygenated the waters and destroyed organic matter before it could accumulate. The absence of carbonate rock in shale beds reflects the absence of organisms that might have secreted carbonate skeletons, also likely due to an anoxic environment. As a result, about 95% of organic matter in sedimentary rocks is found in shales and other mudrocks. Individual shale beds typically have an organic matter content of about 1%, but the richest source rocks may contain as much as 40% organic matter.{{sfn|Blatt|Middleton|Murray|1980|pp=396-397}}

The organic matter in shale is converted over time from the original proteins, [[polysaccharide]]s, [[lipid]]s, and other organic molecules to [[kerogen]], which at the higher temperatures found at greater depths of burial is further converted to [[graphite]] and petroleum.{{sfn|Blatt|Middleton|Murray|1980|pp=397}}

==Historical mining terminology==
Before the mid-19th century, the terms [[slate]], shale and [[schist]] were not sharply distinguished.<ref>R. W. Raymond (1881) "Slate" in [https://books.google.com/books?id=FpqFAAAAIAAJ&pg=PA78 ''A Glossary of Mining and Metallurigical Terms''], American Institute of Mining Engineers. p. 78.</ref>  In the context of underground [[coal mining]], shale was frequently referred to as slate well into the 20th century.<ref>Albert H. Fay (1920) "Slate" in [https://books.google.com/books?id=y6hCAAAAIAAJ&pg=PA622 ''A Glossary of the Mining and Mineral Industry''], United States Bureau of Mines. p. 622.</ref> Black shale associated with coal seams is called black metal.<ref>{{cite book |title= Dictionary geotechnical engineering: English German |last= Herbert |first= Bucksch |year= 1996 |publisher= [[Springer Science+Business Media|Springer]] |isbn= 978-3540581642 |page= 61 |url= https://books.google.com/books?id=te9FMHjOR4oC&pg=PA61}}</ref>

==See also==<!-- New links in alphabetical order please -->
{{Commons category|Shale}}
* {{annotated link|Bakken Formation}}
* {{annotated link|Barnett Shale}}
* {{annotated link|Bearpaw Formation}}
* {{annotated link|Burgess Shale}}
* {{annotated link|Emu Bay Shale}}
* {{annotated link|Marcellus Formation}}
* {{annotated link|Mazon Creek fossil beds}}
* {{annotated link|Oil shale}}
* {{annotated link|Shale gas}}
* {{annotated link|Wheeler Shale}}
* {{annotated link|Bringelly Shale}}
* {{annotated link|Ashfield Shale}}

==References==
{{reflist|40em}}

==External links==
{{commonscatinline}}
{{Rock type}}
[[Category:Shale| ]]
[[Category:Industrial minerals]]