| Sedimentary rock | |
 Shale | |
| Composition | |
|---|---|
| Clay minerals andquartz |
Shale is a fine-grained,clasticsedimentary rock formed frommud that is a mix of flakes ofclay minerals (hydrous aluminium phyllosilicates, e.g.,kaolin,Al2Si2O5(OH)4) and tiny fragments (silt-sized particles) of other minerals, especiallyquartz andcalcite.[1] Shale is characterized by its tendency to split into thin layers (laminae) less than one centimeter in thickness. This property is calledfissility.[1] Shale is the most common sedimentary rock.[2]
The termshale is sometimes applied more broadly, as essentially a synonym formudrock, rather than in the narrower sense of clay-rich fissile mudrock.[3]
Shale typically exhibits varying degrees of 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 indistinguishablebedding planes.[4] Non-fissilerocks of similar composition and particle size (less than 0.0625 mm) are described asmudstones (1/3 to 2/3 silt particles) orclaystones (less than 1/3 silt). Rocks with similar particle sizes but with less clay (greater than 2/3 silt) and therefore grittier aresiltstones.[4][5]
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 offerric oxide (hematite – reds),iron hydroxide (goethite – browns andlimonite – yellow), ormicaceous minerals (chlorite,biotite andillite – greens).[4] The color shifts from reddish to greenish asiron in theoxidized (ferric) state is converted to iron in the reduced (ferrous) state.[6] Black shale results from the presence of greater than one percentcarbonaceous material and indicates a reducing environment.[4] Pale blue to blue-green shales typically are rich incarbonate minerals.[7]
Clays are the major constituent of shales and other mudrocks. The clay minerals represented are largelykaolinite,montmorillonite and illite. Clay minerals of LateTertiary mudstones are expandablesmectites, whereas in older rocks (especially in mid-to earlyPaleozoic shales) illites predominate. The transformation of smectite to illite producessilica,sodium,calcium,magnesium, iron and water. These released elements formauthigenicquartz,chert,calcite,dolomite,ankerite, hematite andalbite, all trace to minor (except quartz) minerals found in shales and other mudrocks.[4] A typical shale is composed of about 58% clay minerals, 28% quartz, 6%feldspar, 5% carbonate minerals, and 2%iron oxides.[8] Most of the quartz isdetrital (part of the original sediments that formed the shale) rather than authigenic (crystallized within the shale after deposition).[9]
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 inanoxic conditions, contain reduced freecarbon along withferrous iron (Fe2+) andsulfur (S2−).Amorphousiron sulfide, along with carbon, produce the black coloration.[4] Because amorphous iron sulfide gradually converts topyrite, 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.[7]
Most shales are marine in origin,[10] and thegroundwater in shale formations is often highlysaline. There is evidence that shale acts as a semipermeable medium, allowing water to pass through while retaining dissolved salts.[11][12]
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 andlagoonal deposits, inriver deltas, onfloodplains and offshore below thewave base.[13] Thick deposits of shale are found near ancientcontinental margins[13] andforeland basins.[14] Some of the most widespread shale formations were deposited byepicontinental seas. Black shales[8] are common inCretaceous strata on the margins of theAtlantic Ocean, where they were deposited infault-bounded silled basins associated with the opening of the Atlantic during the breakup ofPangaea. 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.[15]
Most clay must be deposited as aggregates and floccules, since the settling rate of individual clay particles is extremely slow.[16]Flocculation is very rapid once the clay encounters highly saline sea water.[17] 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 calledsyneresis).[18] Clay pelletization by organisms thatfilter feed is important where flocculation is inhibited. Filter feeders produce an estimated 12 metric tons of clay pellets per square kilometer per year along theU.S. Gulf Coast.[19]
As sediments continue to accumulate, the older, more deeply buried sediments begin to undergodiagenesis. This mostly consists ofcompaction andlithification of the clay and silt particles.[20][21] Early stages of diagenesis, described aseogenesis, take place at shallow depths (a few tens of meters) and are characterized bybioturbation and mineralogical changes in the sediments, with only slight compaction.[22]Pyrite may be formed in anoxic mud at this stage of diagenesis.[8][23]
Deeper burial is accompanied bymesogenesis, 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 asclay mineral grains) are deformed, andpore space is reduced.[24] In addition to this physical compaction, chemical compaction may take place viapressure solution. Points of contact between grains are under the greatest strain, and the strained mineral is moresoluble than the rest of the grain. As a result, the contact points are dissolved away, allowing the grains to come into closer contact.[21]
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.[25] Fissility likely develops early in the compaction process, at relatively shallow depth, since fissility does not seem to vary with depth in thick formations.[26] 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 ofhydrocarbon molecules to the faces of the clay particles, which weakens the binding between particles.[27]
Lithification follows closely on compaction, as increased temperatures at depth hasten deposition ofcement 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 toillite at temperatures of about 55 to 200 °C (130 to 390 °F), releasing water in the process.[8] Other alteration reactions include the alteration of smectite tochlorite and ofkaolinite to illite at temperatures between 120 and 150 °C (250 and 300 °F).[8] Because of these reactions, illite composes 80% ofPrecambrian shales, versus about 25% of young shales.[28]
Unroofing of buried shale is accompanied bytelogenesis, the third and final stage of diagenesis.[22] As erosion reduces the depth of burial, renewed exposure tometeoric water produces additional changes to the shale, such as dissolution of some of the cement to producesecondary porosity. Pyrite may be oxidized to producegypsum.[21]
Black shales are dark, as a result of being especially rich inunoxidizedcarbon. Common in some Paleozoic andMesozoicstrata, black shales were deposited inanoxic, reducing environments, such as in stagnant water columns.[8] Some black shales contain abundant heavy metals such asmolybdenum,uranium,vanadium, andzinc.[8][29][30][31] The enriched values are of controversial origin, having been alternatively attributed to input fromhydrothermal fluids during or after sedimentation or to slow accumulation fromsea water over long periods of sedimentation.[30][32][33]
Fossils, animaltracks orburrows and evenraindrop impressions are sometimes preserved on shale bedding surfaces. Shales may also containconcretions consisting of pyrite,apatite, or various carbonate minerals.[34]
Shales that are subject to heat and pressure ofmetamorphism alter into a hard, fissile,metamorphic rock known asslate. With continued increase inmetamorphic grade the sequence isphyllite, thenschist and finallygneiss.[35]
Shale is the most commonsource rock for hydrocarbons (natural gas andpetroleum).[8] 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.[36]
The organic matter in shale is converted over time from the original proteins,polysaccharides,lipids, and other organic molecules tokerogen, which at the higher temperatures found at greater depths of burial is further converted tographite and petroleum.[37]
Before the mid-19th century, the termsslate, shale andschist were not sharply distinguished.[38] In the context of undergroundcoal mining, shale was frequently referred to as slate well into the 20th century.[39] Black shale associated with coal seams is called black metal.[40]
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