Shale is a clastic or detrital sedimentary rock formed mainly by grains that are less than 0.002mm in size (clays) arranged in a laminated texture, mineralogically it is composed of clayey and micaceous minerals such as kaolinite, chlorite, montmorillonite, muscovite, illite, as well , quartz, feldspars and iron oxides.
The shale It is the sedimentary rock that has the largest extension on the earth’s surface, however, due to its composition and grain size it is very difficult to study it and identify its components with the naked eye and even under the microscope.
Properties and characteristics
Shale-forming minerals
The shales They are composed mainly of clay minerals and micas such as kaolinite, montmorillonite, illite, chlorite, smectite.
In addition, they contain quantities of quartz, potassium feldspars, and plagioclase. It is also common for them to have iron oxides (hematite, goethite and ferrihydrite).
Depending on where they have been deposited, they can have significant amounts of organic matter, carbonates (calcite) and sulfides such as pyrite.
Shale texture
The shales They are generated by very well sorted sediments, that is to say that the granulometry is mostly the size of clay (less than 0.002mm), where it will be rare to find larger grains.
Structure
The shale It is mainly associated with lamination and strata less than 10 cm thick that occurs at the time of sedimentation in the sedimentary basin.
However, there is a fissile texture in these rocks that is not always formed by sedimentation, it also occurs as a result of mechanical diagenetic compaction processes that occur due to high pressure at greater depths of coverage, leading to the destruction of the compact, sheet-like particle structure.
Lamination and fissility in both cases is favored by the high content of micaceous clay minerals.
It is also common to find cross laminations and traces of living beings that have lived in clayey sediments.
Sedimentary environment
The shales They need a “calm” sedimentary environment where there is no high influence of tides or river currents.
Therefore, the best sedimentation environment is the Lacustrine, where the small clay grains have time to gradually sink to the bottom.
In addition, they can form in lacustrine and deltaic coastal environments where there is no major influence of marine currents or waves.
Also small glasses shales They can occur in the sedimentation sectors of the rivers, that is, in the internal parts of the meanders.
Color
The color of the shales will strictly depend on its mineralogical composition.
It is so shales black and gray have high organic matter content and formed in an oxygen deficient environment.
The shales Red or reddish stones contain mainly iron oxides, such as hematite, goethite and ferrihydrite.
When it is brown or yellowish, it may be due to the content of ferrihidta or goethite.
Those of green color are formed by the content of smectite, glauconite and chlorite mainly.
Shale types
Oil shale
It is that rock that contains a high percentage of organic material in the form of kerogen (up to 1/3 of the rock in kerogen).
Large amounts of liquid and gaseous hydrocarbons could be extracted from this rock by heating the rock and treating it with some solvents.
However, at present it is not possible to adequately control the emissions and waste products that would be generated by the extraction of hydrocarbons from the oil shales.
Therefore, the large deposits of these rocks are not fully exploited.
Black and gray shale
They are those rocks that have at least 2% organic matter in their composition, even in the form of graphite.
This implies that the rock was formed in an environment deficient in oxygen, which also favors the generation of pyrite, which is a very common mineral in rocks. black shales.
This type of rock in a suitable geological environment can be the source rocks of liquid hydrocarbons and gases, especially when they have a percentage of organic matter greater than 2%.
When the color is gray it is because the percentage of organic matter is less than 2% and also because they may contain carbonates.
Red, brown and yellow shales.
This type of shales They develop in environments enriched in oxygen, the same that acts on iron oxide particles or iron hydroxide minerals such as hematite, goethite or ferrihydrite.
Thus, hematite is the mineral that produces reddish colors, while goethite produces brown colors and ferrihydrite generates yellow colors.
Green shale
It is related to the presence of chlorite, glauconite and smectite, which could have formed mainly in coastal lacustrine environments.
Shale Uses
- The shales Both due to their extension and their mineralogical composition, they have a variety of important uses in industry:
- In the conventional oil and natural gas industry, they are important because they can be the source rocks of hydrocarbons, especially when their organic matter content exceeds 2%.
- In the unconventional oil and natural gas industry, what is done is hydraulic fracturing (increasing the permeability) of the shales bituminous to obtain the hydrocarbons trapped in the almost impermeable rock.
- Clay products obtained by mixing water with shale ground, this process produces clay that is later used to create bricks, makeup, and other products.
- Crushed rock to be used as low quality road ballast.
- In the construction industry it is used at shales to generate cement that is later used in civil engineering works.