Shale
Shale is a prevalent type of sedimentary rock found globally, characterized by its fine-grained composition primarily made up of silt and clay particles. It is the most common of sedimentary rocks, forming approximately 75% of this category throughout geologic history. Shale is defined by its tendency to break along distinct bedding planes, and variations include mudstone and claystone, depending on the presence and size of its constituent particles. This rock plays a crucial role in the hydrocarbon industry, serving as a significant source of crude oil and natural gas, particularly in regions like the Gulf of Mexico, which has rich organic deposits.
In addition to its geological significance, shale is widely used in various industries. It acts as a filler in construction materials and is a key ingredient in products such as cosmetics, toothpaste, and ceramics. Some specialized forms of shale, like slate, are utilized for roofing and paving. The unique properties of shale, including its low permeability, also make it an effective barrier to the migration of hydrocarbons, helping to retain these valuable resources underground. Overall, shale's diverse applications and abundance make it an important subject of study within geology and various industrial sectors.
Shale
Where Found
Shale is found throughout the world. It is the most common of the three principal types of sedimentary rock, that category of rock formed by consolidation of rock fragments or by chemical precipitation. In the geologic record, for every approximate five units of shale known, three units of sandstone and two units of limestone (the remaining two common categories of sedimentary rock) are also known.
Primary Uses
Shale is used as a filler in numerous construction materials. It is also used in everyday products such as cosmetics and toothpaste, and as an energy source.
Technical Definition
Shale is a fine-grained consolidated rock principally composed of silt-size (particles between 0.0039 and 0.0625 millimeter in diameter) and clay-size (less than 0.0039 millimeter in diameter) rock detritus. Shale is generally characterized by a tendency to break along well-defined bedding planes.
Description, Distribution, and Forms
The classification of shale is generally based on the presence or absence of well-defined bedding (lamination) planes. Fine-grained rock lacking this characteristic is termed mudstone, while a similar rock composed entirely of clay-size material is known as claystone. The ubiquity of shale is explained by its representing approximately 75 percent of all sedimentary rock produced throughout the entirety of geologic time.
Because of their fine-grained nature, shales cannot be conveniently examined mineralogically. Bulk chemistry and X-ray studies show, however, that the average shale is composed principally of the following oxides: silica (approximately 58 percent), aluminum (approximately 15 percent), iron (approximately 7 percent), and calcium, potassium, and carbon (each approximately 3 percent).
History
Shale rich in organic material deposited by the Mississippi River over the past several tens of millions of years caused the Gulf of Mexico to be one of the richest hydrocarbon provinces in the world. Throughout ten of the eastern United States and three western states, the Chattanooga Shale and the Green River Shale are identified as significant oil shale resources.
Obtaining Shale
Shale is a prime geologic source of crude oil and natural gas (hydrocarbon). Hydrocarbon originates from organic matter that accumulates in varieties of shale generally deposited under marine conditions. The preserved organic matter is converted to petroleum and natural gas by burial and related postdepositional changes through the passage of geologic time. The general lack of permeability of shale will later form a barrier to the upper subsurface migration (and thus to possible loss by surface evaporation) of generated hydrocarbon.
Uses of Shale
Kaolinite-rich shale supplies the basic material for a wide range of ceramic products, from pottery and fine porcelain to sewer pipe. Shale rich in barite is employed in the hydrocarbon industry to prevent oil and natural gas blowouts during the drilling of exploratory boreholes. Clay-rich shales are also employed in the cosmetics, insulator, printing ink, medicine, and toothpaste industries. The highly indurated form of shale known as slate is used in the construction industry as roofing and paving material. One major economic importance of shale is associated with the worldwide distribution of oil shale, a dark-colored rock containing 5 to more than 25 percent solid organic material, from which oil can be extracted by distillation. Shale rich in organic material also acts directly as a primary source of crude oil and natural gas.
Bibliography
Natural Resources Canada.
U.S. Geological Survey.