Shale forms via compaction from particles in slow or quiet water, such as river deltas, lakes, swamps, or the ocean floor. Heavier particles sink and form sandstone and limestone , while clay and fine silt remain suspended in water. Over time, compressed sandstone and limestone become shale. Shale typically occurs in a broadsheet, several meters thick. Depending on the geography, lenticular formations may also form.
Sometimes animal tracks , fossils , or even imprints of raindrops are preserved in shale layers. The clay clasts or particles in shale are less than 0. The clay comes from decomposition of feldspar. Shale consists of at least 30 percent clay, with varying amounts of quartz , feldspar, carbonates, iron oxides, and organic matter.
Oil shale or bituminous also contains kerogen , a mixture of hydrocarbons from deceased plants and animals. Shale is classified based on its mineral content. There is siliceous shale silica , calcareous shale calcite or dolomite , limonitic or hematitic shale iron minerals , carbonaceous or bituminous shale carbon compounds , and phospatic shale phosphate. The color of shale depends on its composition. Shale with a higher organic carbon content tends to be darker in color and may be black or gray.
The presence of ferric iron compounds yields red, brown, or purple shale. Ferrous iron yields black, blue, and green shale. Shale containing a lot of calcite tends to be pale gray or yellow. The grain size and composition of minerals in shale determine its permeability, hardness, and plasticity. In general, shale is fissile and readily splits into layers parallel to the bedding plane, which is the plane of clay flake deposition. Shale is laminated , meaning the rock consists of many thin layers that are bound together.
Shale has many commercial uses. It is a source material in the ceramics industry to make brick, tile, and pottery. Shale used to make pottery and building materials requires little processing besides crushing and mixing with water. Crushing shale and heating it with limestone makes cement for the construction industry. Heat drives off water and breaks limestone into calcium oxide and carbon dioxide. Carbon dioxide is lost as a gas, leaving calcium oxide and clay, which harden when mixed with water and dried.
These deposits are known as conventional reservoirs since the fluid flows easily through the pores of the rock. Although drilling to extract oil and natural gas is possible, a large amount is still trapped inside the pore spaces of the shale.
This oil and gas is difficult to remove, and it takes unconventional methods to access this oil and gas. Pore spaces in shale are extremely tiny, and because of this gas cannot easily move out of the shale. However, some gas can be released through hydraulic fracturing , a method where water and chemicals are pumped down a well under pressure to fracture the shale.
In addition, horizontal drilling was found to help with the release of natural gas trapped in shale. Oil shale is rock that contains large amounts of kerogen. This type of shale forms when kerogen is buried at a depth of kilometers and can yield both liquid and gaseous hydrocarbons if the rock is heated or treated with solvents.
Because of this the extensive oil shale deposits around the world are not actively used. Fossil Fuels. Chambers Dictionary of Science and Technology. Chambers Harrap Publishers Ltd.
Rocks: Materials of the Lithosphere - Summary. Retrieved on Jenson Vanities James Martin Vanities. Design Services What's New? What is Limestone? What is Marble? What is Sandstone? What is Shale? What is Slate? What is Soapstone? What is Travertine? New Store. Click image to enlarge Sample of drill cuttings of shale while drilling an oil well in Louisiana. Limey shale overlaid by limestone, Cumberland Plateau, Tennessee.
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