Shale is an abundant, fine-grained, clastic sedimentary rock. The rock forms when silt and clay-sized mineral particles compress. Its composition and properties place the rock in the “mudstones category.” It is distinguished from other mudstones owing to its fissility and lamination. A rock is said to be laminated when it is made up of numerous thin layers. Fissility refers to a rock’s ability to brake along lamination lines. In other words, the rock tends to split along flat planes of weakness.
Shale is usually gray to black. It is usually composed of clay minerals and quartz grains. However, additional elements in the rock often give it different colors. For example, ferric oxides impart red, brown, or orange color. The exact color due to iron oxide depends on the type of oxide in the rock. Hematite (ferric oxide) gives a red color, goethite (iron hydroxide) gives brown, and limonite imparts a yellow color. The presence of micaceous minerals gives the rock a green color. Black and blue colors are often due to organic and carbonate elements.
Shale and other mudstones are primarily composed of clay minerals. The major clay minerals include kaolinite, illite, and montmorillonite. A typical shale is made of clay minerals (58%), quartz (28%), feldspar (6%), carbonate minerals (5%), and iron oxides (2%).
A typical mudstone like shale is usually made up of 95% organic matter. The interesting fact about shale’s composition is that this percentage is usually less than 1% of its weight.
Most shales are formed in marine environments, with groundwater highly saline. Shale acts as a semipermeable membrane, allowing water to pass through and retaining dissolved stains.
Some shales are rich in unoxidized carbon. These are known as black shales, and some of them may also contain heavy metals like uranium, vanadium, zinc, and molybdenum. A few shales also contain fossils, animal tracks, burrows, or raindrop impressions.
Shales are usually deposited in slow-moving water, and their reserves are often found in lakes, lagoonal deposits, river deltas, floodplains, and offshore water resources below the wave base. The fine and small particles that compose shale remain suspended in the water body for a long, and the larger particles settle at the bottom.
The fresh sediments continue to accumulate over time, and the older and deeply buried sediments undergo diagenesis. Diagenesis is the process in which chemical and physical changes happen to rocks. This process consists of compaction and lithification of the clay and silt particles that are deeply buried. Diagenesis is mostly related to physical compaction. Chemical compaction also takes due to increased pressure and temperature on the deeply buried rocks.
Shale develops its fissility (tendency to brake) during diagenesis. The particles become strongly oriented into parallel layers. These parallel layers give shale its unique and distinct fiber.
Shale rocks that undergo metamorphism are altered into slate. The rock can be further converted to phyllite, then schist, and then gneiss if the metamorphism continues.
Hydraulic Properties of Shale
A rock’s hydraulic properties include permeability and porosity, the rock’s ability to hold or transmit fluids like water, oil, or natural gas. Shale has small interstitial spaces; it doesn’t allow movement easily. It often serves as a blockage for oil and natural gas and may block groundwater movement.
The interstitial spaces are small, but they still hold a lot of oil, gas, or water inside the rock. The drillers utilize horizontal drilling and hydraulic fracturing to increase the rock’s opening and permeability.
Where is Shale Found?
Shale reserves have been discovered in a few countries. The largest reserves are in the US, followed by Russia, Congo, Brazil, Italy, Morocco, Jordan, Canada, China, Pakistan, and Australia.
Shale is an economically precious rock. Its mining and processing are usually very costly, but the products are worth the investment.
Black shales (rich in unoxidized carbon) are an important source of rock for oil and natural gas. The organic material inside the rock was transformed into oil and gas when the rock was buried. This process may take up to hundreds of years. The oil and gas move upwards due to lower density and are trapped in porous overlying rocks like sandstone. These types of resources are known as conventional reservoirs.
Shale is used to produce clay. Clay is used in brick houses and roads and manufacturing pots. Many items made from natural clay years ago are now made with shale.
Shale is also used to make cement with limestone. The materials at heated at extremely high temperatures to evaporate water. The heat also brakes limestone into calcium oxide and carbon dioxide. The gas and water vapors evaporate, leaving powdered shale and calcium oxide behind.