What is a Salt Dome?
A salt dome is a geological formation deep under the surface of the Earth. It is a vertical column of salt which protrudes upwards into the layer of sediments called the caprock. Salt formations are typically horizontal, but when in a sedimentary basin where a thick layer of salt is covered by a layer of sediments of heavy mineral, salt formations can rise vertically.
How is Salt Domes Formed?
A phenomenon known as diapirism takes place beneath the surface of the Earth. It allows lighter material to force its way upwards through denser material. In the case of a salt dome, salt has a lower density compared to the rocks above it. When a gravitational force, tectonic force, and a combination of various other forces act upon salt, this results in the deformation of salt. As a result, the salt flows upwards like a viscous liquid.
For the formation of a salt dome, the pressure on the salt must be high enough to allow the salt to penetrate through the layer of sediment above it. Once the salt begins to flow, the process will continue as long as the pressure on the salt is more than the forces acting against it, but once the forces are in equilibrium, the flow of salt will stop.
When conditions are favorable, salt domes can grow thousands of feet above their core. They can also reach the surface of the Earth and result in the formation of salt glaciers. The formation of salt domes does not occur overnight. It is a long process which can take centuries. The process begins in an isolated marine inlet. When the water evaporates, it leaves concentrated salt. Geologists believe that the evaporation process should occur multiple times. This will help achieve the required concentration of salt essential for creating a salt dome.
Once the large deposits of salt are created, wind and other external forces will come into play. Over time, sediments are deposited on the salt. Once the sediments have completely covered the salt formation, the forces acting on the salt will become stronger. Since salt is a mineral with lighter density, it will find its way upwards through the sediments.
Size of the Salt Domes
Salt domes are often very large structures. The diameter of the salt dome can range from ½ a mile to over five miles. The origin or the parent rock from which the salt dome rises is typically found thousands of feet below the surface. A typical salt dome rises at least a mile from its origin and some are known to be higher than six miles.
Economic Significance of Salt Domes
Salt domes are important due to various reasons. They serve as a reservoir for oil and gas. They are also a critical source for sulfur and salt. Once the salt is extracted, the remaining salt dome holes can be used as an effective underground storage and waste disposal site for hazardous waste.
Oil and Gas Reservoirs
Salt domes have a unique significance in the oil and gas industry. As the salt is rising vertically, it exerts pressure on the rocks above it. The caprock, which is the layer of sediment above the salt, is arched upwards. This upward arch serves as a structural trap and can reserve oil and gas. As the caprock is arched upwards, oil and gas migrate towards the salt dome. Salt is an effective trap rock as oil can come to rest up along the salt.
This upward arching of the rock and collection of oil and gas in the arch allows better flow rates of hydrocarbons. This helps in the extraction and recovery of fossil fuels trapped along the salt dunes. The extraction of hydrocarbons along the salt dunes is more cost-effective and environmentally friendly compared to conventional drilling. A single salt dome can have numerous reservoirs of oil and gas at various depths and locations around the salt dome.
A Source of Salt and Sulfur
Since salt domes are salt formations, they can be exploited through mining. Salt is used as an essential raw material in numerous industries. It is widely used in the chemical industry and for treating snow-covered highways. Apart from commercial use, salt also has domestic applications as well.
Another important mineral that can be recovered from salt domes is sulfur. Sulfur is not a part of the salt dome but an important component of the caprock. It occurs as a crystalline material and is known to be formed as a result of bacterial activity. The caprock of certain salt domes contains enough sulfur that can be recovered economically.
The process of recovering sulfur from salt dome involving drilling a well and treating the rock with superheated water and air. However, as of now, this method of recovering sulfur is not cost-effective. Therefore most of the sulfur which is produced today is a byproduct of refining hydrocarbons.
Underground Storage and Waste Disposal
When the salt, sulfur and hydrocarbons are extracted, this leaves large holes which can be used for underground storage. Some of the mines which are developed into salt domes are sealed and can be used for the storage of oil and gas. In the US and Russia, salt domes also serve as state reservoirs of helium gas. This is because salt is the only type of rock that has very low permeability. As a result, it can hold the tiny atoms of helium gas.
Since salt has low levels of permeability, it also restricts the flow of any type of liquid into the soil. These properties make salt domes very effective for the disposal of hazardous radioactive waste.
Salt domes are a distinct geological formation with great economic value. Though it takes centuries to form, once formed, humans can continue to benefit from them for years.