What is a salt dome in geology ?
BORNET Rémi (2024)
Salt dome : definition
A salt dome is a mass composed mainly of salt (halite) with a thin top (the "cap rock") of anhydrite, gypsum and sulphur. It constitutes an irregular column with a swollen head, which can reach 10 km in height (the diapir), which rises in the crust until sometimes piercing the surface, one then speaks of "glaciers" of salt.
A salt dome arises from a layer of evaporite buried at great depth. Under the weight of the rocks above them (lithostatic pressure), these less dense evaporites deform and migrate towards the surface, forming columnar or mushroom-shaped structures. This rise can be accelerated by tectonic movements : this is the phenomenon of halokinesis. The diameter of these domes generally exceeds one kilometer.
The salt domes are very important reserves of halite but also of sulfur, formed in the "cap rock" by bacterial destruction of anhydrite, and effective traps for hydrocarbons. Due to their tightness, they are also used by humans for the storage of petroderivatives and radioactive waste.
A salt dome arises from a layer of evaporite buried at great depth. Under the weight of the rocks above them (lithostatic pressure), these less dense evaporites deform and migrate towards the surface, forming columnar or mushroom-shaped structures. This rise can be accelerated by tectonic movements : this is the phenomenon of halokinesis. The diameter of these domes generally exceeds one kilometer.
The salt domes are very important reserves of halite but also of sulfur, formed in the "cap rock" by bacterial destruction of anhydrite, and effective traps for hydrocarbons. Due to their tightness, they are also used by humans for the storage of petroderivatives and radioactive waste.
Salt domes as hydrocarbon traps
Due to the pressure of the overlying sediments, the salt, which is ductile and less dense, slowly rises through the harder rock layers. It deforms the surrounding rocks, causing folds, faults and fractures. The process of diapirism can last for millions of years, significantly changing the geometry of the subsurface. The main mechanism by which salt domes trap hydrocarbons is related to their impermeability. As the diapir rises through the sedimentary layers, it forms a natural barrier preventing the migration of oil. Hydrocarbons form in deeply buried source rocks, and over time, they migrate to areas where the pressure is lower. The geological structures formed by diapirs combined with their impermeability then constitute excellent reservoirs where these hydrocarbons are recoverable. Salt domes are one of the best natural traps there is. They are therefore the subject of intense research.
Geographic distribution of salt domes
Salt domes are found in many regions of the world. They are primarily found in ancient sedimentary basins where conditions favorable to the precipitation of evaporites existed. Some notable regions include :
The Gulf of Mexico Basin : This is one of the most studied regions for salt domes, due to their association with hydrocarbon deposits.
The North Sea : Another region rich in salt domes, with significant potential for oil exploration.
Iran and Qatar : Known for their large oil fields associated with salt diapirs.
