Salt dome

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Astronaut photography of Jashak salt dome in the Zagros Mountains in Bushehr Province, Iran (the white area in the middle) ZagrosMtns SaltDome ISS012-E-18774.jpg
Astronaut photography of Jashak salt dome in the Zagros Mountains in Bushehr Province, Iran (the white area in the middle)
Salt dome in Fars Province, Iran Salt dome Hadi Karimi.jpg
Salt dome in Fars Province, Iran

A salt dome is a type of structural dome formed when a thick bed of evaporite minerals (mainly salt, or halite) found at depth intrudes vertically into surrounding rock strata, forming a diapir. It is important in petroleum geology because salt structures are impermeable and can lead to the formation of a stratigraphic trap.

Dome (geology) Deformational feature in structural geology of symmetrical anticlines that intersect each other at their respective apices.

A dome is a feature in structural geology consisting of symmetrical anticlines that intersect each other at their respective apices. Intact, domes are distinct, rounded, spherical-to-ellipsoidal-shaped protrusions on the Earth's surface. However, a transect parallel to Earth's surface of a dome features concentric rings of strata. Consequently, if the top of a dome has been eroded flat, the resulting structure in plan view appears as a bullseye, with the youngest rock layers at the outside, and each ring growing progressively older moving inwards. These strata would have been horizontal at the time of deposition, then later deformed by the uplift associated with dome formation.

Evaporite A water-soluble mineral sediment formed by evaporation from an aqueous solution

Evaporite is the term for a water-soluble mineral sediment that results from concentration and crystallization by evaporation from an aqueous solution. There are two types of evaporite deposits: marine, which can also be described as ocean deposits, and non-marine, which are found in standing bodies of water such as lakes. Evaporites are considered sedimentary rocks and are formed by chemical sediments.

Halite mineral form of sodium chloride

Halite, commonly known as rock salt, is a type of salt, the mineral (natural) form of sodium chloride (NaCl). Halite forms isometric crystals. The mineral is typically colorless or white, but may also be light blue, dark blue, purple, pink, red, orange, yellow or gray depending on the amount and type of impurities. It commonly occurs with other evaporite deposit minerals such as several of the sulfates, halides, and borates. The name halite is derived from the Ancient Greek word for salt, ἅλς (háls).

Contents

Formation

The formation of a salt dome begins with the deposition of salt in a restricted marine basin. Because the flow of salt-rich seawater into the basin is not balanced by outflow, much to all water lost from the basin is via evaporation, resulting in the precipitation and deposition of salt evaporites. The rate of sedimentation of salt is significantly larger than the rate of sedimentation of clastics, [1] but it is recognized that a single evaporation event is rarely enough to produce the vast quantities of salt needed to form a layer thick enough for salt diapirs to be formed. This indicates that a sustained period of episodic flooding and evaporation of the basin must occur, as can be seen from the example of the Mediterranean Messinian salinity crisis. At the present day, evaporite deposits can be seen accumulating in basins that merely have restricted access but do not completely dry out; they provide an analogue to some deposits recognized in the geologic record, such as the Garabogazköl basin in Turkmenistan.[ citation needed ]

The Messinian Salinity Crisis (MSC), also referred to as the Messinian Event, and in its latest stage as the Lago Mare event, was a geological event during which the Mediterranean Sea went into a cycle of partly or nearly complete desiccation throughout the latter part of the Messinian age of the Miocene epoch, from 5.96 to 5.33 Ma. It ended with the Zanclean flood, when the Atlantic reclaimed the basin.

Geologic record

The geologic record in stratigraphy, paleontology and other natural sciences refers to the entirety of the layers of rock strata — deposits laid down by volcanism or by deposition of sediment derived from weathering detritus including all its fossil content and the information it yields about the history of the Earth: its past climate, geography, geology and the evolution of life on its surface. According to the law of superposition, sedimentary and volcanic rock layers are deposited on top of each other. They harden over time to become a solidified (competent) rock column, that may be intruded by igneous rocks and disrupted by tectonic events.

Garabogazköl bay in Turkmenistan

The Garabogazköl Aylagy or Kara-Bogaz-Gol is a shallow inundated depression in the northwestern corner of Turkmenistan. It forms a lagoon of the Caspian Sea with a surface area of about 18,000 km2 (6,900 sq mi). It is separated from the Caspian Sea proper, which lies immediately to the west, by a narrow, rocky ridge having a very narrow opening in the rock through which the Caspian waters flow, cascading down into Garabogazköl, leading to the Turkmen language name of the bay, "Mighty Strait Lake". The water volume of the bay fluctuates seasonally with the Caspian Sea; at times it becomes a large bay of the Caspian Sea, while at other times its water level drops drastically.

Over time, the layer of salt is covered with deposited sediment, becoming buried under an increasingly large overburden. The overlying sediment will undergo compaction, causing an increase in density and therefore a decrease in buoyancy. Unlike clastics, pressure has a significantly smaller effect on the density of salt due to its crystal structure and this eventually leads to it becoming more buoyant than the sediment above it. The ductility of salt initially allows it to plastically deform and flow laterally, decoupling the overlying sediment from the underlying sediment. Since the salt has a larger buoyancy than the sediment above—and if a significant faulting event affects the lower surface of the salt—the salt can begin to flow vertically, forming a salt pillow. [2] The vertical growth of these salt pillows creates pressure on the upward surface, causing extension and faulting [3] (see salt tectonics).

Sediment Particulate solid matter that is deposited on the surface of land

Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand and silt can be carried in suspension in river water and on reaching the sea bed deposited by sedimentation and if buried, may eventually become sandstone and siltstone.

The density, or more precisely, the volumetric mass density, of a substance is its mass per unit volume. The symbol most often used for density is ρ, although the Latin letter D can also be used. Mathematically, density is defined as mass divided by volume:

Buoyancy An upward force that opposes the weight of an object immersed in fluid

In physics, buoyancy or upthrust, is an upward force exerted by a fluid that opposes the weight of an immersed object. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus the pressure at the bottom of a column of fluid is greater than at the top of the column. Similarly, the pressure at the bottom of an object submerged in a fluid is greater than at the top of the object. The pressure difference results in a net upward force on the object. The magnitude of the force is proportional to the pressure difference, and is equivalent to the weight of the fluid that would otherwise occupy the volume of the object, i.e. the displaced fluid.

Possible forces that drive the flow of salt are differential loading on the source layer and density contrasts in the overburdening sediment. [4] Forces that resist this flow are the mass of the roof block and the block's inherent resistance to faulting, i.e., strength. [4] To accommodate common density contrast between the overburden sediment and the salt, beginning active diapirism, the diapir height must be more than two-thirds to three-quarters the thickness of the overburden. [4] If the diapir is narrow its height must be greater. [4]

Eventually, over millions of years, the salt will pierce and break through the overlying sediment, first as a dome-shaped, and then a mushroom-shaped, fully formed salt diapir. If the rising salt diapir breaches the surface, it can become a flowing salt glacier. In cross section, these large domes may be anywhere from 1 to 10 kilometres (0.62 to 6.21 mi) across, and extend as deep as 6.5 kilometres (4.0 mi).

Salt glacier

A salt glacier is a rare flow of salt that is created when a rising diapir in a salt dome breaches the surface of the Earth. The name ‘salt glacier’ was given to this phenomenon due to the similarity of movement when compared with ice glaciers. The causes of these formations is primarily due to salt’s unique properties and its surrounding geologic environment. A rising body of salt is referred to as a diapir; which rises to the surface and feeds the salt glacier. Salt structures are usually composed of halite, anhydrite, gypsum and clay minerals. Clays may be brought up with the salt, turning it dark. These salt flows are rare on earth. In a more recent discovery, scientists have found that they also occur on Mars, but are composed of sulfates.

Imaging of undersea salt domes in the Gulf of Mexico Map00365 (27670433694).jpg
Imaging of undersea salt domes in the Gulf of Mexico

Structure

Typical structures of active diapirism are a central crestal graben flanked by flaps that rotate upward and outward. [4] Reverse faults can separate the flaps from the overburden. Normal faults create the crestal graben and propagate downward. [4] New faults form farther outward as the dome arch becomes more intense. [4] These structures occur beneath the surface and are not necessarily associated with the dome at the surface. Emergence of the dome will not occur unless the dome is very wide or tall relative to the overburden's thickness. [4]

Graben Depressed block of planetary crust bordered by parallel faults

In geology, a graben is a depressed block of the crust of a planet bordered by parallel faults.

In geology, a fault is a planar fracture or discontinuity in a volume of rock, across which there has been significant displacement as a result of rock-mass movement. Large faults within the Earth's crust result from the action of plate tectonic forces, with the largest forming the boundaries between the plates, such as subduction zones or transform faults. Energy release associated with rapid movement on active faults is the cause of most earthquakes.

Recognizing salt domes in seismic data

Left shows a generalized structure of a salt dome. Right shows two fault regimes that make up the crestal graben, the dotted line indicates the top of the salt. Salt Dome.JPG
Left shows a generalized structure of a salt dome. Right shows two fault regimes that make up the crestal graben, the dotted line indicates the top of the salt.

If a salt dome has not pierced the surface they can be found located beneath the surface in various ways. The unique surficial structures can be observed as indicating the salt dome beneath the surface. Salt domes can also be interpreted from seismic reflection where the stark density contrast between the salt and surrounding sediments outlines the salt structures. [4] Salt domes can also be associated with sulfur springs and natural gas vents. [5]

Examples

The first salt dome was discovered in 1890 when an exploratory oil well was drilled into Spindletop Hill near Beaumont TX. [5]

Major occurrences of salt domes are found along the Gulf Coast of the USA in Texas and Louisiana. [6] One example of an island formed by a salt dome is Avery Island in Louisiana. At present ocean levels it is no longer surrounded by the sea but it is surrounded by bayous on all sides. The Gulf Coast is home to over 500 currently discovered salt domes. [5]

Another example of an emergent salt dome is at Onion Creek, Utah / Fisher Towers near Moab, Utah, U.S. These two images show a Cretaceous age salt body that has risen as a ridge through several hundred meters of overburden, predominantly sandstone. As the salt body rose, the overburden formed an anticline (arching upward along its center line) which fractured and eroded to expose the salt body.

End-on view of emergent salt dome between remnants of displaced overburden Ken Leonard - Fisher Towers - IMAG0059.JPG
End-on view of emergent salt dome between remnants of displaced overburden
Lateral view of emergent salt dome from ridge of remnant of displaced overburden Ken Leonard - Fisher Towers - Salt Dome P IMAG0067.JPG
Lateral view of emergent salt dome from ridge of remnant of displaced overburden

The term "salt dome" is also sometimes inaccurately used to refer to dome-shaped silos used to store rock salt for melting snow on highways. These domes are actually called monolithic domes and are used to store a variety of bulk goods. [7]

Commercial uses

The rock salt that is found in salt domes is mostly impermeable. As the salt moves up towards the surface, it can penetrate and/or bend strata of existing rock with it. As these strata are penetrated, they are generally bent slightly upwards at the point of contact with the dome, and can form pockets where petroleum and natural gas can collect between impermeable strata of rock and the salt. The strata immediately above the dome that are not penetrated are pushed upward, creating a dome-like reservoir above the salt where petroleum can also gather. These oil pools can eventually be extracted, and, indeed, form a major source of the petroleum produced along the coast of the Gulf of Mexico. [8]

The caprock above the salt domes is sometimes the site of deposits of native sulfur, which is recovered by the Frasch process.

Other uses include storing oil, natural gas, hydrogen gas, or even hazardous waste in large caverns formed after salt mining, as well as excavating the domes for uses in everything from table salt to the granular material used to prevent roadways from icing over.

See also

Related Research Articles

Sedimentary rock Rock formed by the deposition and subsequent cementation of material

Sedimentary rocks are types of rock that are formed by the deposition and subsequent cementation of mineral or organic particles on the floor of oceans or other bodies of water at the Earth's surface. Sedimentation is the collective name for processes that cause these particles to settle in place. The particles that form a sedimentary rock are called sediment, and may be composed of geological detritus (minerals) or biological detritus. Before being deposited, the geological detritus was formed by weathering and erosion from the source area, and then transported to the place of deposition by water, wind, ice, mass movement or glaciers, which are called agents of denudation. Biological detritus was formed by bodies and parts of dead aquatic organisms, as well as their fecal mass, suspended in water and slowly piling up on the floor of water bodies. Sedimentation may also occur as dissolved minerals precipitate from water solution.

Niger Delta Basin (geology)

The Niger Delta Basin, also referred to as the Niger Delta province, is an extensional rift basin located in the Niger Delta and the Gulf of Guinea on the passive continental margin near the western coast of Nigeria with suspected or proven access to Cameroon, Equatorial Guinea and São Tomé and Príncipe. This basin is very complex, and it carries high economic value as it contains a very productive petroleum system. The Niger delta basin is one of the largest subaerial basins in Africa. It has a subaerial area of about 75,000 km2, a total area of 300,000 km2, and a sediment fill of 500,000 km3. The sediment fill has a depth between 9–12 km. It is composed of several different geologic formations that indicate how this basin could have formed, as well as the regional and large scale tectonics of the area. The Niger Delta Basin is an extensional basin surrounded by many other basins in the area that all formed from similar processes. The Niger Delta Basin lies in the south westernmost part of a larger tectonic structure, the Benue Trough. The other side of the basin is bounded by the Cameroon Volcanic Line and the transform passive continental margin.

Geology of the Canyonlands area

The exposed geology of the Canyonlands area is complex and diverse; 12 formations are exposed in Canyonlands National Park that range in age from Pennsylvanian to Cretaceous. The oldest and perhaps most interesting was created from evaporites deposited from evaporating seawater. Various fossil-rich limestones, sandstones, and shales were deposited by advancing and retreating warm shallow seas through much of the remaining Paleozoic.

Anticline geological term

In structural geology, an anticline is a type of fold that is an arch-like shape and has its oldest beds at its core. A typical anticline is convex up in which the hinge or crest is the location where the curvature is greatest, and the limbs are the sides of the fold that dip away from the hinge. Anticlines can be recognized and differentiated from antiforms by a sequence of rock layers that become progressively older toward the center of the fold. Therefore, if age relationships between various rock strata are unknown, the term antiform should be used.

Diapir A type of geologic intrusion in which a more mobile and ductily deformable material is forced into brittle overlying rocks

A diapir is a type of geologic intrusion in which a more mobile and ductily deformable material is forced into brittle overlying rocks. Depending on the tectonic environment, diapirs can range from idealized mushroom-shaped Rayleigh–Taylor-instability-type structures in regions with low tectonic stress such as in the Gulf of Mexico to narrow dikes of material that move along tectonically induced fractures in surrounding rock. The term was introduced by the Romanian geologist Ludovic Mrazek, who was the first to understand the principle of salt intrusion and plasticity. The term "diapir" may be applied to igneous structures, but it is more commonly applied to non-igneous, relatively cold materials, such as salt domes and mud diapirs.

Salt tectonics

Salt tectonics is concerned with the geometries and processes associated with the presence of significant thicknesses of evaporites containing rock salt within a stratigraphic sequence of rocks. This is due both to the low density of salt, which does not increase with burial, and its low strength.

In petroleum geology, a structural trap is a type of geological trap that forms as a result of changes in the structure of the subsurface, due to tectonic, diapiric, gravitational and compactional processes. These changes block the upward migration of hydrocarbons and can lead to the formation of a petroleum reservoir.

The salt tectonics off the Louisiana gulf coast can be explained through two possible methods. The first method attributes spreading of the salt because of sedimentary loading while the second method points to slope instability as the primary cause of gliding of the salt. The first method results in the formation of growth faults in the overlying sediment. Growth faults are normal faults that occur simultaneously with sedimentation, causing them to have thicker sediment layers on the downthrown sides of the faults. In the second method both the salt and the sediment are moving, making it more likely to migrate.

Growth fault

Growth faults are syndepositional or syn-sedimentary extensional faults that initiate and evolve at the margins of continental plates. They extend parallel to passive margins that have high sediment supply. Their fault plane dips mostly toward the basin and has long-term continuous displacement. Figure one shows a growth fault with a concave upward fault plane that has high updip angle and flattened at its base into zone of detachment or décollement. This angle is continuously changing from nearly vertical in the updip area to nearly horizontal in the downdip area.

Salt surface structures

Salt surface structures are extensions of salt tectonics that form at the Earth's surface when either diapirs or salt sheets pierce through the overlying strata. They can occur in any location where there are salt deposits, namely in cratonic basins, synrift basins, passive margins and collisional margins. These are environments where mass quantities of water collect and then evaporate; leaving behind salt and other evaporites to form sedimentary beds. When there is a difference in pressure, such as additional sediment in a particular area, the salt beds – due to the unique ability of salt to behave as a fluid under pressure – form into new structures. Sometimes, these new bodies form subhorizontal or moderately dipping structures over a younger stratigraphic unit, which are called allochthonous salt bodies or salt surface structures.

Columbus Basin

The Columbus Basin is a foreland basin located off the south eastern coast of Trinidad within the East Venezuela Basin (EVB). Due to the intensive deformation occurring along the Caribbean and South American plates in this region, the basin has a unique structural and stratigraphic relationship. The Columbus Basin has been a prime area for hydrocarbon exploration and production as its structures, sediments and burial history provide ideal conditions for generation and storage of hydrocarbon reserves. The Columbus Basin serves as a depocenter for the Orinoco River delta, where it is infilled with 15 km of fluvio-deltaic sediment. The area has also been extensively deformed by series of north west to southeast normal faults and northeast to southwest trending anticline structures.

Geology of the southern North Sea

The North Sea basin is located in northern Europe and lies between the United Kingdom, and Norway just north of The Netherlands and can be divided into many sub-basins. The Southern North Sea basin is the largest gas producing basin in the UK continental shelf, with production coming from the lower Permian sandstones which are sealed by the upper Zechstein salt. The evolution of the North Sea basin occurred through multiple stages throughout the geologic timeline. First the creation of the Sub-Cambrian peneplain, followed by the Caledonian Orogeny in the late Silurian and early Devonian. Rift phases occurred in the late Paleozoic and early Mesozoic which allowed the opening of the northeastern Atlantic. Differential uplift occurred in the late Paleogene and Neogene. The geology of the Southern North Sea basin has a complex history of basinal subsidence that had occurred in the Paleozoic, Mesozoic, and Cenozoic. Uplift events occurred which were then followed by crustal extension which allowed rocks to become folded and faulted late in the Paleozoic. Tectonic movements allowed for halokinesis to occur with more uplift in the Mesozoic followed by a major phase of inversion occurred in the Cenozoic affecting many basins in northwestern Europe. The overall saucer-shaped geometry of the southern North Sea Basin indicates that the major faults have not been actively controlling sediment distribution.

Angola Basin

The Angola Basin is located along the West African South Atlantic Margin which extends from Cameroon to Angola. It is characterized as a passive margin that began spreading in the south and then continued upwards throughout the basin. This basin formed during the initial breakup of the supercontinent Pangaea during the early Cretaceous, creating the Atlantic Ocean and causing the formation of the Angola, Cape, and Argentine basins. It is often separated into two units: the Lower Congo Basin, which lies in the northern region and the Kwanza Basin which is in the southern part of the Angola margin. The Angola Basin is famous for its "Aptian Salt Basins," a thick layer of evaporites that has influenced topography of the basin since its deposition and acts as an important petroleum reservoir.

North German basin A passive-active rift basin in central and west Europe

The North German Basin is a passive-active rift basin located in central and west Europe, lying within the southeastern most portions of the North Sea and the southwestern Baltic Sea and across terrestrial portions of northern Germany, Netherlands, and Poland. The North German Basin is a sub-basin of the Southern Permian Basin, that accounts for a composite of intra-continental basins composed of Permian to Cenozoic sediments, which have accumulated to thicknesses around 10–12 kilometres (6–7.5 mi). The complex evolution of the basin takes place from the Permian to the Cenozoic, and is largely influenced by multiple stages of rifting, subsidence, and salt tectonic events. The North German Basin also accounts for a significant amount of Western Europe's natural gas resources, including one of the worlds largest natural gas reservoir, the Groningen gas field.

The Prairie Evaporite Formation, also known as the Prairie Formation, is a geologic formation of Middle Devonian (Givetian) age that consists primarily of halite and other evaporite minerals. It is present beneath the plains of northern and eastern Alberta, southern Saskatchewan and southwestern Manitoba in Canada, and it extends into northwestern North Dakota and northeastern Montana in the United States.

Tarfaya Basin

The Tarfaya Basin is a structural basin located in southern Morocco that extends westward into the Moroccan territorial waters in the Atlantic Ocean. The basin is named for the city of Tarfaya located near the border of Western Sahara, a region governed by the Kingdom of Morocco. The Canary Islands form the western edge of the basin and lie approximately 100 km to the west.

Salt deformation

Salt deformation is the change of shape of natural salt bodies in response to forces and mechanisms that controls salt flow. Such deformation can generate large salt structures such as underground salt layers, salt diapirs or salt sheets at the surface. Strictly speaking, salt structures are formed by rock salt that is composed of pure halite (NaCl) crystal. However, most halite in nature appears in impure form, therefore rock salt usually refers to all rocks that composed mainly of halite, sometimes also as a mixture with other evaporites such as gypsum and anhydrite. Earth's salt deformation generally involves such mixed materials.

References

  1. Schreiber, B.C. and Hsü, K.J. (1980) Evaporites. In Developments in Petroleum Geology, Vol. 2 (Ed. G.D. Hobson), pp. 87–138. Elsevier Science, Amsterdam.
  2. RGD 1993. Geological Atlas of the subsurface of The Netherlands: Explanations to map sheet IV Texel-Purmerend (1:250,000). Rijks Geologische Dienst (Haarlem): 127 pp.
  3. Dronkert, H. & Remmelts, G. 1996. Influence of salt structures on reservoir rocks in Block L2, Dutch continental shelf. In: Rondeel, H.E., Batjes, D.A.J., Nieuwenhuijs, W.H. (eds): Geology of gas and oil under the Netherlands, Kluwer (Dordrecht): 159–166.
  4. 1 2 3 4 5 6 7 8 9 Schultz-Ela, D.D; Jackson, M.P.A; Vendeville, B.C. (January 12, 1992). "Mechanics of Active Salt Diapirism" (PDF). Tectonophysics. Amsterdam. 228: 275–312. doi:10.1016/0040-1951(93)90345-k.
  5. 1 2 3 "What is a Salt Dome? How do they form?". geology.com. Retrieved 2015-12-17.
  6. C.Michael Hogan. 2011. Sulfur. Encyclopedia of Earth, eds. A.Jorgensen and C.J.Cleveland, National Council for Science and the environment, Washington DC
  7. Salt dome formation
  8. Salt dome at Schlumberger's Oilfield Glossary