A report on Caprock

Horseshoe Falls, part of the Niagara Escarpment
Sealing of a carbonate platform (reservoir rock) by shales (caprock). The erosion of the shale renders the structure of the trap visible.

More resistant rock type overlying a less resistant rock type, analogous to an upper crust on a cake that is a harder than the underlying layer.

- Caprock
Horseshoe Falls, part of the Niagara Escarpment

6 related topics with Alpha

Overall

A structure map, looking downward, generated by contour map software for an 8,500-ft-deep gas and oil reservoir in the Erath field, Erath, Louisiana. The left-to-right gap near the top indicates a fault line between the blue and green contour lines and the purple, red, and yellow lines. The thin red circular line in the middle indicates the top of the oil reservoir. Because gas rises above oil, this latter line marks the gas-and-oil contact zone.

Petroleum reservoir

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Subsurface accumulation of hydrocarbons contained in porous or fractured rock formations.

Subsurface accumulation of hydrocarbons contained in porous or fractured rock formations.

A structure map, looking downward, generated by contour map software for an 8,500-ft-deep gas and oil reservoir in the Erath field, Erath, Louisiana. The left-to-right gap near the top indicates a fault line between the blue and green contour lines and the purple, red, and yellow lines. The thin red circular line in the middle indicates the top of the oil reservoir. Because gas rises above oil, this latter line marks the gas-and-oil contact zone.
An oil field with dozens of wells. This is the Summerland Oil Field, near Santa Barbara, California, before 1906
Eagle Ford Shale flares visible from space (green and infrared wavelengths), in the arc between "1" and "2", amid cities in southeast Texas in 2012.
Location of the gas fields of Iran
Vučkovec Gas Field facility, Croatia
The drillship Discoverer Enterprise is shown in the background, at work during exploratory phase of a new offshore field. The Offshore Support Vessel Toisa Perseus is shown in the foreground, illustrating part of the complex logistics of offshore oil and gas exploration and production.
Structural trap within an Anticline
Structural trap along a fault plane
Structural-stratigraphic trap in a tilted block draped by mudstones
Stratigraphic trap under an unconformity
Stratigraphic trap in a fossilized coral reef (yellow) sealed by mudstones (green)
Stratigraphic trap around an evaporite (pink) salt dome

In conventional reservoirs, the naturally occurring hydrocarbons, such as crude oil or natural gas, are trapped by overlying rock formations with lower permeability, while in unconventional reservoirs, the rocks have high porosity and low permeability, which keeps the hydrocarbons trapped in place, therefore not requiring a cap rock.

Astronaut photography of Jashak salt dome in the Zagros Mountains in Bushehr Province, Iran (the white area in the middle)

Salt dome

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Type of structural dome formed when salt intrudes into overlying rocks in a process known as diapirism.

Type of structural dome formed when salt intrudes into overlying rocks in a process known as diapirism.

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
Imaging of undersea salt domes in the Gulf of Mexico
End-on view of emergent Onion Creek salt dome between remnants of displaced overburden
Lateral view of emergent salt dome from ridge of remnant of displaced overburden

The caprock above the salt domes can contain deposits of native sulfur (recovered by the Frasch process).

Niagara Escarpment (in red)

Niagara Escarpment

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Long escarpment, or cuesta, in Canada and the United States that runs predominantly east–west from New York through Ontario, Michigan, Wisconsin, and Illinois.

Long escarpment, or cuesta, in Canada and the United States that runs predominantly east–west from New York through Ontario, Michigan, Wisconsin, and Illinois.

Niagara Escarpment (in red)
Rattlesnake Point near Milton, Ontario
The Niagara River over thousands of years carves the Niagara Gorge over and through the Niagara Escarpment

The escarpment's caprock is dolomitic limestone, also known as dolostone, which is more resistant and overlies weaker, more easily eroded shale as a weathering-resistant "cap".

Ground-level view of mesas in the Canyonlands National Park, Utah, known as the "Islands in the Sky"

Mesa

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Isolated, flat-topped elevation, ridge or hill, which is bounded from all sides by steep escarpments and stands distinctly above a surrounding plain.

Isolated, flat-topped elevation, ridge or hill, which is bounded from all sides by steep escarpments and stands distinctly above a surrounding plain.

Ground-level view of mesas in the Canyonlands National Park, Utah, known as the "Islands in the Sky"
Aerial view of mesas in Monument Valley, on the Colorado Plateau
Cockburn Range, Kimberley, Western Australia, Australia
Har Qatum, a mesa located on the southern edge of Makhtesh Ramon, Israel
Mount Conner, a mesa located in Northern Territory, Australia
Amadiya, Iraq, a city in its entirety built on a mesa
Mesa in Colorado
A mesa in Noctis Labyrinthus on Mars, viewed by HiRISE

The caprock can consist of either sedimentary rocks such as sandstone and limestone; dissected lava flows; or a deeply eroded duricrust.

Bandiagara Escarpment in Mali.

Scarp retreat

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Geological process through which the location of an escarpment changes over time.

Geological process through which the location of an escarpment changes over time.

Bandiagara Escarpment in Mali.
Components of a scarp where the caprock slopes back from the rim, as is common
Aerial view of retreating scarp in Namibia.
Illawarra escarpment in Australia

If it is protected by a strong caprock, or if it contains vertical fractures, it may retain its steep profile as it retreats.

Anhydrite, Chihuahua, Mexico

Anhydrite

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Mineral with the chemical formula CaSO4.

Mineral with the chemical formula CaSO4.

Anhydrite, Chihuahua, Mexico
Crystal structure of anhydrite
Relief carving of an anhydrite kiln, made from a piece of anhydrite, by Ophelia Gordon Bell

Massive amounts of anhydrite occur when salt domes form a caprock.