Caprock

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Caprock or cap rock is a more resistant rock type overlying a less resistant rock type, [1] analogous to an upper crust on a cake that is harder than the underlying layer.

Contents

Description

Horseshoe Falls, part of the Niagara Escarpment. The dark thin layer in the foreground, where water is not yet running, is the caprock. DSCN4413 horseshoefalls e.jpg
Horseshoe Falls, part of the Niagara Escarpment. The dark thin layer in the foreground, where water is not yet running, is the caprock.

The Niagara Escarpment, over which Niagara Falls flows, is an example of a scarp or escarpment. At Niagara Falls, the caprock is the riverbed above the falls, and is what prevents the river from eroding the face of the falls very quickly. The Niagara caprock is made of dolomitic limestone. Other common types of caprock are sandstone and mafic rock.

In processes such as scarp retreat, the caprock controls the rate of erosion of the scarp. As the softer rock is cut away, periodically the caprock shears off. Caprock is also found in salt domes and on the top of mesa formations.

Petroleum

In the petroleum industry, caprock is any nonpermeable formation that may trap oil, gas or water, preventing it from migrating to the surface. This caprock can prevent hydrocarbons from migrating to the surface, allowing them to accumulate in a reservoir of oil, gas and water. These structures, also known as petroleum traps, are a primary target for the petroleum industry.

Salt dome caprock

The tops of salt domes such as in the Gulf of Mexico dissolve in a characteristic manner, and can range between 0–1500 ft thick. [2] The halite (salt) is removed first, leaving behind gypsum and anhydrite. The anhydrite and gypsum react with organic material to form calcite. The classic Murray 1966 paper [3] describes the generalized sequence as sediments-calcite-gypsum-anhydrite-salt.

See also

Related Research Articles

<span class="mw-page-title-main">Niagara Escarpment</span> Escarpment in Canada and the United States

The Niagara Escarpment is a long escarpment, or cuesta, in Canada and the United States that starts from the south shore of Lake Ontario westward, circumscribes the top of the Great Lakes Basin running from New York through Ontario, Michigan, and Wisconsin. The escarpment is the cliff over which the Niagara River plunges at Niagara Falls, for which it is named.

<span class="mw-page-title-main">Permian Basin (North America)</span> Large sedimentary basin in the US

The Permian Basin is a large sedimentary basin in the southwestern part of the United States. It is the highest producing oil field in the United States, producing an average of 4.2 million barrels of crude oil per day in 2019. This sedimentary basin is located in western Texas and southeastern New Mexico. It reaches from just south of Lubbock, past Midland and Odessa, south nearly to the Rio Grande River in southern West Central Texas, and extending westward into the southeastern part of New Mexico. It is so named because it has one of the world's thickest deposits of rocks from the Permian geologic period. The greater Permian Basin comprises several component basins; of these, the Midland Basin is the largest, Delaware Basin is the second largest, and Marfa Basin is the smallest. The Permian Basin covers more than 86,000 square miles (220,000 km2), and extends across an area approximately 250 miles (400 km) wide and 300 miles (480 km) long.

<span class="mw-page-title-main">Salt dome</span> Structural dome formed of salt or halite

A salt dome is a type of structural dome formed when salt intrudes into overlying rocks in a process known as diapirism. Salt domes can have unique surface and subsurface structures, and they can be discovered using techniques such as seismic reflection. They are important in petroleum geology as they can function as petroleum traps.

<span class="mw-page-title-main">Anhydrite</span> Mineral, anhydrous calcium sulfate

Anhydrite, or anhydrous calcium sulfate, is a mineral with the chemical formula CaSO4. It is in the orthorhombic crystal system, with three directions of perfect cleavage parallel to the three planes of symmetry. It is not isomorphous with the orthorhombic barium (baryte) and strontium (celestine) sulfates, as might be expected from the chemical formulas. Distinctly developed crystals are somewhat rare, the mineral usually presenting the form of cleavage masses. The Mohs hardness is 3.5, and the specific gravity is 2.9. The color is white, sometimes greyish, bluish, or purple. On the best developed of the three cleavages, the lustre is pearly; on other surfaces it is glassy. When exposed to water, anhydrite readily transforms to the more commonly occurring gypsum, (CaSO4·2H2O) by the absorption of water. This transformation is reversible, with gypsum or calcium sulfate hemihydrate forming anhydrite by heating to around 200 °C (400 °F) under normal atmospheric conditions. Anhydrite is commonly associated with calcite, halite, and sulfides such as galena, chalcopyrite, molybdenite, and pyrite in vein deposits.

<span class="mw-page-title-main">Petroleum reservoir</span> Subsurface pool of hydrocarbons

A petroleum reservoir or oil and gas reservoir is a subsurface accumulation of hydrocarbons contained in porous or fractured rock formations.

<span class="mw-page-title-main">Cutler Formation</span> Geologic formation in the Four Corners, US

The Cutler Formation or Cutler Group is a rock unit that is exposed across the U.S. states of Arizona, northwest New Mexico, southeast Utah and southwest Colorado. It was laid down in the Early Permian during the Wolfcampian epoch.

<span class="mw-page-title-main">Geology of Texas</span> Overview of the geology of the U.S. state of Texas

Texas contains a wide variety of geologic settings. The state's stratigraphy has been largely influenced by marine transgressive-regressive cycles during the Phanerozoic, with a lesser but still significant contribution from late Cenozoic tectonic activity, as well as the remnants of a Paleozoic mountain range.

<span class="mw-page-title-main">Petroleum trap</span> Geological structure allowing accumulation of hydrocarbons in a reservoir

In petroleum geology, a trap is a geological structure affecting the reservoir rock and caprock of a petroleum system allowing the accumulation of hydrocarbons in a reservoir. Traps can be of two types: stratigraphic or structural. Structural traps are the most important type of trap as they represent the majority of the world's discovered petroleum resources.

The Leduc Formation is a stratigraphic unit of Late Devonian (Frasnian) age in the Western Canada Sedimentary Basin. It takes its name from the city of Leduc, and it was formally described from the B.A. Pyrz No. 1 well in central Alberta, between the depths of 1,623.7 m (5,327 ft) and 1,807.5 m (5,930 ft), by Imperial Oil Limited in 1950. Supplementary information came from a complete section of the formation that was cored in Imperial Oil's Leduc No. 530 well between 1,633 m (5,358 ft) and 1,863 m (6,112 ft).

<span class="mw-page-title-main">Wind River Basin</span> Basin in Wyoming, United States

The Wind River Basin or Shoshone Basin is a semi-arid intermontane foreland basin in central Wyoming, United States. It is bounded by Laramide uplifts on all sides. On the west is the Wind River Range and on the North are the Absaroka Range and the Owl Creek Mountains. The Casper Arch separates the Wind River from the Powder River Basin to the east and the Sweetwater Uplift lies to the south. The basin contains a sequence of 10,000–12,000 feet of predominantly marine sediments deposited during the Paleozoic and Mesozoic Eras. During the Laramide over 18,000 feet of Eocene lacustrine and fluvial sediments were deposited within the basin. Following the Eocene an additional 3,000 feet of sediments were deposited before, and as the basin was uplifted in the late Tertiary.

<span class="mw-page-title-main">Cedar Mesa Sandstone</span>

Cedar Mesa Sandstone is a sandstone member of the Cutler Formation, found in southeast Utah, southwest Colorado, northwest New Mexico, and northeast Arizona.

<span class="mw-page-title-main">Paradox Formation</span>

In geology, the Paradox Formation Is a Pennsylvanian age formation which consists of abundant evaporites with lesser interbedded shale, sandstone, and limestone. The evaporites are largely composed of gypsum, anhydrite, and halite. The formation is found mostly in the subsurface, but there are scattered exposures in anticlines in eastern Utah and western Colorado. These surface exposures occur in the Black Mesa, San Juan and Paradox Basins and the formation is found in the subsurface in southwestern Colorado, southeastern Utah, northeastern Arizona and northeastern New Mexico.

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.

<span class="mw-page-title-main">Seven Rivers Formation</span> Geologic formation in the western United States

The Seven Rivers Formation is a geologic formation in southeastern New Mexico and west Texas. It preserves fossils dating back to the late Guadalupian Epoch of the Permian period.

<span class="mw-page-title-main">Tansill Formation</span> Geologic formation in the western United States

The Tansill Formation is a geologic formation in southeastern New Mexico and west Texas, United States. It preserves fossils dating back to the late Guadalupian Age of the Permian period.

<span class="mw-page-title-main">Theron Wasson</span>

Theron Rhodes Wasson (1887–1970) was a leading American petroleum geologist and engineer, who pioneered the use of geophysical surveys to find oil and gas.

<span class="mw-page-title-main">Greater Green River Basin</span> River basin in southwestern Wyoming, United States

The Greater Green River Basin (GGRB) is a 21,000 square mile basin located in Southwestern Wyoming. The Basin was formed during the Cretaceous period sourced by underlying Permian and Cretaceous deposits. The GGRB is host to many anticlines created during the Laramide Orogeny trapping many of its hydrocarbon resources. It is bounded by the Rawlins Uplift, Uinta Mountains, Sevier overthrust belt, Sierra Madre Mountains, and the Wind River Mountain Range. The Greater Green River Basin is subdivided into four smaller basins, the Green River Basin, Great Divide Basin, Washakie Basin, and Sand Wash Basin. Each of these possesses hydrocarbons that have been economically exploited. There are 303 named fields throughout the basin, the majority of which produce natural gas; the largest of these gas fields is the Jonah Field.

<span class="mw-page-title-main">Alaska North Slope basin</span>

The Alaskan North Slope (ANS) is a foreland basin located on the northern edge of the Brooks Range. The Alaska North Slope is bounded on the north by the Beaufort Sea and runs from the Canadian border to the maritime boundary with Russia in the west. The western edge extends into the Chukchi Sea and Chukchi platform where the basin is at its widest. As the basin moves east it narrows towards the Canadian border. The basin is 1000 km long, 600 km at its widest, and covers a total area of 240,000 km2.

The Grayburg Formation is a geologic formation in west Texas and southern New Mexico. It preserves fossils dating back to the Guadalupian Epoch of the Permian Period.

The Castile Formation is a geologic formation in west Texas and southeastern New Mexico, United States. It was deposited in the Ochoan Stage of the Permian period.

References

  1. Kearey, Philip (2001). Dictionary of Geology, 2nd ed., Penguin Reference, London, New York, etc., p. 41.. ISBN   978-0-14-051494-0.
  2. Walker, Charles W. (Dec 1976). "Origins of Gulf Coast Salt-Dome Cap Rock". AAPG Bulletin . 60 (12): 2162–2166. doi:10.1306/c1ea3aa0-16c9-11d7-8645000102c1865d . Retrieved 2010-09-07.
  3. Murray, Grover E. (Mar 1966). "Salt structures of Gulf of Mexico basin--a review". AAPG Bulletin. 50 (3): 439–478. doi:10.1306/5d25b49d-16c1-11d7-8645000102c1865d . Retrieved 2010-09-07.