The Alaskan North Slope (ANS) is a

foreland basin A foreland basin is a structural basin that develops adjacent and parallel to a mountain belt. Foreland basins form because the immense mass created by crustal thickening associated with the evolution of a mountain belt causes the lithospher ...

located on the northern edge of the

Brooks Range The Brooks Range (Gwich’in language, Gwich'in: ''Gwazhał'') is a mountain range in far northern North America stretching some from west to east across northern Alaska into Canada's Yukon Territory. Reaching a peak elevation of on Mount Isto, ...

. The

Alaska North Slope The Alaska North Slope is the region of the U.S. state of Alaska located on the northern slope of the Brooks Range along the coast of two marginal seas of the Arctic Ocean, the Chukchi Sea being on the western side of Point Barrow, and the Beau ...

is bounded on the north by the

Beaufort Sea The Beaufort Sea ( ; ) is a marginal sea of the Arctic Ocean, located north of the Northwest Territories, Yukon, and Alaska, and west of the Canadian Arctic Archipelago. The sea is named after Sir Francis Beaufort, a Hydrography, hydrographer. T ...

and runs from the Canadian border to the maritime boundary with

Russia Russia, or the Russian Federation, is a country spanning Eastern Europe and North Asia. It is the list of countries and dependencies by area, largest country in the world, and extends across Time in Russia, eleven time zones, sharing Borders ...

in the west. The western edge extends into the

Chukchi Sea The Chukchi Sea (, ), sometimes referred to as the Chuuk Sea, Chukotsk Sea or the Sea of Chukotsk, is a marginal sea of the Arctic Ocean. It is bounded on the west by the Long Strait, off Wrangel Island, and in the east by Point Barrow, Alaska, ...

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.

Basin formation

The ANS is broken into three provinces the

, the Northern Foothills, and the Coastal Plain. The foothills province has severe to gently deformed rock. The southern edge, closest to the

, consists of folded and faulted

Lower Cretaceous Lower may refer to: * ''Lower'' (album), 2025 album by Benjamin Booker * Lower (surname) * Lower Township, New Jersey *Lower Receiver (firearms) * Lower Wick Gloucestershire, England See also * Nizhny {{Disambiguation ...

deposits. This region is home to the deepest section of the basin the Colville trough, with a maximum depth around 30,000 ft. The northern foothills province is made up of numerous elongated

detachment fold A detachment fold, in geology, occurs as layer parallel thrusting along a decollement (or detachment) develops without upward propagation of a fault; the accommodation of the strain produced by continued displacement along the underlying thrust re ...

s in Mid to Late

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...

deposits. The coastal plain is a flat, low relief, area that is covered by

Pleistocene The Pleistocene ( ; referred to colloquially as the ''ice age, Ice Age'') is the geological epoch (geology), epoch that lasted from to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was fin ...

deposits lacking structure until the Barrow Arch. The Barrow Arch is a hinge line fault zone that marks a northern high in the ANS. The Barrow Arch runs east to west along the length of the basin and separates the ANS

from the

Canadian Basin The Canada Basin is a deep oceanic basin within the Arctic Ocean. It is part of the Amerasian Basin and lies off the coast of Alaska and northwest Canada between the Chukchi Plateau north of Alaska and the Alpha Ridge north of Ellesmere Island ...

to the north. Sediments have a subtle dip of 3°-6° southward into the Colville trough. The Colville Trough is the deepest part of the ANS Basin. It is on the southern edge of the foothills province just below the

. The Colville Trough was formed in the

by extreme

subsidence Subsidence is a general term for downward vertical movement of the Earth's surface, which can be caused by both natural processes and human activities. Subsidence involves little or no horizontal movement, which distinguishes it from slope mov ...

that resulted from the

Orogeny Orogeny () is a mountain-mountain formation, building process that takes place at a convergent boundary, convergent plate margin when plate motion compresses the margin. An or develops as the compressed plate crumples and is tectonic uplift, u ...

Tectonic setting

The ANS foreland basin was broken down into three

depositional In geology, depositional environment or sedimentary environment describes the combination of physical, chemical, and biological processes associated with the deposition of a particular type of sediment and, therefore, the rock types that will be ...

and

tectonic Tectonics ( via Latin ) are the processes that result in the structure and properties of the Earth's crust and its evolution through time. The field of ''planetary tectonics'' extends the concept to other planets and moons. These processes ...

sequences that define the deposits and structure of the basin. From the

Mississippian Mississippian may refer to: * Mississippian (geology), a subperiod of the Carboniferous period in the geologic timescale, roughly 360 to 325 million years ago * Mississippian cultures, a network of precontact cultures across the midwest and Easte ...

Jurassic The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately 143.1 Mya. ...

a carbonate ramp built out to the south on the

passive margin A passive margin is the transition between Lithosphere#Oceanic lithosphere, oceanic and Lithosphere#Continental lithosphere, continental lithosphere that is not an active plate continental margin, margin. A passive margin forms by sedimentatio ...

of the Arctic Alaskan Plate. The Arctic Alaskan Plate was then

rift In geology, a rift is a linear zone where the lithosphere is being pulled apart and is an example of extensional tectonics. Typical rift features are a central linear downfaulted depression, called a graben, or more commonly a half-graben ...

ed and rotated in the Early to Mid Cretaceous. This

ing created the Barrow Arch that separates the ANS from the

. As the plate continued to rotate it collided with the island arch chain, initiating the

. As the

uplifted

dropped the carbonate ramp and created a massive amount of accommodation space in the foothills region. Sedimentation couldn't keep up with the

forming a shallow

anoxic Anoxia means a total depletion in the level of oxygen, an extreme form of hypoxia or "low oxygen". The terms anoxia and hypoxia are used in various contexts: * Anoxic waters, sea water, fresh water or groundwater that are depleted of dissolved ox ...

sea during the

Late Cretaceous The Late Cretaceous (100.5–66 Ma) is the more recent of two epochs into which the Cretaceous Period is divided in the geologic time scale. Rock strata from this epoch form the Upper Cretaceous Series. The Cretaceous is named after ''cre ...

Tertiary Tertiary (from Latin, meaning 'third' or 'of the third degree/order..') may refer to: * Tertiary period, an obsolete geologic period spanning from 66 to 2.6 million years ago * Tertiary (chemistry), a term describing bonding patterns in organic ch ...

. These sediments, along with the

carbonate and shallow marine deposits, form the

source Source may refer to: Research * Historical document * Historical source * Source (intelligence) or sub source, typically a confidential provider of non open-source intelligence * Source (journalism), a person, publication, publishing institute ...

and

reservoir rock A petroleum reservoir or oil and gas reservoir is a subsurface accumulation of hydrocarbons contained in porous or fractured rock formations. Such reservoirs form when kerogen (ancient plant matter) is created in surrounding rock by the presenc ...

throughout the ANS

Stratigraphy

The

stratigraphy Stratigraphy is a branch of geology concerned with the study of rock layers (strata) and layering (stratification). It is primarily used in the study of sedimentary and layered volcanic rocks. Stratigraphy has three related subfields: lithost ...

of the ANS is broken up into three major sequences the Ellesmerian, Beaufortian, and Brookian. ANS StratColumn

Ellesmerian Sequence

The Ellesmerian Sequence was deposited from the

Triassic The Triassic ( ; sometimes symbolized 🝈) is a geologic period and system which spans 50.5 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.4 Mya. The Triassic is t ...

. Carbonate and shallow marine deposits accumulated on a southern facing

. The Ellesmerian sequence thickens towards the south into the Colville Trough and west into the Hannah Trough, located in the

. The Ellesmerian sequence hosts important

reservoir A reservoir (; ) is an enlarged lake behind a dam, usually built to water storage, store fresh water, often doubling for hydroelectric power generation. Reservoirs are created by controlling a watercourse that drains an existing body of wa ...

and

source rock In petroleum geology, source rock is a sedimentary rock which has generated hydrocarbons or which has the potential to generate hydrocarbons. Source rocks are one of the necessary elements of a working petroleum system. They are organic-rich sedim ...

units that are exploited throughout the region for

hydrocarbon In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and Hydrophobe, hydrophobic; their odor is usually fain ...

s. The Lisburne Group is a carbonate dominated formation that has great

potential as well as possible sourcing layers. The most important and actively producing formation is the Sadlerochit. It is a

sandstone Sandstone is a Clastic rock#Sedimentary clastic rocks, clastic sedimentary rock composed mainly of grain size, sand-sized (0.0625 to 2 mm) silicate mineral, silicate grains, Cementation (geology), cemented together by another mineral. Sand ...

dominated

that the

Prudhoe Bay Prudhoe Bay is a town located in North Slope Borough in the U.S. state of Alaska. As of the 2020 census, the population of the CDP was 1,310 people, down from 2,174 residents in the 2010 census, and up from just 5 residents in 2000; however ...

field gets the most

production from.

Beaufortian Sequence

The Beaufortian Sequence was deposited from the

. This sequence of deposition is dominated by

shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of Clay mineral, clay minerals (hydrous aluminium phyllosilicates, e.g., Kaolinite, kaolin, aluminium, Al2Silicon, Si2Oxygen, O5(hydroxide, OH)4) and tiny f ...

and consists of syn-rift deposits supplied by local

erosion Erosion is the action of surface processes (such as Surface runoff, water flow or wind) that removes soil, Rock (geology), rock, or dissolved material from one location on the Earth's crust#Crust, Earth's crust and then sediment transport, tran ...

. There are multiple

unconformities An unconformity is a buried erosion surface, erosional or non-depositional surface separating two Rock (geology), rock masses or Stratum, strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer ...

present throughout this sequence. The most important is the

Unconformity An unconformity is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval ...

(LCU). The LCU is an important migration pathway for regional

s and is capped by

mudstone Mudstone, a type of mudrock, is a fine-grained sedimentary rock whose original constituents were clays or muds. Mudstone is distinguished from ''shale'' by its lack of fissility.Blatt, H., and R.J. Tracy, 1996, ''Petrology.'' New York, New York, ...

s that act as a

stratigraphic trap 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 tra ...

. The most important

formations found in the Beaufortian sequence are the Kingak Shale and the Pebble Shale. The Pebble Shale Unit caps the LCU and is also a great hydrocarbon source unit.

Brookian Sequence

The Brookian Sequence was deposited from the

to the

. This sequence is dominated by

siliciclastic Siliciclastic (or ''siliclastic'') rocks are clastic noncarbonate sedimentary rocks that are composed primarily of silicate minerals, such as quartz or clay minerals. Siliciclastic rock types include mudrock, sandstone Sandstone is a Clastic ...

deposits sourced by both shallow marine and terrestrial deposits. This sequence is the thickest deposit of sediment and it coincides with the formation of the

. Multiple

units as well as

sources are present throughout. The Brookian sequence hosts multiple important formations for

exploration like the Hue Shale, Torok, Nanushuk, Seabee, and Schrader Bluff. The

Torok Formation The Torok Formation is a geologic formation in the National Petroleum Reserve in Alaska (NPR-A). It preserves fossils dating back to the Cretaceous period. Geology The Torok Formation lies 2,000–4,000 feet deeper than the Nanushuk Formation. T ...

was deposited on the floor of the deep basin.

Hydrocarbon production

The Alaskan North Slope is one of the most prolific oil producing basins in North America. It dominates the Arctic in oil production with about 15 billion barrels produced, 12 billion barrels coming from the

field. Most of the production comes from the coastal area along the Barrow Arch where

s are structurally and

stratigraphically Stratigraphy is a branch of geology concerned with the study of rock layers (strata) and layering (stratification). It is primarily used in the study of sedimentary and layered volcanic rocks. Stratigraphy has three related subfields: lithostrati ...

trapped migrating up the LCU. New fields are being found further from the structure associated with the Barrow Arch and they are more

trapped by

Cenozoic The Cenozoic Era ( ; ) is Earth's current geological era, representing the last 66million years of Earth's history. It is characterized by the dominance of mammals, insects, birds and angiosperms (flowering plants). It is the latest of three g ...

deposits. As of 2006, the top three oil-producing fields in the Alaskan North Slope were: *

- 12 billion bbl *

Kuparuk River The Kuparuk River ( Iñupiaq: ''Kuukpaaġruk'') is a river in Alaska's North Slope that enters a bay on the Beaufort Sea between Beechey Point and Prudhoe Bay. The north-flowing river is about long, and its delta is about wide. Its Eskimo name ...

- 2 billion bbl * Duck Island- 446 million bbl As of 2006, the majority of the oil produced came from reserves in the Ellesmerian sequence. The central part of the location dominated production, specifically the Prudhoe Bay field, where there has been much

structural trap In petroleum geology, a trap is a geological structure affecting the reservoir rock and caprock of a petroleum system allowing the accumulation of Hydrocarbon, hydrocarbons in a petroleum reservoir, reservoir. Traps can be of two types: stratigra ...

ping associated to the

ing and formation of the Barrow Arch. Oil pools continue to be discovered further from the arch and are predominantly stratigraphically trapped. Most of them were sourced by hydrocarbon reserves from the Beaufortian sequence. After 2013, discoveries included pools into the Brookian sequence. In 2016, Caelus Energy drilled 2 wells it called "Tulimaniq" into a large Brookian submarine fan complex spanning more than 300 square miles.

References

{{reflist Geography of North Slope Borough, Alaska Geology of Alaska