The Leduc Formation is a
stratigraphic
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 ...
unit of
Late Devonian
The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, wher ...
(
Frasnian
The Frasnian is one of two faunal stages in the Late Devonian Period. It lasted from million years ago to million years ago. It was preceded by the Givetian Stage and followed by the Famennian Stage.
Major reef-building was under way during th ...
)
age
Age or AGE may refer to:
Time and its effects
* Age, the amount of time someone or something has been alive or has existed
** East Asian age reckoning, an Asian system of marking age starting at 1
* Ageing or aging, the process of becoming older ...
in the
Western Canada Sedimentary Basin
The Western Canadian Sedimentary Basin (WCSB) underlies of Western Canada including southwestern Manitoba, southern Saskatchewan, Alberta, northeastern British Columbia and the southwest corner of the Northwest Territories. This vast sedimentary ...
.
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
Central Alberta is a region located in the Canadian province of Alberta.
Central Alberta is the most densely populated rural area in the province. Agriculture and energy are important to the area's economy.
Geography
Central Alberta is bordere ...
, between the depths of and , 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 and .
[Glass, D.J. (editor) 1997. Lexicon of Canadian Stratigraphy, vol. 4, Western Canada including eastern British Columbia, Alberta, Saskatchewan and southern Manitoba. Canadian Society of Petroleum Geologists, Calgary, 1423 p. on CD-ROM. .]
The Leduc Formation is a major source of oil
An oil is any nonpolar chemical substance that is composed primarily of hydrocarbons and is hydrophobic (does not mix with water) & lipophilic (mixes with other oils). Oils are usually flammable and surface active. Most oils are unsaturated ...
and gas
Gas is one of the four fundamental states of matter (the others being solid, liquid, and plasma).
A pure gas may be made up of individual atoms (e.g. a noble gas like neon), elemental molecules made from one type of atom (e.g. oxygen), or ...
in central Alberta, and the drilling of the highly successful Leduc No. 1
Leduc No. 1 was a major crude oil discovery made near Leduc, Alberta, Canada on February 13, 1947. It provided the geological key to Alberta's most prolific conventional oil reserves and resulted in a boom in petroleum exploration and developmen ...
well in 1947 ushered in a new era in the Western Canadian petroleum industry.
Lithology
The Leduc Formation consists of fossil reefs
A reef is a ridge or shoal of rock, coral or similar relatively stable material, lying beneath the surface of a natural body of water. Many reefs result from natural, abiotic processes— deposition of sand, wave erosion planing down rock out ...
that are highly porous
Porosity or void fraction is a measure of the void (i.e. "empty") spaces in a material, and is a fraction of the volume of voids over the total volume, between 0 and 1, or as a percentage between 0% and 100%. Strictly speaking, some tests measure ...
, which makes them excellent reservoirs
A reservoir (; from French ''réservoir'' ) is an enlarged lake behind a dam. Such a dam may be either artificial, built to store fresh water or it may be a natural formation.
Reservoirs can be created in a number of ways, including control ...
for oil and gas. They were deposited as limestone
Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...
and 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 (parallel layering).Blatt, H., and R.J. Tracy, 1996, ''Petrology. ...
in shallow water reef environments. Stromatoporoids were the primary reef-building organisms, and rock-types range from skeletal 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 (parallel layering).Blatt, H., and R.J. Tracy, 1996, ''Petrology. ...
s and floatstone
Floatstone is a type of carbonate rock.
The original Dunham classification (Dunham, 1962) of limestone
Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed m ...
s to finer grained muddy packstone
Under the Dunham classification (Dunham, 1962Dunham, R.J. (1962) Classification of carbonate rocks according to depositional texture. In: Classification of Carbonate Rocks (Ed. W.E. Ham), Am. Assoc. Pet. Geol. Mem., 1, 108–121.) system of limesto ...
s and wackestone
Under the Dunham classification (Dunham, 1962Dunham, R.J., 1962. Classification of carbonate rocks according to depositional texture. In: W.E. Ham (Ed.), Classification of Carbonate Rocks. American Association of Petroleum Geologists Memoir. Amer ...
s.
Many, but not all, of the reefs were later subjected to dolomitization
Dolomitization is a geological process by which the carbonate mineral dolomite is formed when magnesium ions replace calcium ions in another carbonate mineral, calcite. It is common for this mineral alteration into dolomite to take place due to ev ...
during diagenesis
Diagenesis () is the process that describes physical and chemical changes in sediments first caused by water-rock interactions, microbial activity, and compaction after their deposition. Increased pressure and temperature only start to play a ...
, which increased their porosity, and they now consist of dolomite Dolomite may refer to:
*Dolomite (mineral), a carbonate mineral
*Dolomite (rock), also known as dolostone, a sedimentary carbonate rock
*Dolomite, Alabama, United States, an unincorporated community
*Dolomite, California, United States, an unincor ...
rather than limestone. The dolomitization that took place in the region has increased the porosity primarily in the more deeply buried lagoonal back reef facies. The pre-existing porosity has also been preserved well due to the dolomitization. Porosity in the region is dominated by vuggy, moldic, intercrystalline, as well as fracture
Fracture is the separation of an object or material into two or more pieces under the action of stress. The fracture of a solid usually occurs due to the development of certain displacement discontinuity surfaces within the solid. If a displa ...
types of porosity. Generally, the mean porosity of the Leduc Formation is 5.2%, with the permeability of the rocks in the region being an even spread.
Anhydrite
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 ...
is also common in the Leduc Formation, along with the replacement dolomitization.
Oil and gas production
The Leduc Formation is a major source of oil and natural gas in central Alberta. The Leduc No. 1 well drilled in 1947 produced 50 thousand cubic metres (more than 300 thousand barrels) of oil, marking the beginning of the post-war Albertan oil boom, and contributed to a large population boom in the cities of Calgary
Calgary ( ) is the largest city in the western Canadian province of Alberta and the largest metro area of the three Prairie Provinces. As of 2021, the city proper had a population of 1,306,784 and a metropolitan population of 1,481,806, makin ...
and Edmonton
Edmonton ( ) is the capital city of the Canadian province of Alberta. Edmonton is situated on the North Saskatchewan River and is the centre of the Edmonton Metropolitan Region, which is surrounded by Alberta's central region. The city ancho ...
. ["Woods, Hon. Lt-Col James Hossack, (12 July 1867–20 May 1941), Pres. Calgary Herald", ''Who Was Who'', Oxford University Press, 2007-12-01, retrieved 2019-04-08] The discovery and subsequent production from the wells also led to an economic boom in Alberta, which now puts Calgary among the forefront of producers of oil in North America.
The Strachan and Ricinus West gas fields, discovered in 1967 and 1969, are also in Late Devonian
The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, whe ...
Leduc-age reefs. The reefs were found using seismic common-depth point (CDP) techniques, which were being developed and used in the Western Canada Sedimentary Basin. The well that was crucial to the discovery of these two formations was drilled in 1955, and yielded gas as well as some salt water. Reef buildup of the Strachan and Ricinus gas field reefs are and respectively.
Distribution and thickness
The Leduc Formation occurs as discrete, discontinuous reef "buildups" in a line following the Woodbend shelf margin from Drumheller
Drumheller is a town on the Red Deer River in the badlands of Central Alberta, east-central Alberta, Canada. It is northeast of Calgary and south of Stettler, Alberta, Stettler. The Drumheller portion of the Red Deer River valley, often ref ...
in central Alberta to the Peace River
The Peace River (french: links=no, rivière de la Paix) is a river in Canada that originates in the Rocky Mountains of northern British Columbia and flows to the northeast through northern Alberta. The Peace River joins the Athabasca River in th ...
Arch area in northern Alberta
Northern Alberta is a geographic region located in the Canadian province of Alberta.
An informally defined cultural region, the boundaries of Northern Alberta are not fixed. Under some schemes, the region encompasses everything north of the cent ...
. The formation is absent in inter-reef areas, and buildups can reach from to in thickness.
Relationship to other units
In central Alberta the Leduc Formation conformably overlies the platform limestones and dolomites of the Cooking Lake and Beaverhill Lake Formations. In northern Alberta near the Peace River Arch it rests on older red bed
Red beds (or redbeds) are sedimentary rocks, typically consisting of sandstone, siltstone, and shale, that are predominantly red in color due to the presence of ferric oxides. Frequently, these red-colored sedimentary strata locally contain th ...
s or on the Granite Wash. The Leduc reefs are surrounded by shales
Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especiall ...
of the Duvernay and Ireton Formations and the Woodbend Group
The Woodbend Group is a stratigraphical unit of Frasnian age in the Western Canadian Sedimentary Basin.
It was first described in the ''British American Pyrcz No. 1'' well by Imperial Oil geological staff in 1950.
Lithology
The Formation is co ...
that were deposited in non-reefal, open marine environments.
The Leduc reefs are the same ages as, but not contiguous with, the reefs of the Cairn Formation
The Cairn Formation is a geologic formation of Late Devonian (Frasnian) age in the Western Canada Sedimentary Basin. It was named for the Cairn River near its junction with the Southesk River in Jasper National Park by D.J. McLaren in 1955.McLaren, ...
farther west. Because the Leduc reefs are not exposed at the surface, the extensive outcrop
An outcrop or rocky outcrop is a visible exposure of bedrock or ancient superficial deposits on the surface of the Earth.
Features
Outcrops do not cover the majority of the Earth's land surface because in most places the bedrock or superficial ...
s of the Cairn reefs in the Canadian Rockies
The Canadian Rockies (french: Rocheuses canadiennes) or Canadian Rocky Mountains, comprising both the Alberta Rockies and the British Columbian Rockies, is the Canadian segment of the North American Rocky Mountains. It is the easternmost part ...
have been studied to increase the understanding of the Leduc reefs.[Bloy, G.R., Hunter, I.G. and Leggett, S.R. 1984. Depositional environments of the Devonian Cairn Formation, Rocky Mountain front ranges, Canmore, Alberta. Carbonates in subsurface and outcrop: 1984 CSPG Core Conference, Canadian Society of Petroleum Geologists, p. 148.]
Near the Leduc Formation there is also the Swan Hills Formation. These two formations hold some similarities, such as the types of rocks and certain diagenetic processes. Rock types in both regions are dominated by limestones and dolomites. Dolomitization has taken place in both formations however it is more dominant in the Leduc Formation. Evaporites
An evaporite () is a water-soluble sedimentary mineral deposit 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 ...
such as Anhydrites are also more present in the Leduc Formation, however they are also present in the Swan Hills Formation.
The Swan Hills Formation also holds some differences to the Leduc Formation. The porosity types in both formations differ, and the types of fossilized biota also differ. Vuggy, moldic, intercrystalline, and fracture porosities are present in the Leduc Formation whereas the primary porosities in the Swan Hills Formation are interparticle and interfossil. One of the primary fossilized biota in the Leduc Formation are stromatoporoids, whereas the primary fossil type in the Swan Hills Formation are Amphipora.
Fossil content
The following fossils have been reported from the formation:[Leduc Formation]
at Fossilworks
Fossilworks is a portal which provides query, download, and analysis tools to facilitate access to the Paleobiology Database
The Paleobiology Database is an online resource for information on the distribution and classification of fossil animals ...
.org
;Bivalves
* '' Megalodon sp.''
* '' Modiomorpha sp.''
* '' Turbinopsis sp.''
;Brachiopods
* '' Conocardium sp.''
* '' Cranaena sp.''
* '' Schizophoria sp.''
;Gastropods
* '' Cypricardinia sp.''
* '' Leptodesma (Leiopteria) sp.''
* Archaeogastropoda indet.
;Corals
* '' Actinostroma clathratum''
* '' A. redwaterense''
* '' Anostylostroma laxum''
* '' Atelodictyon cf. stelliferum''
* '' Euryamphipora platyformis''
* '' Ferestromatopora dubia''
* '' Hammatostroma nodosum''
* '' Macgeea parva''
* '' Peneckiella floydensis''
* '' Stachyodes costulata''
* '' Stromatopora cygnea''
* '' Stromatoporella cf. subvesiculosa''
* '' Stromatoporella damnoniensis''
* '' Stromatoporella cf. mirabilis''
* '' Synthetostroma vesiculosum''
* '' Trupetostroma warreni''
* '' Trupetostroma cf. coalescens''
* '' Alveolites sp.''
* '' Amphipora sp.''
* '' Charactophyllum sp.''
* '' Macgeea sp.''
* '' Metriophyllum sp.''
* '' Phacellophyllum sp.''
* '' Phillipsastrea sp.''
* '' Stachyodes sp.''
* '' Syringoporella sp.''
* '' Syringopora sp.''
* '' Tabulophyllum sp.''
* '' Thamnopora sp.''
* '' Zaphrentis sp.''
* '' ?Trupetostroma sp.''
References
{{Western Canadian Sedimentary Basin, Northwest_Plains=yes, South AB=yes
Geologic formations of Alberta
Devonian Alberta
Frasnian Stage
Dolomite formations
Limestone formations
Reef deposits
Shallow marine deposits
Reservoir rock formations
Devonian southern paleotropical deposits
Western Canadian Sedimentary Basin
Fossiliferous stratigraphic units of North America
Paleontology in Alberta