Chalk Formation
Encyclopedia
The Chalk Group is a lithostratigraphic unit
(a certain number of rock strata) in the northwestern part of Europe
. It is characterised by thick deposits of chalk
, a soft porous white limestone
, deposited in a marine environment during the Upper Cretaceous
period.
Chalk is a limestone that consists of coccolith
biomicrite. A biomicrite is a limestone
composed of fossil
debris ("bio") and calcium carbonate
mud ("micrite"). The majority of the fossil debris comprising chalk consists of the microscopic plates, which are called coccoliths, of microscopic green algae known as coccolithophores. In addition to the coccoliths, the fossil debris includes a variable, but minor, percentage of the fragments of foraminifera
, ostracod
s and mollusks. The coccolithophore
s lived in the upper part of the water column. When they died, the microscopic calcium carbonate
plates, which formed their shells settled downward through the ocean water and accumulated on the ocean bottom to form a thick layer of calcareous ooze
, which eventually became the Chalk Group.
The Chalk Group usually shows few signs of bedding, other than lines of flint
nodules which become common in the upper part. Nodules of the mineral pyrite
also occur and are usually oxidized to brown iron oxide
on exposed surfaces.
, the southern cliffs on the Isle of Wight
and the quarries
and motorway cuttings at Blue Bell Hill, Kent
, (which has been classified as a Site of Special Scientific Interest) and at the Stokenchurch Gap
on the Oxfordshire
/Buckinghamshire
border where the M40 motorway
cuts through the Aston Rowant NNR
.
The Lower Chalk is usually relatively soft and greyish in colour. It is also the most fossiliferous (especially for ammonite
fossils). The Lower Chalk strata
usually begins with marl called the Glauconitic or Chloritic Marl
, named after the grains of the green mineral
s glauconite
and chlorite
which it contains. The remainder of the Lower Chalk is argillaceous
in its lower part (the Chalk Marl) and becomes progressively purer above. In the central Chilterns
, the two parts are separated by the hard Totternhoe Stone
, which forms a prominent scarp in some places. There are few, if any, flint nodules present. The thickness of the Lower Chalk strata varies, averaging around 200 feet (60 m), depending upon the location. The Lower Chalk often contains fossils such as the ammonites Schloenbachia, Scaphites, and Mantelliceras, the belemnite
Actinocamax
, and the bivalves
Inoceramus and Ostrea.
The Middle Chalk averages about 200 feet (60 m) in thickness. Fossils found in the Middle Chalk include the brachiopod
Terebratulina and the echinoid
Conulus. However, though fossils have been found, they are generally sparse.
The Upper Chalk by comparison is softer than the Middle Chalk and the flint nodules it contains are far more abundant in the South of England
, although in Yorkshire
the Middle Chalk has the highest concentration of flints. It may contain ammonite and gastropod fossils in some nodular layers. The thickness of the Upper Chalk strata varies greatly, often averaging around 300 feet (95 m). In the Upper Chalk fossils may be abundant and include the bivalve
Spondylus, the brachiopod
s Terebratulina and Gibbithyris, the echinoids
Sternotaxis, Micraster, Echinocorys, and Tylocidaris, the crinoid
Marsupites, and the small sponge
Porosphaera.
The youngest beds of the Upper Chalk formation in England
are found on the coast of Norfolk
. Other fossils commonly found in the Chalk Formation include: solitary coral
s (such as Parasmilia), marine worm tubes (such as Rotularia), bryozoans, scattered fragments of starfish and fish remains (including shark
teeth such as Cretolamna and Squalicorax
).
linking England
and France
was constructed by tunnelling through the Chalk Marl (a prominent formation of the Lower Chalk).
parts of the Chalk Group are basically continuous and crop out as a slightly northwest dipping monocline
in a belt from the German
city of Aachen
to the city of Mons
, where they join Cretaceous deposits of the Paris Basin
. North of Namur
the Cretaceous is overlain by younger Paleocene
and Eocene
deposits of the Landen Group
.
In the Low Countries
, the Chalk Group is divided into five formations, from top to base:
In Belgium, the Houthem Formation is sometimes not included in the Chalk Group because it is not a Cretaceous formation. Some stratigraphers therefore prefer to put it in the lower Paleogene
Hesbaye Group
.
in the North Sea Central Graben, mainly in Norwegian
and Danish
sectors and to a lesser extent in the British
sector (UKCS).
Across the north central and northern North Sea, the Chalk Group is a major seal unit, overlying a number of blocks of reservoir rocks and preventing their fluid contents from migrating upwards. North of the line of the Mid-North Sea - Ringkobing - Fyn structural high, the Chalk Group is still recognisable in drilled samples, but becomes increasingly muddy northwards. North of the Beryl Embayment (59°30' N 01°30'E), the Chalk Group is a series of slightly to moderately calcareous mudstones grouped under the name of the Shetland Group. With the exception of some thin sandy units in the inner Moray Firth
, this sequence has neither source potential nor reservoir capcaity and is not generally considered a drilling target. Its thickness and homogeneity does make it a common target for carrying out directional drilling
manoeuvers.
In the Shearwater and ETAP areas (around 56°30' N 02°30'E , UKCS quadrants 22,23,29 and 30), the Chalk Group can be significantly overpressured. Further south in UKCS quadrant 30 and Norwegian quandrants 1 and 2, this overpressure helps preserve porosity and enables the chalk to be an effective reservoir.
are redeposited allochthon
ous beds. These include debris flows and turbidite
flows. Porosities
can be very high when preserved from diagenesis
by early hydrocarbon
charge. However, when these hydrocarbons are produced, diagenesis and compaction can re-start which has led to several metres of subsidence at seabed, the collapse of a number of wells, and some extremely expensive remedial work to lift the platforms and re-position them.
Lithostratigraphy
Lithostratigraphy is a sub-discipline of stratigraphy, the geological science associated with the study of strata or rock layers. Major focuses include geochronology, comparative geology, and petrology...
(a certain number of rock strata) in the northwestern part of Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...
. It is characterised by thick deposits of chalk
Chalk
Chalk is a soft, white, porous sedimentary rock, a form of limestone composed of the mineral calcite. Calcite is calcium carbonate or CaCO3. It forms under reasonably deep marine conditions from the gradual accumulation of minute calcite plates shed from micro-organisms called coccolithophores....
, a soft porous white limestone
Limestone
Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....
, deposited in a marine environment during the Upper Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...
period.
Chalk is a limestone that consists of coccolith
Coccolith
Coccoliths are individual plates of calcium carbonate formed by coccolithophores which are arranged around them in a coccosphere.- Formation and composition :...
biomicrite. A biomicrite is a limestone
Limestone
Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....
composed of fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
debris ("bio") and calcium carbonate
Calcium carbonate
Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...
mud ("micrite"). The majority of the fossil debris comprising chalk consists of the microscopic plates, which are called coccoliths, of microscopic green algae known as coccolithophores. In addition to the coccoliths, the fossil debris includes a variable, but minor, percentage of the fragments of foraminifera
Foraminifera
The Foraminifera , or forams for short, are a large group of amoeboid protists which are among the commonest plankton species. They have reticulating pseudopods, fine strands of cytoplasm that branch and merge to form a dynamic net...
, ostracod
Ostracod
Ostracoda is a class of the Crustacea, sometimes known as the seed shrimp because of their appearance. Some 65,000 species have been identified, grouped into several orders....
s and mollusks. The coccolithophore
Coccolithophore
Coccolithophores are single-celled algae, protists, and phytoplankton belonging to the division of haptophytes. They are distinguished by special calcium carbonate plates of uncertain function called coccoliths , which are important microfossils...
s lived in the upper part of the water column. When they died, the microscopic calcium carbonate
Calcium carbonate
Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...
plates, which formed their shells settled downward through the ocean water and accumulated on the ocean bottom to form a thick layer of calcareous ooze
Ooze
Ooze may refer to:* Ooze, class of pelagic sediments, or substance in it, at bottoms of lakes or oceans* Oozes are deep ocean sediments containing at least 30% biogenous material.Published games:* Titles:** Ooze, game of Action 52 series...
, which eventually became the Chalk Group.
The Chalk Group usually shows few signs of bedding, other than lines of flint
Flint
Flint is a hard, sedimentary cryptocrystalline form of the mineral quartz, categorized as a variety of chert. It occurs chiefly as nodules and masses in sedimentary rocks, such as chalks and limestones. Inside the nodule, flint is usually dark grey, black, green, white, or brown in colour, and...
nodules which become common in the upper part. Nodules of the mineral pyrite
Pyrite
The mineral pyrite, or iron pyrite, is an iron sulfide with the formula FeS2. This mineral's metallic luster and pale-to-normal, brass-yellow hue have earned it the nickname fool's gold because of its resemblance to gold...
also occur and are usually oxidized to brown iron oxide
Iron oxide
Iron oxides are chemical compounds composed of iron and oxygen. All together, there are sixteen known iron oxides and oxyhydroxides.Iron oxides and oxide-hydroxides are widespread in nature, play an important role in many geological and biological processes, and are widely utilized by humans, e.g.,...
on exposed surfaces.
England
In England, the Chalk Group is divided into three formations: the Upper Chalk, the Middle Chalk and the Lower Chalk. Famous outcrops include the White Cliffs of DoverWhite cliffs of Dover
The White Cliffs of Dover are cliffs which form part of the British coastline facing the Strait of Dover and France. The cliffs are part of the North Downs formation. The cliff face, which reaches up to , owes its striking façade to its composition of chalk accentuated by streaks of black flint...
, the southern cliffs on the Isle of Wight
Isle of Wight
The Isle of Wight is a county and the largest island of England, located in the English Channel, on average about 2–4 miles off the south coast of the county of Hampshire, separated from the mainland by a strait called the Solent...
and the quarries
Quarry
A quarry is a type of open-pit mine from which rock or minerals are extracted. Quarries are generally used for extracting building materials, such as dimension stone, construction aggregate, riprap, sand, and gravel. They are often collocated with concrete and asphalt plants due to the requirement...
and motorway cuttings at Blue Bell Hill, Kent
Kent
Kent is a county in southeast England, and is one of the home counties. It borders East Sussex, Surrey and Greater London and has a defined boundary with Essex in the middle of the Thames Estuary. The ceremonial county boundaries of Kent include the shire county of Kent and the unitary borough of...
, (which has been classified as a Site of Special Scientific Interest) and at the Stokenchurch Gap
Stokenchurch Gap
The Stokenchurch Gap, also known as the Aston Rowant Cutting or Aston Hill cutting , or locally "The Canyon" is a steep chalk cutting, constructed through the Chiltern Hills in Oxfordshire, England during the early 1970s for the M40 motorway...
on the Oxfordshire
Oxfordshire
Oxfordshire is a county in the South East region of England, bordering on Warwickshire and Northamptonshire , Buckinghamshire , Berkshire , Wiltshire and Gloucestershire ....
/Buckinghamshire
Buckinghamshire
Buckinghamshire is a ceremonial and non-metropolitan home county in South East England. The county town is Aylesbury, the largest town in the ceremonial county is Milton Keynes and largest town in the non-metropolitan county is High Wycombe....
border where the M40 motorway
M40 motorway
The M40 motorway is a motorway in the British transport network that forms a major part of the connection between London and Birmingham. Part of this road forms a section of the unsigned European route E05...
cuts through the Aston Rowant NNR
Aston Rowant NNR
Aston Rowant Nature Reserve is located on the western escarpment of the Chiltern Hills near Stokenchurch on the Oxfordshire - Buckinghamshire border in the Chilterns Area of Outstanding Natural Beauty The reserve is in several sections, divided by the M40 motorway in the Aston Rowant Cutting...
.
The Lower Chalk is usually relatively soft and greyish in colour. It is also the most fossiliferous (especially for ammonite
Ammonite
Ammonite, as a zoological or paleontological term, refers to any member of the Ammonoidea an extinct subclass within the Molluscan class Cephalopoda which are more closely related to living coleoids Ammonite, as a zoological or paleontological term, refers to any member of the Ammonoidea an extinct...
fossils). The Lower Chalk strata
Stratum
In geology and related fields, a stratum is a layer of sedimentary rock or soil with internally consistent characteristics that distinguish it from other layers...
usually begins with marl called the Glauconitic or Chloritic Marl
Marl
Marl or marlstone is a calcium carbonate or lime-rich mud or mudstone which contains variable amounts of clays and aragonite. Marl was originally an old term loosely applied to a variety of materials, most of which occur as loose, earthy deposits consisting chiefly of an intimate mixture of clay...
, named after the grains of the green mineral
Mineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...
s glauconite
Glauconite
Glauconite is an iron potassium phyllosilicate mineral of characteristic green color with very low weathering resistance and very friable.It crystallizes with a monoclinic geometry...
and chlorite
Chlorite group
The chlorites are a group of phyllosilicate minerals. Chlorites can be described by the following four endmembers based on their chemistry via substitution of the following four elements in the silicate lattice; Mg, Fe, Ni, and Mn....
which it contains. The remainder of the Lower Chalk is argillaceous
Argillaceous minerals
Argillaceous minerals may appear silvery upon optical reflection and are minerals containing substantial amounts of clay-like components . Argillaceous components are fine-grained aluminosilicates, and more particularly clay minerals such as kaolinite, montmorillonite-smectite, illite, and...
in its lower part (the Chalk Marl) and becomes progressively purer above. In the central Chilterns
Chiltern Hills
The Chiltern Hills form a chalk escarpment in South East England. They are known locally as "the Chilterns". A large portion of the hills was designated officially as an Area of Outstanding Natural Beauty in 1965.-Location:...
, the two parts are separated by the hard Totternhoe Stone
Totternhoe Stone
thumb|250 px|right|The characteristic checkerboard design of many Bedfordshire churches built with alternating chalk and flint blocks: [[St Mary's ]]....
, which forms a prominent scarp in some places. There are few, if any, flint nodules present. The thickness of the Lower Chalk strata varies, averaging around 200 feet (60 m), depending upon the location. The Lower Chalk often contains fossils such as the ammonites Schloenbachia, Scaphites, and Mantelliceras, the belemnite
Belemnoidea
Belemnoids are an extinct group of marine cephalopod, very similar in many ways to the modern squid and closely related to the modern cuttlefish. Like them, the belemnoids possessed an ink sac, but, unlike the squid, they possessed ten arms of roughly equal length, and no tentacles...
Actinocamax
Actinocamax
Actinocamax is a genus of belemnite, an extinct group of cephalopods....
, and the bivalves
Bivalvia
Bivalvia is a taxonomic class of marine and freshwater molluscs. This class includes clams, oysters, mussels, scallops, and many other families of molluscs that have two hinged shells...
Inoceramus and Ostrea.
The Middle Chalk averages about 200 feet (60 m) in thickness. Fossils found in the Middle Chalk include the brachiopod
Brachiopod
Brachiopods are a phylum of marine animals that have hard "valves" on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, while the front can be opened for feeding or closed for protection...
Terebratulina and the echinoid
Sea urchin
Sea urchins or urchins are small, spiny, globular animals which, with their close kin, such as sand dollars, constitute the class Echinoidea of the echinoderm phylum. They inhabit all oceans. Their shell, or "test", is round and spiny, typically from across. Common colors include black and dull...
Conulus. However, though fossils have been found, they are generally sparse.
The Upper Chalk by comparison is softer than the Middle Chalk and the flint nodules it contains are far more abundant in the South of England
Southern England
Southern England, the South and the South of England are imprecise terms used to refer to the southern counties of England bordering the English Midlands. It has a number of different interpretations of its geographic extents. The South is considered by many to be a cultural region with a distinct...
, although in Yorkshire
Yorkshire
Yorkshire is a historic county of northern England and the largest in the United Kingdom. Because of its great size in comparison to other English counties, functions have been increasingly undertaken over time by its subdivisions, which have also been subject to periodic reform...
the Middle Chalk has the highest concentration of flints. It may contain ammonite and gastropod fossils in some nodular layers. The thickness of the Upper Chalk strata varies greatly, often averaging around 300 feet (95 m). In the Upper Chalk fossils may be abundant and include the bivalve
Bivalvia
Bivalvia is a taxonomic class of marine and freshwater molluscs. This class includes clams, oysters, mussels, scallops, and many other families of molluscs that have two hinged shells...
Spondylus, the brachiopod
Brachiopod
Brachiopods are a phylum of marine animals that have hard "valves" on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, while the front can be opened for feeding or closed for protection...
s Terebratulina and Gibbithyris, the echinoids
Sea urchin
Sea urchins or urchins are small, spiny, globular animals which, with their close kin, such as sand dollars, constitute the class Echinoidea of the echinoderm phylum. They inhabit all oceans. Their shell, or "test", is round and spiny, typically from across. Common colors include black and dull...
Sternotaxis, Micraster, Echinocorys, and Tylocidaris, the crinoid
Crinoid
Crinoids are marine animals that make up the class Crinoidea of the echinoderms . Crinoidea comes from the Greek word krinon, "a lily", and eidos, "form". They live both in shallow water and in depths as great as 6,000 meters. Sea lilies refer to the crinoids which, in their adult form, are...
Marsupites, and the small sponge
Sea sponge
Sponges are animals of the phylum Porifera . Their bodies consist of jelly-like mesohyl sandwiched between two thin layers of cells. While all animals have unspecialized cells that can transform into specialized cells, sponges are unique in having some specialized cells, but can also have...
Porosphaera.
The youngest beds of the Upper Chalk formation in England
England
England is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...
are found on the coast of Norfolk
Norfolk
Norfolk is a low-lying county in the East of England. It has borders with Lincolnshire to the west, Cambridgeshire to the west and southwest and Suffolk to the south. Its northern and eastern boundaries are the North Sea coast and to the north-west the county is bordered by The Wash. The county...
. Other fossils commonly found in the Chalk Formation include: solitary coral
Coral
Corals are marine animals in class Anthozoa of phylum Cnidaria typically living in compact colonies of many identical individual "polyps". The group includes the important reef builders that inhabit tropical oceans and secrete calcium carbonate to form a hard skeleton.A coral "head" is a colony of...
s (such as Parasmilia), marine worm tubes (such as Rotularia), bryozoans, scattered fragments of starfish and fish remains (including shark
Shark
Sharks are a type of fish with a full cartilaginous skeleton and a highly streamlined body. The earliest known sharks date from more than 420 million years ago....
teeth such as Cretolamna and Squalicorax
Squalicorax
Squalicorax is a genus of extinct lamniform shark known to have lived during the Cretaceous period. A fully articulated 1.9 m long fossil skeleton of Squalicorax has been found in Kansas, evidence of its presence in the Western Interior Seaway...
).
The English Channel
The Channel TunnelChannel Tunnel
The Channel Tunnel is a undersea rail tunnel linking Folkestone, Kent in the United Kingdom with Coquelles, Pas-de-Calais near Calais in northern France beneath the English Channel at the Strait of Dover. At its lowest point, it is deep...
linking England
England
England is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...
and France
France
The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...
was constructed by tunnelling through the Chalk Marl (a prominent formation of the Lower Chalk).
The Low Countries
The Dutch and BelgianBelgium
Belgium , officially the Kingdom of Belgium, is a federal state in Western Europe. It is a founding member of the European Union and hosts the EU's headquarters, and those of several other major international organisations such as NATO.Belgium is also a member of, or affiliated to, many...
parts of the Chalk Group are basically continuous and crop out as a slightly northwest dipping monocline
Monocline
A monocline is a step-like fold in rock strata consisting of a zone of steeper dip within an otherwise horizontal or gently-dipping sequence.-Formation:Monoclines may be formed in several different ways...
in a belt from the German
Germany
Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...
city of Aachen
Aachen
Aachen has historically been a spa town in North Rhine-Westphalia, Germany. Aachen was a favoured residence of Charlemagne, and the place of coronation of the Kings of Germany. Geographically, Aachen is the westernmost town of Germany, located along its borders with Belgium and the Netherlands, ...
to the city of Mons
Mons
Mons is a Walloon city and municipality located in the Belgian province of Hainaut, of which it is the capital. The Mons municipality includes the old communes of Cuesmes, Flénu, Ghlin, Hyon, Nimy, Obourg, Baudour , Jemappes, Ciply, Harmignies, Harveng, Havré, Maisières, Mesvin, Nouvelles,...
, where they join Cretaceous deposits of the Paris Basin
Paris Basin
Paris Basin may refer to:*As a hydrological basin, it is largely the basin of the River Seine* Paris Basin , the geological basin...
. North of Namur
Namur (city)
Namur is a city and municipality in Wallonia, in southern Belgium. It is both the capital of the province of Namur and of Wallonia....
the Cretaceous is overlain by younger Paleocene
Paleocene
The Paleocene or Palaeocene, the "early recent", is a geologic epoch that lasted from about . It is the first epoch of the Palaeogene Period in the modern Cenozoic Era...
and Eocene
Eocene
The Eocene Epoch, lasting from about 56 to 34 million years ago , is a major division of the geologic timescale and the second epoch of the Paleogene Period in the Cenozoic Era. The Eocene spans the time from the end of the Palaeocene Epoch to the beginning of the Oligocene Epoch. The start of the...
deposits of the Landen Group
Landen Group
The Landen Group is a lithostratigraphic unit in the Belgian subsurface. The group consists of two formations of Thanetian to Ypresian age...
.
In the Low Countries
Low Countries
The Low Countries are the historical lands around the low-lying delta of the Rhine, Scheldt, and Meuse rivers, including the modern countries of Belgium, the Netherlands, Luxembourg and parts of northern France and western Germany....
, the Chalk Group is divided into five formations, from top to base:
- the Houthem FormationHouthem FormationThe Houthem Formation, named after the Dutch town of Houthem, is a geological formation that crops out in the south of Belgian and Dutch Limburg. It has also been found in borings in the northeastern part of the Campine Basin...
, consisting of Paleocene calcareous arenites; - the Maastricht FormationMaastricht FormationThe Maastricht Formation , named after the city of Maastricht, the Netherlands, is a geological formation in the Netherlands and Belgium whose strata date back to the Late Cretaceous, within 500k years of the K-Pg boundary, dated to 65.5 million years ago. The formation is part of the Chalk Group...
, consisting of MaastrichtianMaastrichtianThe Maastrichtian is, in the ICS' geologic timescale, the latest age or upper stage of the Late Cretaceous epoch or Upper Cretaceous series, the Cretaceous period or system, and of the Mesozoic era or erathem. It spanned from 70.6 ± 0.6 Ma to 65.5 ± 0.3 Ma...
chalk and calcareous areniteAreniteArenite is a sedimentary clastic rock with sand grain size between 0.0625 mm and 2 mm and contain less than 15% matrix. The related adjective is arenaceous...
s; - the Gulpen Formation, consisting of CampanianCampanianThe Campanian is, in the ICS' geologic timescale, the fifth of six ages of the Late Cretaceous epoch . The Campanian spans the time from 83.5 ± 0.7 Ma to 70.6 ± 0.6 Ma ...
to Maastrichtian chalk; - the Vaals Formation, consisting of alternating Campanian clays, glauconiferous sands and silts;
- the Aachen FormationAachen FormationThe Aachen Formation is a geologic formation in the southern Netherlands and northern Belgium. It is part of the Chalk Group and named after the German city of Aachen.The formation crops out in southern Belgian and Dutch Limburg and adjacent areas in Germany...
, consisting of SantonianSantonianThe Santonian is an age in the geologic timescale or a chronostratigraphic stage. It is a subdivision of the Late Cretaceous epoch or Upper Cretaceous series. It spans the time between 85.8 ± 0.7 mya and 83.5 ± 0.7 mya...
glauconiferous sands and silts.
In Belgium, the Houthem Formation is sometimes not included in the Chalk Group because it is not a Cretaceous formation. Some stratigraphers therefore prefer to put it in the lower Paleogene
Paleogene
The Paleogene is a geologic period and system that began 65.5 ± 0.3 and ended 23.03 ± 0.05 million years ago and comprises the first part of the Cenozoic Era...
Hesbaye Group
Hesbaye Group
The Hesbaye Group is a stratigraphic group in the subsurface of northeastern Belgium. This group was deposited during the early Paleocene epoch and is subdivided into two formations: the Houthem Formation and the Opglabbeek Formation...
.
The North Sea
The chalk is also an important petroleum reservoirOil reservoir
A petroleum reservoir, or oil and gas reservoir, is a subsurface pool of hydrocarbons contained in porous or fractured rock formations. The naturally occurring hydrocarbons, such as crude oil or natural gas, are trapped by overlying rock formations with lower permeability...
in the North Sea Central Graben, mainly in Norwegian
Norway
Norway , officially the Kingdom of Norway, is a Nordic unitary constitutional monarchy whose territory comprises the western portion of the Scandinavian Peninsula, Jan Mayen, and the Arctic archipelago of Svalbard and Bouvet Island. Norway has a total area of and a population of about 4.9 million...
and Danish
Denmark
Denmark is a Scandinavian country in Northern Europe. The countries of Denmark and Greenland, as well as the Faroe Islands, constitute the Kingdom of Denmark . It is the southernmost of the Nordic countries, southwest of Sweden and south of Norway, and bordered to the south by Germany. Denmark...
sectors and to a lesser extent in the British
United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...
sector (UKCS).
Across the north central and northern North Sea, the Chalk Group is a major seal unit, overlying a number of blocks of reservoir rocks and preventing their fluid contents from migrating upwards. North of the line of the Mid-North Sea - Ringkobing - Fyn structural high, the Chalk Group is still recognisable in drilled samples, but becomes increasingly muddy northwards. North of the Beryl Embayment (59°30' N 01°30'E), the Chalk Group is a series of slightly to moderately calcareous mudstones grouped under the name of the Shetland Group. With the exception of some thin sandy units in the inner Moray Firth
Moray Firth
The Moray Firth is a roughly triangular inlet of the North Sea, north and east of Inverness, which is in the Highland council area of north of Scotland...
, this sequence has neither source potential nor reservoir capcaity and is not generally considered a drilling target. Its thickness and homogeneity does make it a common target for carrying out directional drilling
Directional drilling
Directional drilling is the practice of drilling non-vertical wells. It can be broken down into three main groups: Oilfield Directional Drilling, Utility Installation Directional Drilling Directional drilling (or slant drilling) is the practice of drilling non-vertical wells. It can be broken down...
manoeuvers.
In the Shearwater and ETAP areas (around 56°30' N 02°30'E , UKCS quadrants 22,23,29 and 30), the Chalk Group can be significantly overpressured. Further south in UKCS quadrant 30 and Norwegian quandrants 1 and 2, this overpressure helps preserve porosity and enables the chalk to be an effective reservoir.
Reservoir stratigraphy
- Chalk Unit 6 - Ekofisk formation - DanianDanianThe Danian is the oldest age or lowermost stage of the Paleocene epoch or series, the Paleogene period or system and the Cenozoic era or erathem. The beginning of the Danian age is at the Cretaceous-Tertiary extinction event...
age (major reservoir in the Ekofisk Field and others) - Chalk Unit 5 - Tor formation - MaastrichtianMaastrichtianThe Maastrichtian is, in the ICS' geologic timescale, the latest age or upper stage of the Late Cretaceous epoch or Upper Cretaceous series, the Cretaceous period or system, and of the Mesozoic era or erathem. It spanned from 70.6 ± 0.6 Ma to 65.5 ± 0.3 Ma...
age, (major reservoir in many fields including Joanne oil filed (UKCS), Valhall oil field (NCS)) - Chalk Unit 4 - Hod formation - Late CampanianCampanianThe Campanian is, in the ICS' geologic timescale, the fifth of six ages of the Late Cretaceous epoch . The Campanian spans the time from 83.5 ± 0.7 Ma to 70.6 ± 0.6 Ma ...
age (mostly non reservoir) - Chalk Unit 3 - Hod formation - SantonianSantonianThe Santonian is an age in the geologic timescale or a chronostratigraphic stage. It is a subdivision of the Late Cretaceous epoch or Upper Cretaceous series. It spans the time between 85.8 ± 0.7 mya and 83.5 ± 0.7 mya...
to Middle Campanian - Chalk Unit 2 - Hod formation - Middle TuronianTuronianThe Turonian is, in the ICS' geologic timescale, the second age in the Late Cretaceous epoch, or a stage in the Upper Cretaceous series. It spans the time between 93.5 ± 0.8 Ma and 89.3 ± 1 Ma...
to ConiacianConiacianThe Coniacian is an age or stage in the geologic timescale. It is a subdivision of the Late Cretaceous epoch or Upper Cretaceous series and spans the time between 89.3 ± 1 Ma and 85.8 ± 0.7 Ma... - Turonian shale - Early Turonian (non reservoir)
- Plenus Marl - Late CenomanianCenomanianThe Cenomanian is, in the ICS' geological timescale the oldest or earliest age of the Late Cretaceous epoch or the lowest stage of the Upper Cretaceous series. An age is a unit of geochronology: it is a unit of time; the stage is a unit in the stratigraphic column deposited during the corresponding...
(non reservoir) - Chalk Unit 1 - Hidra Formation - Cenomanian (non reservoir)
Reservoir geology
The majority of Chalk reservoirsOil reservoir
A petroleum reservoir, or oil and gas reservoir, is a subsurface pool of hydrocarbons contained in porous or fractured rock formations. The naturally occurring hydrocarbons, such as crude oil or natural gas, are trapped by overlying rock formations with lower permeability...
are redeposited allochthon
Allochthon
thumb|right|250px|Schematic overview of a thrust system. The hanging wall block is called a [[nappe]]. If an [[erosion]]al hole is created in the nappe that is called a [[window |window]]...
ous beds. These include debris flows and turbidite
Turbidite
Turbidite geological formations have their origins in turbidity current deposits, which are deposits from a form of underwater avalanche that are responsible for distributing vast amounts of clastic sediment into the deep ocean.-The ideal turbidite sequence:...
flows. Porosities
Porosity
Porosity or void fraction is a measure of the void spaces in a material, and is a fraction of the volume of voids over the total volume, between 0–1, or as a percentage between 0–100%...
can be very high when preserved from diagenesis
Diagenesis
In geology and oceanography, diagenesis is any chemical, physical, or biological change undergone by a sediment after its initial deposition and during and after its lithification, exclusive of surface alteration and metamorphism. These changes happen at relatively low temperatures and pressures...
by early hydrocarbon
Hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....
charge. However, when these hydrocarbons are produced, diagenesis and compaction can re-start which has led to several metres of subsidence at seabed, the collapse of a number of wells, and some extremely expensive remedial work to lift the platforms and re-position them.
See also
- Chalk streamChalk streamChalk streams have characteristics which set them apart from watercourses associated with other rock types.Aside from those with an interest in the geological and ecological disciplines, the term chalk stream is most widely used among a small group of fly fishermen ,...
- DownlandDownlandA downland is an area of open chalk hills. This term is especially used to describe the chalk countryside in southern England. Areas of downland are often referred to as Downs....
- Geology of the United Kingdom
- Southern England Chalk FormationSouthern England Chalk FormationThe Chalk Formation of Southern England is a system of chalk downland in the south of England. The formation is perhaps best known for Salisbury Plain, the location of Stonehenge, the Isle of Wight and the twin ridgeways of the North Downs and South Downs....