- Chalk Group
The Chalk Group (often just called the Chalk) 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, ostracods and mollusks. The coccolithophores 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.
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 Dover, 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 minerals 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 brachiopods 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 corals (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).
The English Channel
The Low Countries
The Dutch (Dutch: Krijtkalk-Groep) and Belgian (Dutch: Krijt-Groep) 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.
- the Houthem Formation, consisting of Paleocene calcareous arenites;
- the Maastricht Formation, consisting of Maastrichtian chalk and calcareous arenites;
- the Gulpen Formation, consisting of Campanian to Maastrichtian chalk;
- the Vaals Formation, consisting of alternating Campanian clays, glauconiferous sands and silts;
- the Aachen Formation, consisting of Santonian 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 Hesbaye Group.
The North Sea
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.
- Chalk Unit 6 - Ekofisk formation - Danian age (major reservoir in the Ekofisk Field and others)
- Chalk Unit 5 - Tor formation - Maastrichtian age, (major reservoir in many fields including Joanne oil filed (UKCS), Valhall oil field (NCS))
- Chalk Unit 4 - Hod formation - Late Campanian age (mostly non reservoir)
- Chalk Unit 3 - Hod formation - Santonian to Middle Campanian
- Chalk Unit 2 - Hod formation - Middle Turonian to Coniacian
- Turonian shale - Early Turonian (non reservoir)
- Plenus Marl - Late Cenomanian (non reservoir)
- Chalk Unit 1 - Hidra Formation - Cenomanian (non reservoir)
The majority of Chalk reservoirs are redeposited allochthonous 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.
References and further reading
- Hancock, J.M. (1975). The Petrology of the Chalk. London: Proceedings of the Geologists Association, vol 86. pp. 449–535.
- Harris, C.S. (2007). "Chalk Facts – website on everything Chalk". http://www.geologyshop.co.uk/chalk.htm.
- Lousley, J.E. (1969). Wild Flowers of Chalk and Limestone. London: Collins.
- Scholle, P.A.; Bedout, D.G., and Moore, C.H. (1983). Carbonate Depositional Environments. American Association of Petroleum Geologist Memoir 33.
- Smith, A.B.; Batten, D.J. (2002). Fossils of the Chalk (Second Edition ed.). The Palaeontological Association.
- ^ As discussed in Chalk Facts by C. S. Harris and Scholle et al. (1983)
- ^ Hack, Robert; Azzam, Rafig; Charlier, Robert; 2004: Engineering Geology for Infrastructure Planning in Europe, Lecture Notes in Earth Sciences 104, Springer, ISBN 978-3-540-21075-7, p. 491
- ^ Evans, D.; et al. (2003). The Millennium Atlas of the North Sea. Millennium Atlas Co.. Chapter 13, Upper Cretaceous, Chalk Group. ISBN 1-86239-119-X.
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