Chalk Formation

The Chalk Formations of Europe are thick deposits of chalk, a soft porous white limestone, deposited in a marine environment during the upper Cretaceous Period. They appear most prominently in England. The formations are divided into three parts: The Upper Chalk, the Middle Chalk, and the Lower Chalk. The famous White cliffs of Dover, England are a good example of a Chalk Formation deposit. Another good example displaying the sequence of the Chalk Formation are the southern cliffs on the Isle of Wight, England and the quarries and motorway cutting at Blue Bell Hill, Kent, England (which has been classified as a Site of Special Scientific Interest).

As discussed in [http://www.geologyshop.co.uk/chalk.htm Chalk Facts] by C. S. Harris and Scholle et al. (1983), the Chalk Formation consists mostly 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 this 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 Formation.

The Chalk Formation 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.

tratigraphy and paleontology

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").

Chalk petroleum reservoirs

The chalk is also an important petroleum reservoir in the North Sea Central Graben, mainly in Norwegian and Danish sectors and to a lesser extent in the United Kingdom 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.

Reservoir stratigraphy

*Chalk Unit [cite book
last = Evans
first = D.
coauthors = et al
editor =
title = The Millennium Atlas of the North Sea
url =
edition =
series =
date =
year = 2003
publisher = Millennium Atlas Co.
isbn = 1-86239-119-X
doi =
pages = Chapter 13, Upper Cretaceous, Chalk Group ] 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)

Reservoir geology

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.

ee also

* Chalk stream
* Downland
* Geology of the United Kingdom
* Southern England Chalk Formation

References and further reading

* cite web
last = Harris
first = C.S.
coauthors =
title = Chalk Facts – website on everything Chalk
year = 2007
publisher =
location =
url = http://www.geologyshop.co.uk/chalk.htm
doi =

* cite book
last = Lousley
first = J.E.
coauthors =
title = Wild Flowers of Chalk and Limestone
year = 1969
publisher = Collins
location = London
url =
doi =
pages =
isbn=

* cite book
last = Scholle
first = P.A.
coauthors = Bedout, D.G., and Moore, C.H.
title = Carbonate Depositional Environments
year = 1983
publisher = American Association of Petroleum Geologist Memoir 33
location =
url =
doi =
pages =
isbn =

* cite book
last = Smith
first = A.B.
coauthors = Batten, D.J.
editor =
title = Fossils of the Chalk
url =
edition = Second Edition
series =
date =
year = 2002
publisher = The Palaeontological Association
isbn =
doi =
pages =