a prominent sub-unit of the Grey Chalk Subgroup). The North Sea 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 Continental Shelf 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 capacity 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 Eastern Trough Area Project 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 quadrants 1 and 2, this overpressure helps preserve porosity and enables the Chalk to be an effective reservoir. Reservoir stratigraphy Chalk Unit 6 - Ekofisk Formation - Danian age (major reservoir in the Ekofisk oil field and others) Chalk Unit 5 - Tor Formation - Maastrichtian age, (major reservoir in many fields including Joanne oil field (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 restart 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