The term Hidra Formation was introduced by Deegan & Scull (1977), in the Central North Sea area (Table 1), for the unit of chalky limestones with interbedded argillaceous chalks lying between the Cromer Knoll Group below and the Plenus Marl Formation above (since renamed the Black Band by Johnson & Lott (1993). This same unit is recognizable in the wells of the Southern North Sea Basin and it was recommended by Lott & Knox (1994) that the usage of the term Hidra Formation was therefore extended. The formation is defined formally in the Central Graben (Norwegian Sector) by Isaksen & Tonstad (1989).
From the Hidra High in Norwegian blocks 1/3 and 2/1, which was named after an island of Hidra off the southwest coast of Norway.
Norwegian Sector: In the type well the Hidra Formation consists of white to light grey, hard chalks with thin interbeds of grey to black shale in the lower part of the formation. Locally, the formation is more marly with interbedded marly chalk and marl. The chalks are occasionally softer with abundant glauconite and pyrite. The colour may be white, grey, green, brown or pink. Traces of pink waxy tuff occur in places. The formation is generally highly bioturbated.UK Sector: The Hidra Formation consists of micritic chalky limestones with interbedded argillaceous chalks and mudstones (Johnson & Lott, 1993; Lott & Knox, 1994). The unit becomes increasingly argillaceous in its lower part in the Southern North Sea, with thin beds of detrital clay become increasingly common towards the base of the formation. At the base of the formation in UK well 22/1-2A, hard, black, carbonaceous and argillaceous limestones are present. It also becomes more argillaceous to the north, where in the North Viking Graben it passes laterally into the Svarte Formation (Shetland Group). The chalky limestones are white to pale grey and occasionally pink, red-brown or green. The argillaceous chalks and mudstones are pale to dark grey or red-brown. Chert is only rarely recorded. Thin, pale grey, glauconitic and calcareous, quartzose sandstones are locally present in the Inner Moray Firth (Andrews et al., 1990) and South Viking Graben (Johnson & Lott, 1993). Pink, waxy, tuffaceous beds have been recorded in the Central North Sea (Deegan & Scull, 1977).
Norwegian Sector: The formation is 70 m thick in the type well, and 55.5 m in BO-1 in the Danish Sector. Seismic interpretation suggests that the formation reaches a maximum thickness of about 150 m in the northwestern part of the Central Trough in the Norwegian Sector.
UK Sector: In the Central North Sea the formation is typically 30-70 m thick, 45 m in well 22/1-2A and 30.5 m in well 29/25-1. However, is up to about 160 m in the Erskine and Fisher Bank basins of the Central Graben (Johnson & Lott, 1993; Gatliff et al., 1994) and up to 120m in the southern and eastern parts of the Moray Firth (Andrews et al., 1990). The formation onlaps the flanks of the Forties-Montrose and Jaeren highs and is absent from the western flanks of the Central Graben, on the West Central Ridge and Argyll Shelf (Gatliff et al., 1994). The Hidra Formation ranges up to c.40 m in thickness in the Southern North Sea (Lott & Knox, 1994).
The Hidra Formation occurs throughout most of the Central North Sea and South Viking Graben, but is locally absent over some contemporary structural highs (30/1c-2A, Panel 12S, Johnson & Lott, 1993). The Hidra Formation occurs throughout most of the Southern North Sea area but is absent, due to subsequent inversion and erosion over the Sole Pit Basin and over some contemporary structural highs and salt diapirs (Lott & Knox, 1994).
In the Norwegian Sector, it is missing above highs such as the Sørvestlandet, Mandal, Jæren, Utsira and Sele Highs, the Grensen Ridge, as well as many of the salt diapirs.
Well name: 1/3-1 (Norwegian Sector)WGS84 coordinates: Lat. 56º 51’ 21”N Long. 02º 51’ 05”E
|Central North Sea (Johnson & Lott, 1993)|
15/22-4: 3115-3266.5 m (10219-10717 ft)
|Lat. 58º 15’ 52.3”N
Lat. 58º 48’ 27.0”N
Lat. 57º 56’ 12.2”N
Lat. 56º 49’ 51.95”N
|Long. 00º 13’ 49.2”E
Long. 01º 16’ 44.5”E
Long. 01є 02' 55.8"E
Long.01º 43’ 20.37”E
Long. 01°51'48.80"N, No cores.
|Southern North Sea (Lott & Knox, 1994)|
|43/8a-2: 1261.51295.5 m (41394250 ft)
49/25-2: 19491989.5 m (63946527 ft)
Lat. 53º 11’ 51.2”N
Long. 02º 51’ 19.49”E
BO-1: 2275.5 to 2220 m
||Long. 04°34'18.66"E, Cored through the upper 35 m
Norwegian and Danish Sectors: The upper boundary is defined by theBlodøks Formation. The boundary is characterised by a change from the chalk lithology to mainly mudstone. This is seen as an abrupt change to higher gamma-ray response and a decrease in velocity in the Blodøks Formation. The boundary shows as a glauconitised hardground in the core from Danish well BO-1.UK Sector: The top of the Hidra Formation is normally marked by a sharp downward change from the basal black mudstone of the Herring Formation (Black Band) to white to pale grey chalky limestones (Johnson & Lott, 1993; Lott & Knox, 1994).
Norwegian Sector: The formation usually shows a gamma-ray response that has constant low values and high velocities. These contrast sharply at the lower boundary with the higher gamma-ray response and lower velocity of the Asgard and Sola Formations. The lower boundary is more gradational when the carbonate rich facies of the Rødby Formation is present beneath the Hidra Formation.UK Sector: The base of the Hidra Formation is normally defined by a downward change from interbedded pale to dark grey and pink chalky, argillaceous limestones to red-brown chalky mudstones of the Rødby Formation. In the Central North Sea, this boundary is commonly represented by a minor unconformity, although locally the formation rests unconformably upon Jurassic or older rocks (Gatliff et al., 1994).
The variable proportion of interbedded mudstone in the Hidra Formation of the Central North Sea provides a typically uneven geophysical log character (Gatliff et al., 1994), whereas in the Southern North Sea there is a lower proportion of mudstones and less uneven signature. The top of the Hidra Formation is characterized on wireline log responses by a sharp downward decrease in gamma-ray values and increase in sonic velocity response. Where the Black Band is absent or poorly developed the boundary is much more difficult to pick and biostratigraphic data may be required to define the boundary (e.g. 49/5-1, Lott & Knox, 1994). The lower boundary of the formation is marked on the wireline log responses by a downward increase in gamma-ray values and decrease in velocity into the underlying Rødby Formation (Lott & Knox, 1994). In the Moray Firth, the base of the formation is commonly taken at the top Lower Cretaceous seismic event. This is marked in wells by a relatively undistinguished log break of variable amplitude associated with a thin chalk bed that interrupts the progressive change in gamma-ray and velocity values (Andrews et al., 1990).
In the Central North Sea (Johnson & Lott, 1993) and Southern North Sea (Lott & Knox, 1994) the Rotalipora cushmani foraminiferal biomarker occurs near the top of the Hidra Formation where there is a major influx of planktonic foraminifera, notably Rotalipora cushmani, R. greenhornensis, Hedbergella brittonensis and H. delrioensis. The Lingulogavelinella ciryi inflata/Rotalipora reicheli biomarker occurs in the middle part of the formation and consistent and common Hedbergella planispira and Quinqueloculina antiqua have their FDOs immediately above the base of the Hidra Formation at a similar horizon as the FDO (First Downhole Occurrence) of the ostracod Neocythere steghausi.
Of the nannofossil biomarkers, the following occur within the Hidra Formation: Axopodorhabdus albianus, which forms a good marker for the top of the formation, Gartnerago theta, Gartnerago nanum and Biscutum constans acme.
Palynological recovery in the Hidra Formation is variable, but generally low. The Apteodinium granulatum dinoflagellate cyst biomarker marks the top of the formation and the Epelidosphaeridia spinosa biomarker is present within the formation (Costa & Davey, 1992).
Cenomanian; the base of the unit is taken at the base of UC1, base of Cenomanian at 100.5 Ma. However, in the Erskine Basin (UK Sector), boreholes suggest chalk sedimentation commenced earlier in the basinal areas of the Central Graben (Gatliff et al., 1994).
The Hidra Formation is laterally equivalent to the upper part of the flintless, grey occasionally pink, argillaceous chalks of the Ferriby Chalk Formation of eastern England (Wood & Smith, 1978; Table 4.3). The Hidra Formation and its lateral equivalent Svarte Formation (Shetland Group) correlate approximately with the Grey Chalk Subgroup of the onshore southern England province (Hopson, 2005).
The bioturbated chalky limestones of the Hidra Formation were deposited in open marine environments as fine-grained pelagic carbonate, containing abundant bioclastic, skeletal debris (dominated by coccolith plates) (Johnson & Lott, 1993; Lott & Knox, 1994). Beds of terrigenous mudstone are common towards the base of the formation in the Central North Sea (Johnson & Lott, 1993). Thin calcarenite turbidite beds may be present within the Central Graben (Kennedy, 1987). The upward change from dominantly benthonic to planktonic foraminiferal faunas in the middle of the formation reflects the development of good open-ocean connections in the late Cenomanian (Crittenden et al., 1991).