previous page

Hidra Formation (Hidraformasjonen)

(After NPD Bulletin no. 5)

Chalk Group


Named by Deegan & Scull (1977) after the Hidra High in Norwegian blocks 1/3 and 2/1. The name Hidra is after the island of Hidra on the southern coast of Norway.

Well type section

Norwegian well 1/3-1 from 4441 to 4371 m, coordinates N 56°5'21.00", E 02°51'05.00". No cores.

Well reference sections

UK well 22/1-2A from 3783 to 3738 m, coordinates N 57°56'12.20", E 01°02'55.80". No cores.
UK well 29/25-1 from 2258.5 to 2228 m, coordinates N 56°18'10.00", E 01°51'48.80". No cores.
Danish well BO-1 from 2275.5 to 2220 m, coordinates N 55°48'8.22", E 04°34'18.66". Cored through the upper 35 m.


The formation is 70 m thick in the type well, 45 m in 22/1-2A, 30.5 m in 29/25-1 and 55.5 m in BO-1. 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.


In the type well the 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. At the base of the formation in UK well 22/1-2A, hard, black, carbonaceous and argillaceous limestones are present. Traces of pink waxy tuff occur in places. The formation is generally highly bioturbated.

Characteristics of the lower boundary

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.

Characteristics of the upper boundary

The upper boundary is defined by the stratotype of the Blodøks 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.

Geographical distribution

The formation is found in the central and southern North Sea. 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.

Occurrences of formation tops in wells
Isochore map RØDBY-HIDRA

Biostratigraphy and Stage/Age

Cenomanian; the base of the unit is taken at the base of UC1, base of Cenomanian at 100.5 Ma.

Depositional environment

Open marine with a perioditic or turbiditic origin for the sediments.


Isaksen, D. and Tonstad, K. (eds.) 1989: A revised Cretaceous and Tertiary lithostratigraphic nomenclature for the Norwegian North Sea. NPD-Bulletin No. 5, 59 pp.

home previous page