Hidra Formation (Hidraformasjonen)
(After NPD Bulletin no. 5)
Chalk Group
Name
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.
Thickness
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.
Lithology
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.
References
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.
