Hod Formation (Hodformasjonen)
(From NPD Bulletin no. 5)
Shetland Group
Name
Named by Deegan & Scull (1977) from the Hod Field in Norwegian block 2/11.
The name Hod derives from one of the twelve principal gods in Norse
mythology. Hod was a son of Odin.
Well type section
Norwegian well 1/3-1
from 4343 to 3828 m, coordinates N 56°51'21.00", E 01°51'05.00" (Fig. 24).
No cores.
Well reference sections
UK well 29/25-1 from 2225 to 2012 m, coordinates N
56°18'10.00", E 01°51'48.80" (Fig. 26). No cores.
Norwegian well 2/8-8
from 2601 to 2494 m, coordinates N 56°16'50.28", E 03°24'15.93" (Fig. 28).
36 m of cores discontinuously through the upper 78 m and lowermost 6 m of
the formation.
Thickness
The formation is 515 m thick in the type well, 213 m in UK well 29/25-1
and 107 m in Norwegian well 2/8-8. In the Norwegian sector, seismic
interpretation indicates that the formation may reach a thickness of
more than 700 m in the northwestern part of the Central Trough.
Lithology
In the type well the formation consists of hard, white to light grey,
crypto- to microcrystalline limestones which may become argillaceous or
chalky in places. White, light grey to light brown, soft to hard chalk
facies may dominate the formation or alternate with limestones. The
limestones may be pink or pale orange. Thin, silty, white, light grey to
green or brown, and soft, grey to black, calcareous clay/shale laminae are
occasionally present. Pyrite and glauconite may occur throughout the
formation and the latter may be common in the lower part.
Basal stratotype
The lower boundary is usually marked by a distinct log break to a lower
gamma-ray response and higher velocity from the Blodøks
Formation to the Hod Formation (Fig. 24). The boundary may be less distinct
when the Blodøks Formation is more calcareous (Fig.31).
Characteristics of the upper boundary
The upper boundary towards the Tor Formation is generally
marked by a change in gamma-ray readings to a more constant and slightly
lower level, and also by higher velocity, (Figs. 24 and 31). The upper
boundary may represent an unconformity in the Ekofisk area (e.g. Norwegian
well 2/8-8, Fig. 28).
Distribution
The formation is widely distributed in central and eastern parts of the
central North Sea, passing laterally into sediments of the Herring and
Flounder Formations to the west and the Tryggvason
and Kyrre Formations to the northwest.
Age
Turonian to Campanian.
Depositional environment
Open marine with deposition of cyclic pelagic carbonates (periodites) and
distal turbidites (Skovbro 1983 and d'Heur 1986).
Remarks
An informal, tripartite subdivision of the Hod Formation into lower, middle
and upper members is often possible in the southern part of the Central
Trough in the Norwegian sector. The subdivision is based on the frequent
presence of a higher clay content in the middle of the Hod Formation (Figs. 24,
28 and 29).
Lower member of the Hod Formation: This unit constitutes the largest part of
the Hod Formation and is a sequence of bioturbated laminated chalks with a
low clay content. It occurs in Norwegian wells 1/3-1 from 4343 to 4066 m,
2/8-8 from 2601 to 2538 m and 1/9-1 from 3648 to 3353 m.
Middle member of the Hod Formation: This is a sequence consisting mainly of
periodites, which generally have a greyish colour reflecting a marked
increase in terrigenous clay. It is shown on well logs as an increase in
gamma-ray readings. It occurs in Norwegian wells 1/3-1 from 4066 to 4009 m,
2/8-8 from 2538 to 2518 m and 1/9-1 from 3353 to 3344 m.
Upper member of the Hod Formation: This unit constitutes another sequence
dominated by periodites with minor allochthonous intercalations, but with a
return to a low clay content. It occurs in Norwegian wells 1/3-1 from 4009
to 3828 m, 2/8-8 from 2518 to 2494 m and 1/9-1 from 3344 to 3312 m.
The Herring Formation of Deegan & Scull (1977) includes a similar lithology
and was deposited at the same time as the Hod Formation. It is regarded here
as the lower part of the Hod Formation. The Hod Formation is also equivalent
in age to the Tryggvason and
Kyrre Formations (Fig. 6).
