Åsgard Formation (new) (Åsgardformasjonen)
(From NPD Bulletin no. 5)
Cromer Knoll Group
Name
Named from Norse mythology after the castle of the
Norse gods, where Odin ruled.
Well type section
Norwegian well 2/11-1
from 3555 to 3063 m, coordinates N 56°14'16.98", E 03°27'07.05" (Fig. 12). No cores.
Well reference sections
Norwegian well 17/11-2
from 2410 to 1802 m, coordinates N 58°06'54.91", E 03°22'09.81" (Fig. 13). No cores.
Danish well 1-1 from 3358 to 2986 m, coordinates N 56°03'10", E 04°14'60" (Fig. 14). No cores.
Thickness
The formation is 492 m thick in the type well and 608 m thick in reference well 17/11-2. In the Central Trough area the thickness varies from a few metres to more than 500 m over short distances, showing the complex pattern of small, restricted Early Cretaceous basins. An even thicker sequence was penetrated in the Norwegian-Danish Basin, and especially in the Asta Graben, where more than 700 m were encountered in well 17/12-3. The formation is thickest in the Sogn Graben where it is probably more than 1200 m, as indicated by seismic data.
Lithology
The formation is dominated by light to dark grey, olive-grey, greenish and
brownish, often calcareous claystones, and passes into light grey, light
greenish-grey and light olivegrey marlstones and stringers of limestone.
Mica, pyrite and glauconite are common. The claystones may be silty, and
siltstones or very fine-grained sandstone layers or laminae are present.
Where major sandstone layers occur they are regarded as belonging to the
Ran sandstone units defined below. In a few Norwegian
wells in the central North Sea (eg. 1/9-3, 2/3-1, 2/7-2, 2/10-1, 2/11-1,
7/3-1, 7/8-2, 7/12-4, 7/12-5 and 8/1-1) a sequence of calcareous claystone,
marlstone and limestone interbeds is recognised as the basal part of the
Åsgard Formation (Figs. 12 and 14). This sequence is very difficult to
correlate in the Norwegian sector, even over small distances, and is
therefore regarded as representing local variations in the lowermost part
of the Åsgard Formation. In the Danish sector this sequence is defined as the Leek Member (Jensen et al. 1986).
Basal stratotype
The lower boundary is defined by a marked upward decrease in gamma-ray
response and an increase in velocity in areas where the underlying sediments
are slightly to non-calcareous, organic-rich claystones and shales, usually
belonging to the Mandal,
Flekkefjord, Tau or Draupne
Formations (Figs. 15 and 22). Where the claystones and shales are less
organic rich and more calcareous, the boundary may be more difficult to
identify on logs.
Characteristics of the upper boundary:
The characteristics of the upper boundary vary with the overlying
formations. Where the Tuxen Formation occurs,
the boundary is defined by an upward decrease in the gamma-ray readings
and an increase in velocity, reflecting slightly more calcareous claystones,
marl-stones and limestones compared with the underlying
Åsgard Formation (Figs. 14 and 15). Where the Tuxen Formation is missing
and the Sola Formation is deposited on the Åsgard
Formation, the boundary is defined by an upward increase in gamma-ray
readings and a decrease in velocity (Figs. 17,18 and 23). If both the
Tuxen and Sola Formations are missing, the boundary to the overlying
Rødby Formation is defined by an upward decrease
in gamma-ray readings and an increase in velocity. Locally, the Åsgard
Formation is overlain by the Ran sandstone units
(Fig. 22) and the Agat Formation (Figs. 19 and 20). This boundary is defined by an upward decrease in gamma-ray readings.
Distribution
The Åsgard Formation is very widespread in the North Sea (Figs. 7-11), as are the partial equivalents in the Danish sector (Valhall Formation, Jensen et al. 1986; Vedsted Formation, Larsen 1966), British sector (Speeton Clay, Rhys 1974) and Dutch sector (Vlieland Shale Member, NAM & RGD 1980). In the Norwegian sector, the formation is absent from the highest parts of the Mandal, Jaeren and Utsira Highs, the Lomre Terrace, the Troll area, Tampen .Spur and locally over salt pillows and diapirs in the Central Trough and the Norwegian-Danish Basin.
Occurrences of formation tops in wells
Age
Where the Tuxen Formation occurs, the Åsgard Formation ranges in age from
Late Ryazanian to Late Hauterivian. In areas where neither the Tuxen nor
Sola Formations are recognised, the Åsgard Formation represents a lateral
equivalent and may reach Late Aptian to Early Albian age.
Depositional environment
The formation was deposited in an open marine, low-energy shelf environment
with well-oxygenated bottom water.
Remarks
Deegan & Scull (1977) divided the Cromer Knoll Group into the Rødby and
Valhall Formations. Several lithostratigraphic units have later been
described in the Valhall Formation (Hesjedal & Hamar 1983, Jensen et al. 1986). The remaining claystones and marlstones of the originally defined Valhall Formation constitute the Asgard Formation.
