Sola Formation (Solaformasjonen)
(From NPD Bulletin no. 5)
Cromer Knoll Group
Informally named by Hesjedal & Hamar (1983) after a village in southwestern
Norway. Formally named by Jensen et al. (1986).
Well type section
Danish well 1-1 from 2898 to 2859 m. Coordinates N
56°03'10", E 04°14'60" (Fig. 14). No cores.
Well reference sections
Norwegian well 2/11-1
from 2988 to 2910 m, coordinates N 56°14'16.98", E 03°27'07.05" (Fig. 12). No cores.
Norwegian well 24/12-2
from 4043 to 3985 m, coordinates N 59°12'00.75", E 01o52'53.34" (Fig. 18). No cores.
The thickness in the type well is 39 m, and in reference well 2/11-1 it is 78 m. It generally varies between 20 m and 200 m. The formation is thick in the Viking Graben and Asta Graben, and thin in the East Shetland Basin and parts of the Fiskebank Sub-Basin.
The Sola Formation consists of shales interbedded with stringers of
marlstone and limestone. The carbonate content is lower than that in the
underlying Tuxen and Åsgard
Formations and the overlying Rødby Formation.
The colour is black or dark grey, but olive-grey, brown and red colours
occur. The shales are finely laminated and often very pyritic.
The lower boundary is usually placed on the Tuxen
or Åsgard Formations (Figs. 12, 14, 17 and 18).
Generally, the gamma-ray response increases and the velocity decreases
from the calcareous and sandy sediments up into the shaly and organic rich
Sola Formation. In some wells in the east, on the Horda Platform,
the gamma-ray response does not increase when the boundary from the
Asgard Formation up into the Sola Formation is crossed. In such wells,
a lower, more stable velocity identifies the Sola Formation.
Characteristics of the upper boundary
The upper boundary is most often placed where the carbonate content starts
to increase rapidly into the overlying Rødby
Formation (Figs. 12, 14,18, and 22). In some areas where, the Sola
Formation is overlain by the Ran sandstone units
(Fig. 23), the boundary is defined by an upward decrease in gamma-ray
response and an increase in velocity.
The formation is widespread in the North Sea. It is absent or thin on structural highs, salt-induced structures and in parts of the Central Trough and Norwegian-Danish Basin.
Occurrences of formation tops in wells
The Sola Formation is of Mid Aptian-Early Albian age. A possible Middle (Late) Barremian-Albian age is recorded from Danish wells (Heilmann-Clausen 1986).
The Sola Formation was deposited in a marine environment with alternating
anoxic and oxic bottom conditions. Hesjedal & Hamar (1983) suggested that the formation was deposited during a regressive period, while Rawson & Riley (1982) held the opposite view.