Jorsalfare Formation (new) (Jorsalfareformasjonen)
(From NPD Bulletin no. 5)
Shetland Group
Name
Named after Sigurd "Jorsalfare" Magnusson, a Norwegian king (A.D. 1103-1130).
Well type section
Norwegian well 25/1-1
from 2997 to 2711 m, coordinates N 59°53'17.40", E 02°04'42.70" (Fig. 33).
One core (17 m) in the middle of the formation and another (4 m) at the base.
Well reference sections
Norwegian well 35/3-2
from 1665 to 1520 m, coordinates N 61°51'05.98", E 03°AG2%.2T (Fig. 34).
No cores.
Norwegian well 24/9-1
from 3117 to 2752 m, coordinates N 59°16'09.48", E 01°47'31.18" (Fig. 35).
No cores.
Thickness
The formation is 286 m thick in the type well (25/1-1),
145 m in well 35/3-2 and 365 m in well 24/9-1.
Lithology
The formation generally consists of mudstones interbedded with thin
limestone beds. The mudstones are light to medium grey, often calcareous.
The limestones are white to light grey, fine grained, occasionally sandy
and dolomitic.
Basal stratotype
The lower boundary is defined by a decrease in gamma-ray intensity and
an increase in velocity, reflecting an increase in calcareous content
from the Kyrre Formation into the Jorsalfare
Formation (Figs. 33 and 34). In the Tampen Spur area, however, the boundary
may be difficult to identify due to small differences in calcareous
content. The lower boundary may be unconformable above the Jurassic
sequences (e.g. in the Gullfaks area).
Characteristics of the upper boundary
The upper boundary may be towards the Våle,
Lista or Ty Formations of the
Rogaland Group. When the upper boundary is
towards the shale of the Lista Formation it is usually characterised by
an upward increase in gamma-ray intensity and a distinct drop in velocity
(Fig. 34). When it is towards the Våle Formation it does not show the
same distinct drop in velocity and increase in gamma-ray intensity,
because the overlying lithology consists of limestones or calcareous
mudstones (Fig. 35). Where the upper boundary is towards the Ty Formation
it is identified as a change to sandstone (Fig. 33).
Distribution
The formation is present in the Viking Graben and on the Tampen Spur.
Its boundaries towards the Jorsalfare Formation in the Viking Graben,
the Hardråde Formation on the Horda Platform
and the Tor Formation on the Utsira High are
illustrated in Fig. 32b.
The main characteristics that can be used to distinguish the three
formations are:
a. The Jorsalfare Formation contains shales with thin
limestone beds which are usually no thicker than 5 m.
b. The Tor Formation is dominated by limestones, and has a negligible shale
content.
c. The Hardråde Formation contains thick limestone beds (10-60 m), as well as shales, except in the Troll area where it is thin and may consist of only a single bed of limestone or marly limestone.
Separation of these three formations may be difficult in transitional areas.
Occurrences of formation tops in wells
Isochore map KYRRE-JORSALFARE
Age
Late Campanian to Maastrichtian.
Depositional environment
Open marine.
Remarks
Hardråde and Tor Formations of the Shetland Group, and also with the
informal "formation E" of Deegan & Scull (1977) (Fig. 6).
