Thud Formation (Thudformasjonen)
After Fritsen, A. & Riis, F. 2000. A revised chalk lithostratigraphic nomenclature; NPD Report, unpublished)
Chalk Group
Name
After an alternative name for the Norse god Odin (Sturluson, 1954). Thud means “thin one”.
Well type section
Well 1/3-8 4337-4125 m MD.
Well reference section
None.
Thickness
The thickness of this formation ranges from zero to a few hundred metres in the study wells.
Lithology
Poorly constrained owing to the lack of core coverage. Biofacies in cuttings samples from some wells are
characterised by moderately abundant and diverse planktonic foraminifera and nannofossils, and generally
rare benthonic foraminifera, indicating an affinity with the open marine platform or “shallow water” pelagic
biofacies of the Valhall structure.
Characteristics of the lower boundary
Picked primarily on seismic and biostratigraphic criteria. The seismic criterion is an onlap surface.
The biostratigraphic criterion is penetration of nannozone UC11.
Characteristics of the upper boundary
Picked primarily on a seismic criterion (onlap surface), corresponding to the original informal middle Hod sequence boundary.
Log pattern or seismic characteristics
A complete Thud formation has been penetrated in few wells, because of its basinal setting. In the northern
part of the region, the gamma values in
1/3-1 and
1/3-8 indicate a clay content that is higher than in the
Magne Formation. The relatively argillaceous chalks seem to alternate with clean chalks, giving rise to a
”box-shaped” pattern of the gamma log. The time equivalent to the Thud Formation in the Roar-2 well in
the Danish sector has developed a different log and seismic pattern.
Geographical distribution
It is absent in the 2/2 wells and in wells on structural highs, such as Valhall-Hod and Eldfisk in the Norwegian Sector
and in the Lulu-1 well in the Danish Sector. It is thickest in wells in basinal lows, such as those in the 1/3 area in the
Norwegian Sector and the Roar area in the Danish Sector.
Occurrences of formation tops in wells
Isochore map NARVE-THUD
Biostratigraphy and Stage/Age
Late Early Santonian to earliest Campanian (for practical purposes, Santonian). Microzones FCS18pp-FCS20pp;
nannozones UC12-UC14 pp. The base is taken at base of UC12 near 85.5 Ma.
Remarks
Laps on to Narve Formation on flanks of structures.
