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NORLEX


Tor Formation (Torformasjonen)

After Fritsen, A. & Riis, F. 2000. A revised chalk lithostratigraphic nomenclature; NPD Report, unpublished)

For earlier definition, see NPD Bulletin no. 5.

Chalk Group

Name

Named by Deegan & Scull (1977) from the Tor Field in Norwegian blocks 2/4 and 2/5. Tor was a son of Odin, and one of the principal Gods of Norse mythology.

Well type section

Norwegian well 1/3-1 from 3828 to 3354 m, coordinates N 5651'21.00", E 0251'05.00" (Tonstad and Isaksen 1989). No cores.

Well reference sections

The Mona-1 well is chosen as a one of two reference sections for the Tor Formation. The stratigraphic sequence in Mona-1 is relatively complete with individual zones being represented by a significant thickness. Except for the very top the Tor Formation is cored throughout this well. Discovery well for the Tor Field, well 2/5-1, is chosen as a second reference section for the Tor Formation. Only the upper half of the Tor Formation is cored but the 2/5-1 well has all Tor zones well developed.

Thickness

Tor thicknesses in the study wells vary from 0 m in Baron-2 to 472m in 1/3-8. The thickness variations in the study wells are however not representative of the North Sea Basin Chalk/ Tor Formation distribution as most wells are drilled on structural highs.

Lithology

The Tor Formation is in general very clean with a low content of insolubles (<5%). The clean nature of the chalk is reflected in the light colour and the evidence of pressure solution in the form of dental stylolites. Dental stylolites are typical for clean chalk, while the pressure solution in the more dirty chalks appears as horsetails solution seams. The Tor Formation is composed of mixed pelagic and allochthonous chalks. There is a gradual upward increase in the amount of allochthonous material.

The distribution of sedimentary facies is driven by paleotopography, so that what is seen today is the combined result of primary deposition and secondary processes in the form of reworking and diagenesis. Therefore, to understand the depositional facies, the location of the individual wells with respect to regional structural features (basinal axis, margins and inversion ridges) and more local halokinetic structures, is critical.

Characteristics of the lower boundary

In general the lower Tor boundary is difficult to pick on logs. A decrease in the GR level, small or large, is common, though quite often no change of the GR level is observed. In the stratigraphically more complete wells there seems to be a regional element to the log characteristics. In the northern basin wells the sonic log shows an increase in velocity upon entering the Tor Formation from Magne below. This is associated with either no change or a slight increase in the density reading, i.e lower porosity. From the Lindesnes Ridge and southeastwards the sonic log shows a decrease in velocity crossing from Magne to Tor Formation. This is associated with either no change or a slight decrease in the density reading, i.e higher porosity. Where nannozones UC16-18 are thin or missing a change to lower porosity associated with an increase in velocity is observed at the boundary. These are observations of a general nature and exceptions can be found. The resistivity logs to some degree mirror the sonic and density logs in water bearing chalk. Where the velocity and density go up an increased resistivity is often seen, reflecting a more dense rock, and vice versa. Exceptions are wells where the bottom part of the Tor Formation is porous and oil-bearing. Here a decrease in velocity and density is accompanied by an increase in resistivity. The lower boundary either coincides with the boundary between biozones UC16 and 18 (UC17 is only scarcely present) or is placed midway in UC16.

Characteristics of the upper boundary

In general the Top Tor is of the same age over the study area. The duration of the Tor/Ekofisk hiatus mostly depends on how much is missing from the Ekofisk Formation. Locally all of the Ekofisk is the crestal parts of the Hod and Valhall Fields. Here the Tor Formation, or reworked Tor Formation, is overlain by Paleocene shales (2/8-A-1). The GR generally shows an increase. The Top Tor pick is usually put right at the start of the increase. In accordance with a shift to lower porosities in the bottom part of the Ekofisk Formation the velocity shows a clear increase at Top Tor. However, the uppermost part of the Tor Formation can be well cemented and as a result appear exactly like the Ekofisk on density and velocity logs. In the absence of good biostratigraphic data the usual slight increase in GR should enable an accurate log pick, though. A relative decrease in resistivity across the Tor/Ekofisk boundary is seen. This picture is consistent even though the resistivity in the top part of the Tor Formation is affected by pore fluids (water, oil or gas).

Log pattern or seismic characteristics

The Tor Formation seismic package is characterised by well defined continous reflectors in the upper half, while the lower half most often show less distinct reflectors. This difference is most pronounced in the northern part of the study area, where the formation is also the thickest. It is probable, that the lower half is simply better developed to the north and that these deeper layers have a very thin development southeastwards in the basin resulting in more pronounced reflectors. Neither the Base nor the Top Tor boundary are seismically distinct reflectors and would be difficult to pick without well-tie. Onto the Lindesnes Ridge a major part of the Tor package disappears. The 2/7 and 2/8 wells show that the missing part is from top down.

Geographical distribution

Regional thickness maps based on seismic interpretation show a general thickening of the Upper Cretaceous package from southeast to northwest in the Central Graben system (Britze et al, 1995, Ziegler, 1990, Japsen, 1998). This general observation has an overprint of local east or west depocentres going from north to southeast along the Graben axis. This pattern is confirmed by the study wells also for the Tor Formation alone.

Occurrences of formation tops in wells
Isochore map MAGNE-TOR

Biostratigraphy and Stage/Age

The age of the Tor Formation has in this study been decided to encompass all of the Maastrichtian, i.e. nannofossil zones UC17 through 20 and foram zones FCS22 through FCS23. The base of the Formation is in the middle of zone UC16 near 74 Ma.

At the detailed level the top of zone UC16, which is of late Campanian age, cannot in all wells be brought to coincide with well defined log breaks. This is a problem that remains even after careful integration of all data. UC17 and 18 are Early Maastrichtian. UC19 and 20 are Late Maastrichtian and encompasses the upper part of nannoplankton Zone UC16 through to Zone UC20 and the two foraminiferid Zones FCS22 and FCS23, both of which have been further divided into subzones. There is considerable biostratigraphic evidence for intra-formational reworking within the Tor throughout the study region. Several wells contain sections where Late Campanian and Early Maastrichtian Tor sediments have been re-deposited during Late Maastrichtian. There is also evidence of reworking within Late Maastrichtian. Despite the frequent presence of stacked allochthonous chalk units within the Tor Formation, the ages of individual units can be ascertained on the basis of the characteristic nannofloral and microfaunal associations. Many of the well sections examined include very thin and condensed Late Campanian to Early Maastrichtian intervals, suggesting active structural growth during this period. The maximum phase of allochthonous chalk deposition is during the Late Maastrichtian resulting in considerable variation in formation thickness across the region.

Depositional environment

Open marine with deposition of calcareous debris flows, turbidites and autochthonous periodites.

References

Fritsen, A. and Riis, F. (unpublished): A revised chalk lithostratigraphic nomenclature.

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