| Computer Code: |
TOR |
Preferred Map Code: |
notEntered |
| Status Code: |
Full |
| Age range: |
Campanian Age (KC)
— Maastrichtian Age (KM) |
| Lithological Description: |
The Tor Formation typically consists of homogeneous, white or pale grey, tan or pink chalky limestones. Locally, however, softer argillaceous chalky limestones are interbedded with the harder, purer limestones (e.g. 16/28-7). Cored sections reveal the presence of beds of relatively porous, slumped and chaotically bedded chalky limestone (Kennedy, 1980); these are marked by low velocity peaks and by distinctive porosity and dipmeter log responses. These reworked facies comprise the main reservoir intervals in the oilfields of the Norwegian Central North Sea. |
| Definition of Lower Boundary: |
The base of the Tor Formation is normally marked by a relatively abrupt downward change from hard, white or pale grey chalky limestones to more argillaceous, and commonly red-stained, limestones and mudstones of the Mackerel and Flounder formations. On wireline logs, the boundary corresponds to a downward increase in gamma values and a decrease in velocity. Over intrabasinal and graben-margin ridges, the Tor Formation overlies Lower Cretaceous and older sediments (e.g. 29/23-1). |
| Definition of Upper Boundary: |
The top of the Tor Formation is marked by a downward change from relatively argillaceous chalky limestones of the Ekofisk Formation to relatively clean, low-gamma limestones. Thin argillaceous limestone units are present locally and, in northern parts of the Central Graben, expansion of these argillaceous units leads to some uncertainty as to the correct pick. Thus the top of the Tor Formation in well 21/1-5 may be stratigraphically slightly lower than in more typical sections (see Panel 10W in Knox and Holloway, 1992). |
| Thickness: |
Where it overlies Lower Cretaceous or older sediments on the intrabasinal and graben-marginal ridges, the Tor Formation is generally less than 150 m thick. However, the formation is more than 250 m thick in depocentres within the Central Graben, reaching over 500 m in some sections. |
| Geographical Limits: |
The Tor Formation is the most widespread of the Chalk Group formations, and occurs throughout the Central North Sea, covering even the Forties-Montrose High. It also extends into the South Viking Graben and onto the East Shetland Platform, where it rests on Jurassic and older rocks. It is absent, however, from the graben margin of the Western Platform and parts of the Halibut Horst. The western limit of the Tor Formation is erosional. |
| Parent Unit: |
Chalk Group (CK)
|
| Previous Name(s): |
none recorded or not applicable
|
| Alternative Name(s): |
none recorded or not applicable
|
| Stratotypes: |
| Type Section |
Norwegian Sector North Sea well N1/3-1 (Deegan and Scull, 1977, p.27, fig.30): 3354-3827.5 m (11004-12558 ft) below KB. |
| Reference Section |
North Sea well 16/17- 6: 2964-3249 m (9724-10660 ft) (Johnson and Lott, 1993). |
| Reference Section |
North Sea well 22/01- 2A: 2982-3198 m (9783-10492 ft) (revised depths) (Johnson and Lott, 1993). |
| Reference Section |
North Sea well 29/04a- 1A: 3176-3604 m (10420-11824 ft) (Johnson and Lott, 1993). |
| Reference Section |
North Sea well 29/25- 1: 1869-1995 m (6132-6542 ft) (Johnson and Lott, 1993). |
| Reference(s): |
| Johnson, H and Lott, G K. 1993. 2. Cretaceous of the Central and Northern North Sea. In Knox, R W O'B and Cordey, W G (eds.) Lithostratigraphic nomenclature of the UK North Sea. British Geological Survey, Nottingham. |
| Waters, C N, Gillespie, M R, Smith, K, Auton, C A, Floyd, J D, Leslie, A G, Millward, D, Mitchell, W I, McMillan, A A, Stone, P, Barron, A J M, Dean, M T, Hopson, P M, Krabbendam, M, Browne, M A E, Stephenson, D, Akhurst, M C, and Barnes, R P. 2007. Stratigraphical Chart of the United Kingdom: Northern Britain. (British Geological Survey.) |
| Deegan, C E and Scull, B J. 1977. A standard lithostratigraphic nomenclature for the Central and Northern North Sea. Report of the Institute of Geological Sciences, 77/25; NPD Bulletin No.1. |
| Isaksen, D and Tonstad, K. 1989. A revised Cretaceous and Tertiary lithostratigraphic nomenclature for the Norwegian North Sea. NPD-Bulletin No.5 |
| Knox, R W O'B and Holloway, S. 1992. 1. Paleogene of the Central and Northern North Sea. In: Knox, R W O'B and Cordey, W G (eds.) Lithostratigraphic nomenclature of the UK North Sea. British Geological Survey, Nottingham. |
| Kennedy, W J. 1980. Aspects of chalk sedimentation in the southern Norwegian offshore. In: The sedimentation of North Sea reservoir rocks. Norwegian Petroleum Society (NPF), Geilo. |
| King, C, Bailey, H W, Burton, C A, and King, A D. 1989. Cretaceous of the North Sea. In: Jenkins, D G and Murray, J W (eds) Stratigraphical atlas of fossil foraminifera, 372-417. Ellis Horwood Limited, Chichester, for the British Micropalaeontological Society. |
| Nygaard, E, Lieberkind, K, and Frykman, P. 1983. Sedimentology and reservoir parameters of the Chalk Group in the Danish central graben. Geologie Minjbouw, 62, 177-190. |
| 1:50K maps on which the lithostratigraphical unit is found, and map code used: |
| none recorded or not applicable |
