| Computer Code: |
GRIS |
Preferred Map Code: |
notEntered |
| Status Code: |
Full |
| Age range: |
Mid Eocene (GEM)
— Early Oligocene (GOE) |
| Lithological Description: |
Consists of thinly or thickly bedded, fine to medium grained, locally coarse-grained sandstone, interbedded with mudstone, grey to green-grey and variably silty. The sandstone commonly includes minor amounts of glauconite, and is locally shelly. Wireline-log signatures are mostly of 'blocky' character (e.g. 3/8-2, 16/17-8A, 22/1 a-3), but one or more upward-coarsening sandstone units are present in sandstones at the top of the Horda Formation in some well sections (e.g. 15/26a-3). |
| Definition of Lower Boundary: |
The base of the Grid Sandstone Member is drawn at the base of the lowest significant sandstone unit. Very thin sandstone units isolated by thick mudstones are excluded (e.g. 15/26a-3). Where thicker sandstone units are separated by a substantial thickness of mudstone, they are treated as upper and lower 'leafs' of the Grid Sandstone Member (e.g. 15/28a-3). The sandstone is normally underlain by green-grey or grey-green mudstone, but where the sandstone is limited to the uppermost part of the Horda Formation, grey, silty mudstone may occur immediately beneath. The base of the Grid Sandstone Member normally falls within, or at the base of, Horda unit 2, above the regionally recognizable gamma-ray marker. |
| Definition of Upper Boundary: |
Where the sandstone is restricted to the middle or lower part of the Horda Formation, the top is marked by a downward change from green-grey, variably silty, mudstone to sandstone. Where the sandstone extends into the upper part of the Horda Formation, it is commonly overlain directly by high-gamma, grey-brown, commonly glauconitic, mudstone or siltstone of the basal Lark Formation (e.g. 15/26a-3). In distal sections, however, there is a thin unit of grey to brown-grey, silty Horda mudstone between the Grid Sandstone Member and the basal mudstone of the Lark Formation (e.g. 3/8-2). Elsewhere, an alternation of mudstone and sandstone occurs above the main development of the Grid Sandstone Member (e.g. 16/17-8A). |
| Thickness: |
The thickness of the Grid Sandstone Member is extremely variable, but is greatest in the South Viking Graben. The maximum thickness is over 300 m, although total sand thickness rarely exceeds 100 m, with individual units rarely exceeding 50 m. |
| Geographical Limits: |
The Grid Sandstone Member is found over much of the UK Northern North Sea and extends into adjacent parts of the Norwegian sector (see Isaksen and Tonstad, 1989, fig. 66). Its northern limit is poorly defined. Its western limit, i.e. the western limit of the Brodie Sandstone, is arbitrarily drawn. |
| Parent Unit: |
Horda Formation (HORD)
|
| Previous Name(s): |
none recorded or not applicable
|
| Alternative Name(s): |
none recorded or not applicable
|
| Stratotypes: |
| Type Section |
Norwegian Sector North Sea well N15/3-3: 1470-1840m (4823-6037ft) below KB (Isaksen and Tonstad, 1989). |
| Reference Section |
North Sea well 3/08- 2: 904-991 m (2966-3251 ft) (Knox and Holloway, 1992). |
| Reference Section |
North Sea well 15/26a- 3: 717-814 m (2352-2671 ft) (Knox and Holloway, 1992). |
| Reference Section |
North Sea well 16/17- 8A: 1524-2032.5 m (5000-6668 ft) (Knox and Holloway, 1992). |
| Reference Section |
North Sea well 22/01a- 3: 2191-2222 m (7188-7290 ft) (Knox and Holloway, 1992). |
| Reference(s): |
| 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. |
| Isaksen, D and Tonstad, K. 1989. A revised Cretaceous and Tertiary lithostratigraphic nomenclature for the Norwegian North Sea. NPD-Bulletin No.5 |
| 1:50K maps on which the lithostratigraphical unit is found, and map code used: |
| none recorded or not applicable |
