| Computer Code: |
LEWI |
Preferred Map Code: |
notEntered |
| Status Code: |
Full |
| Age range: |
Ladinian Age (TD)
— Rhaetian Age (TR) |
| Lithological Description: |
The Lewis Member is composed of one or more massive, composite sandstone beds, intercalated within thinly bedded sandstones, mudstones, and occasional thin limestones.
The massive sandstones are white, pale grey, greenish grey, yellowish brown or occasionally reddish brown in colour. In the Beryl Field, such sandstones are heavily bioturbated, mainly fine to medium grained, clean, and well sorted. They are commonly calcareous and locally micaceous, and occasionally have traces of pyrite and carbonaceous detritus, possibly of diagenetic origin. They contain sporadic calcrete horizons and mudstone bands (Knutson and Munro, 1991). The thinly bedded sandstones range between very fine and medium. Some of these contain traces of chlorite. The interbedded mudstones are mainly reddish brown or greenish grey, and they are variably calcareous, grading to marl. |
| Definition of Lower Boundary: |
In the Beryl Field, a 30-45 m thick basal massive sandstone unit ('Lewis Formation unit III' of Knutson and Munro, 1991) rests on a 15 m-thick lacustrine mudstone ('Lewis Formation unit II' of Knutson and Munro, 1991), the boundary being marked by a sharp downward increase in gamma-ray values. In other parts of the Beryl Embayment, the underlying mudstone is less well developed, and the boundary is taken at a downward change from sandstone to interbedded mudstone and sandstone, marked by a change from a consistent to a variable gamma-ray response. |
| Definition of Upper Boundary: |
Except in the west, where it underlies Jurassic or Cretaceous rocks, the Lewis Member is conformably overlain by Rhaetic lacustrine mudstones of the Harris Member. The boundary is taken at the top of the highest significant sandstone bed, marked by a downward decrease in average gamma-ray values. The boundary may be diachronous across the Beryl Embayment. |
| Thickness: |
The member is between 70 m and 150 m thick. In the west, the member is truncated beneath a base-Jurassic unconformity; elsewhere, its limit is defined by shaling out of the basal sandstone bed. |
| Geographical Limits: |
The Lewis Member can be identified across east-central areas of the Beryl Embayment. |
| Parent Unit: |
Cormorant Formation (CORM)
|
| Previous Name(s): |
none recorded or not applicable
|
| Alternative Name(s): |
none recorded or not applicable
|
| Stratotypes: |
| Type Section |
North Sea well 9/13-A2: 3254-3321 m (10675-10895 ft) below KB (Cameron, 1993). |
| Reference Section |
North Sea well 9/12a- 8: 3082-3235.5 m (1011 l-10615 ft) (Cameron, 1993). |
| Reference(s): |
| Frostick, L E, Linsey, T K, and Reid, I. 1992. Tectonic and climatic control of Triassic sedimentation in the Beryl Basin, northern North Sea. Journal of the Geological Society, London 149, 13-26. |
| Knutson, C A, and Munro, I C. 1991. The Beryl Field, Block 9/13, UK North Sea. In: Abbotts, I L (ed.) United Kingdom oil and gas fields 25 years commemorative volume. Geological Society, London, Memoir No. 14, 33-42. |
| Lervik, K S, Spencer, A M, and Warrington, G. 1989. Outline of Triassic stratigraphy and structure in the central and northern North Sea. In: Collinson, J D (ed.) Correlation in hydrocarbon exploration, 173-189. Graham and Trotman, London. |
| Steele, L E, and Adams, G E. 1984. A review of the Northern North Sea's Beryl Field after seven years' production. In: 1984 European Petroleum Conference, London. Society of Petroleum Engineers of AIME, Paper No. 12960. |
| Cameron, T D J. 1993. 4. Triassic, Permian and pre-Permian 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. |
| 1:50K maps on which the lithostratigraphical unit is found, and map code used: |
| none recorded or not applicable |
