| Computer Code: |
CTCO |
Preferred Map Code: |
notEntered |
| Status Code: |
Full |
| Age range: |
Langsettian Substage (CA)
— Duckmantian Substage (CB) |
| Lithological Description: |
The Caister Coal Formation is composed of grey mudstones and silty mudstones, with varying proportions of seat earths, coal seams, and sandstone beds. One or more marine bands of dark grey or black shale may also be present towards the base of the formation, as such are present in onshore Westphalian A sections (refer to Ramsbottom et al., 1978). The A. vanderbeckei marine band has been identified near the top of the formation in cored sections from southern Quadrant 44. However, none of the marine bands forms a distinctive marker on the wireline logs. Coal seams up to 3 m thick constitute up to 5 per cent of the formation, and some may be very widespread. White and grey sandstone beds constitute between 15 and 45 per cent of the formation, and display a range of upward-fining, upward-coarsening, blocky, and ratty wireline-log responses. Bed thickness generally ranges between 1 and 25 m, but the reservoir sandstones of the Murdoch and Caister fields are up to 50 m thick. The thickest sandstones have sharp bases and mainly gradational tops, and are composite, stacked channel units, containing intervals of argillaceous and carbonaceous detritus that accumulated during periods of channel abandonment. Grain size of the sandstones is mainly very fine, fine, or occasionally medium, but some of the composite beds contain coarser pebbly horizons. Most of the sandstones are argillaceous, but the pebbly reservoir sandstones of the Caister and Murdoch fields are clean. Wireline log data enable cyclicity of sedimentation to be resolved on a variety of scales in most sections. Small-scale upward-coarsening cycles capped by a siltstone or sandstone bed, often overlain by a seat earth and coal seam, are commonly between 10 and 30 m, but are occasionally up to 50 m thick. Larger-scale cycles, defined by an upward increase in the proportion and thickness of sandstone beds, are exceptionally up to 300 m thick. |
| Definition of Lower Boundary: |
The Caister Coal Formation rests on the Millstone Grit Formation in all wells, and the boundary is defined with reference to the basal cluster of coal seams in the Upper Carboniferous succession. Specifically, the base of the Caister Coal Formation is defined along the base of the lowest coal seam of the coal measures, marked by the basal low-velocity spike on sonic logs. As so defined, the formational boundary is easily identified in most long sections, but is occasionally less clear where the Westphalian A sequence contains relatively few coal seams (e.g. 48/3-3). |
| Definition of Upper Boundary: |
In large areas of the Southern North Sea the Caister Coal Formation is unconformably overlain by Lower Permian red and grey desert sandstones (Leman Sandstone Formation) or, mainly to the north of 54°N, by reddish-brown sabkha and lacustrine mudstones and evaporites (Silverpit Formation). The uppermost coal measures are locally secondarily reddened in these areas, commonly for as much as 30 m beneath the basal Permian strata. The top of the Caister Coal Formation is a generally sharp facies transition from uniform Permian sandstones or mudstones above the unconformity, to interbedded sandstones and mudstones below. In sections where the facies transition is not clear from wireline logs, it may be located by a downward change from the brighter hues of the Permian red sediments to the duller hues of the secondarily reddened coal measures. The Caister Coal Formation is overlain by the Westoe Coal Formation in remaining areas of the Southern North Sea, the boundary being defined by the upward lithofacies transition from sandy to argillaceous coal measures. This transition is typically sharp (e.g. 44/23-4; 44/27-1), occurring at the top of a large-scale upward-coarsening cycle. Overlying coal measures contain a significantly lower component of sandstone beds. The formational boundary is defined by the top of the highest sandstone bed of the sandy coal measures. This boundary is regionally diachronous. |
| Thickness: |
Up to 750 m (Q 49/4). |
| Geographical Limits: |
The distribution and thickness of the Caister Coal Formation are influenced both by the degree of preservation of Westphalian strata beneath the regional base-Permian unconformity, and by the southward fades change from sandy to argillaceous coal measures within the Westphalian A deposits. Hence, the formation is widespread and thick along the axes of the major WNW-ESE trending Variscan synclines in the north, but is thin or absent south of about 53°30'N in the UK sector. The Caister Coal Formation is up to 550 m thick in the Murdoch and Caister fields and is about 750 m thick in block 49/4. |
| Parent Unit: |
Conybeare Group (CNYB)
|
| Previous Name(s): |
none recorded or not applicable
|
| Alternative Name(s): |
none recorded or not applicable
|
| Stratotypes: |
| Type Section |
North Sea well 44/23- 4: 3637-4138.5 m (11932-13578 ft) below KB (Cameron, 1993). |
| Reference Section |
North Sea well 41/20- 1: 2190.5-2520.5 m (7187-8270 ft) (Cameron, 1993). |
| Reference Section |
North Sea well 44/29- 1A: 3793-4307 m (12443-14131 ft) (Cameron, 1993). |
| Reference Section |
North Sea well 49/04- 1: 3347.5-4087.5 m (10982-13410 ft) (Cameron, 1993). |
| Reference(s): |
| Ramsbottom, W H C, Calver, M A, Eagar, R M C, Hodson, F, Holliday, D W, Stubblefield, C J and Wilson, R B. 1978. A correlation of the Silesian rocks in the British Isles. Geological Society of London, Special Report No.10. |
| Ritchie, J S and Pratsides, P. 1993. The Caister Fields, Block 44/23a, UK North Sea. In: Parker, J R (ed.) Petroleum geology of Northwest Europe: Proceedings of the 4th Conference, 759-769. Geological Society, London. |
| Cameron, T D J. 1993. 5. Carboniferous and Devonian of the Southern 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, Smith, K, Hopson, P M, Wilson, D, Bridge, D M, Carney, J N, Cooper, A H, Crofts, R G, Ellison, R A, Mathers, S J, Moorlock, B S P, Scrivener, R C, McMillan, A A, Ambrose, K, Barclay, W J, and Barron, A J M. 2007. Stratigraphical Chart of the United Kingdom: Southern Britain. British Geological Survey, 1 poster. |
| 1:50K maps on which the lithostratigraphical unit is found, and map code used: |
| none recorded or not applicable |
