Editing A sequence-stratigraphy scheme of the Late Carboniferous, southern North Sea, Anglo-Dutch sector

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Besly, B. M. 1990. Carboniferous. In ''Introduction to the petroleum geology of the North Sea'', K. W. Glennie (ed.), 90–119. London: Geological Society.
 
Besly, B. M. 1990. Carboniferous. In ''Introduction to the petroleum geology of the North Sea'', K. W. Glennie (ed.), 90–119. London: Geological Society.
  
Besly, B. M., S. D. Burley, P. Turner 1993. The late Carboniferous “barren redbed” play of the Silver Pit area, southern North Sea. In ''Petroleum geology of northwest Europe: proceedings of the fourth conference'', J. R. Parker (ed.), 727–40. London: Geological Society.  
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Besly, B. M., S. D. Burley, P. Turner 1993. The late Carboniferous “barren redbed” play of the Silver Pit area, southern North Sea. In ''Petroleum geology of northwest Europe: proceedings of the fourth conference'', J. R. Parker (ed.), 727–40. London: Geological Society. Butzer, K. W. 1971. ''Recent history of an Ethiopian delta: the Omb River and the level of Lake Rudolf''. Research Paper 136, Department of Geography, University of Chicago.
 
 
Butzer, K. W. 1971. ''Recent history of an Ethiopian delta: the Omb River and the level of Lake Rudolf''. Research Paper 136, Department of Geography, University of Chicago.
 
  
 
Calver, M. A. 1968. Distribution of Westphalian marine faunas in northern England and adjoining areas. ''Proceedings of the Yorkshire Geological Society ''37, 1–72.
 
Calver, M. A. 1968. Distribution of Westphalian marine faunas in northern England and adjoining areas. ''Proceedings of the Yorkshire Geological Society ''37, 1–72.
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Collinson, J. D., C. M. Jones, G. A. Blackbourn, B. M. Besly, G. M. Archard, A. H. McMahon 1993. Carboniferous depositional systems of the southern North Sea. In ''Petroleum geology of northwest Europe: proceedings of the fourth conference'', J. R. Parker (ed.), 677–87. London: Geological Society.
 
Collinson, J. D., C. M. Jones, G. A. Blackbourn, B. M. Besly, G. M. Archard, A. H. McMahon 1993. Carboniferous depositional systems of the southern North Sea. In ''Petroleum geology of northwest Europe: proceedings of the fourth conference'', J. R. Parker (ed.), 677–87. London: Geological Society.
  
Davies, J. H. & A. E. Trueman 1927. A revision of the non-marine lamellibranchs of the Coal Measures and their zonal sequence. ''Geological Society of London, Quarterly Journal ''83, 210–57.  
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Davies, J. H. & A. E. Trueman 1927. A revision of the non-marine lamellibranchs of the Coal Measures and their zonal sequence. ''Geological Society of London, Quarterly Journal ''83, 210–57. Davies, S. J. & D. McLean 1996. Spectral gamma-ray and palynological characterization of Kinderscoutian marine bands in the Namurian of the Pennine Basin. ''Yorkshire Geological Society, Proceedings ''51, 103–114.
 
 
Davies, S. J. & D. McLean 1996. Spectral gamma-ray and palynological characterization of Kinderscoutian marine bands in the Namurian of the Pennine Basin. ''Yorkshire Geological Society, Proceedings ''51, 103–114.
 
 
 
Eagar, R. M. C. 1994. Discussion of “Classification of Carboniferous non-marine bivalves: systematics versus stratigraphy” by G. M. Vasey. ''Geological Society of London, Journal ''151, 1030–1033.  
 
  
Elliot, T. 1993. High resolution sequence stratigraphy of clastic basin-fill successions. ''Petroleum Exploration Society of Great Britain, Newsletter'', June 1993, 4.
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Eagar, R. M. C. 1994. Discussion of “Classification of Carboniferous non-marine bivalves: systematics versus stratigraphy” by G. M. Vasey. ''Geological Society of London, Journal ''151, 1030–1033. Elliot, T. 1993. High resolution sequence stratigraphy of clastic basin-fill successions. ''Petroleum Exploration Society of Great Britain, Newsletter'', June 1993, 4.
  
 
Fielding, C. R. 1984. Upper delta plain lacustrine and fluviolacustrine facies from the Westphalian of the Durham coalfield, northeast England. ''Sedimentology ''31, 547–67.
 
Fielding, C. R. 1984. Upper delta plain lacustrine and fluviolacustrine facies from the Westphalian of the Durham coalfield, northeast England. ''Sedimentology ''31, 547–67.
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Guion, P. D. 1984. Crevasse-splay deposits and roof-rock quality in the Threequarters seam (Carboniferous) in the East Midlands coalfield, UK. In ''Sedimentology of coal and coal-bearing sequences'', R. A. Rahmani & R. M. Flores (eds), 291–308. Oxford: Blackwell Scientific.
 
Guion, P. D. 1984. Crevasse-splay deposits and roof-rock quality in the Threequarters seam (Carboniferous) in the East Midlands coalfield, UK. In ''Sedimentology of coal and coal-bearing sequences'', R. A. Rahmani & R. M. Flores (eds), 291–308. Oxford: Blackwell Scientific.
  
Guion, P. D., I. M. Fulton, N. S. Jones 1995. Sedimentary facies of the coal-bearing Westphalian A and B north of the Wales–Brabant high. In ''European coal geology'', M. K. G. Whateley & D. A. Spears (eds), 45–78. Special Publication 82, Geological Society, London.  
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Guion, P. D., I. M. Fulton, N. S. Jones 1995. Sedimentary facies of the coal-bearing Westphalian A and B north of the Wales–Brabant high. In ''European coal geology'', M. K. G. Whateley & D. A. Spears (eds), 45–78. Special Publication 82, Geological Society, London. Hallsworth, C. R. & J. I. Chisholm 2000. Stratigraphic evolution of provenance characteristics in Westphalian sandstones of the Yorkshire coalfield. ''Yorkshire Geological Society, Proceedings ''53, 43–72. Hampson, G. 1995. Discrimination of regionally extensive coals in the Upper Carboniferous of the Pennine Basin, UK using high resolution sequence stratigraphic concepts. In ''European coal geology'', M. K. G. Whateley & D. A. Spears (eds), 79–97. Special Publication 82, Geological Society, London.
 
 
Hallsworth, C. R. & J. I. Chisholm 2000. Stratigraphic evolution of provenance characteristics in Westphalian sandstones of the Yorkshire coalfield. ''Yorkshire Geological Society, Proceedings ''53, 43–72.  
 
 
 
Hampson, G. 1995. Discrimination of regionally extensive coals in the Upper Carboniferous of the Pennine Basin, UK using high resolution sequence stratigraphic concepts. In ''European coal geology'', M. K. G. Whateley & D. A. Spears (eds), 79–97. Special Publication 82, Geological Society, London.
 
  
 
Harland, W. B., A. V. Cox, C. Llewellyn, A. G. Pickton, A. G. Smith, R. Walters 1982. ''A geological timescale''. Cambridge: Cambridge University Press.
 
Harland, W. B., A. V. Cox, C. Llewellyn, A. G. Pickton, A. G. Smith, R. Walters 1982. ''A geological timescale''. Cambridge: Cambridge University Press.
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Read, W. A. & I. H. Forsyth 1989. Allocycles and autocycles in the upper part of the Limestone Coal Group (Pendleian E1) in the Glasgow–Stirling region of the Midland Valley of Scotland. ''Geological Journal ''24, 121–37.
 
Read, W. A. & I. H. Forsyth 1989. Allocycles and autocycles in the upper part of the Limestone Coal Group (Pendleian E1) in the Glasgow–Stirling region of the Midland Valley of Scotland. ''Geological Journal ''24, 121–37.
  
Rippon, J. H. 1984. The Clowne seam, marine band, and overlying sediments in the Coal Measures (Westphalian B) of north Derbyshire. ''Yorkshire Geological Society, Proceedings ''45, 27–43.  
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Rippon, J. H. 1984. The Clowne seam, marine band, and overlying sediments in the Coal Measures (Westphalian B) of north Derbyshire. ''Yorkshire Geological Society, Proceedings ''45, 27–43. Ritchie, J. S. & P. Pratsides 1993. The Caister fields, Block 44/23a, UK North Sea. In ''Petroleum geology of northwest Europe: proceedings of the fourth conference'', J. R. Parker (ed.), 759–69. London: Geological Society.
 
 
Ritchie, J. S. & P. Pratsides 1993. The Caister fields, Block 44/23a, UK North Sea. In ''Petroleum geology of northwest Europe: proceedings of the fourth conference'', J. R. Parker (ed.), 759–69. London: Geological Society.
 
  
 
Ritchie, J. S., D. Pilling, S. Hayes 1998. Reservoir development, sequence stratigraphy and geological modelling of Westphalian fluvial reservoirs of the Caister C field, UK southern North Sea. ''Petroleum Geoscience ''4, 203–211.
 
Ritchie, J. S., D. Pilling, S. Hayes 1998. Reservoir development, sequence stratigraphy and geological modelling of Westphalian fluvial reservoirs of the Caister C field, UK southern North Sea. ''Petroleum Geoscience ''4, 203–211.
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Scott, A. C. 1977. A review of the ecology of Upper Carboniferous plant assemblages, with data from Strathclyde. ''Palaeontology ''20, 447–73.
 
Scott, A. C. 1977. A review of the ecology of Upper Carboniferous plant assemblages, with data from Strathclyde. ''Palaeontology ''20, 447–73.
  
Scott, A. C. 1979. The ecology of Coal Measures floras from northern Britain. ''Proceedings of the Geologists’ Association ''90, 97–116.
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Ross, C. A. 1979. The ecology of Coal Measures floras from northern Britain. ''Proceedings of the Geologists’ Association ''90, 97–116.
  
 
Smith, A. H. V. & M. A. Butterworth 1967. ''Miospores in the coal seams of the Carboniferous of Great Britain''. Special Papers in Palaeontology 1, Palaeontological Association, London.
 
Smith, A. H. V. & M. A. Butterworth 1967. ''Miospores in the coal seams of the Carboniferous of Great Britain''. Special Papers in Palaeontology 1, Palaeontological Association, London.
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==== Description ====
 
==== Description ====
  
Low-sinuosity channel deposits consist mainly of sandstones, locally up to 50–60 m thick, with generally minor fine-grained partings and intraformational conglomerates. These sandstones are typically clean, fine to very coarse-grained and range from well sorted to poorly sorted as grain size increases. Grain size may vary widely within sandstone bodies, and overall upward-fining profiles may be apparent. In places, especially in more distal coastal plain areas, low-sinuosity channel sandstones are entirely fine to very fine grained and well sorted. The base of low-sinuosity channel sandstones is typically erosive and the top sharp. Internally, cross bedding is commonly well developed, with asymptotic sets up to ''c. ''1 m thick forming erosively based cosets 2–5 m thick. Larger planar crossbed sets and downcurrent descending compound cosets (cf. Haszeldine 1983) also occur locally.
+
Low-sinuosity channel deposits consist mainly of sandstones, locally up to 50–60m thick, with generally minor fine-grained partings and intraformational conglomerates. These sandstones are typically clean, fine to very coarse-grained and range from well sorted to poorly sorted as grain size increases. Grain size may vary widely within sandstone bodies, and overall upward-fining profiles may be apparent. In places, especially in more distal coastal plain areas, low-sinuosity channel sandstones are entirely fine to very fine grained and well sorted. The base of low-sinuosity channel sandstones is typically erosive and the top sharp. Internally, cross bedding is commonly well developed, with asymptotic sets up to ''c. ''1m thick forming erosively based cosets 2–5m thick. Larger planar crossbed sets and downcurrent descending compound cosets (cf. Haszeldine 1983) also occur locally.
  
 
==== Interpretation ====
 
==== Interpretation ====
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==== Geometry ====
 
==== Geometry ====
  
Mapping in onshore areas with good well control indicates that facies Cl is typically 20–30 m thick, but locally may reach 50–60 m or more. Lateral continuity is good, with channel sandstones extending for several kilometres perpendicular to palaeoflow and at least several tens of kilometres parallel to palaeoflow. Localized zones of stacked low-sinuosity channel sandstones have resulted from contemporaneous tectonism in places (e.g. Haszeldine 1983, Fielding 1984, Leeder & Hardman 1990, Ritchie & Pratsides 1993), and have major implications for reservoir geometry and prospectivity.
+
Mapping in onshore areas with good well control indicates that facies Cl is typically 20–30m thick, but locally may reach 50– 60m or more. Lateral continuity is good, with channel sandstones extending for several kilometres perpendicular to palaeoflow and at least several tens of kilometres parallel to palaeoflow. Localized zones of stacked low-sinuosity channel sandstones have resulted from contemporaneous tectonism in places (e.g. Haszeldine 1983, Fielding 1984, Leeder & Hardman 1990, Ritchie & Pratsides 1993), and have major implications for reservoir geometry and prospectivity.
  
 
==== Wireline log characteristics ====
 
==== Wireline log characteristics ====
  
Facies Cl is characterized by blocky and consistent gamma-ray and sonic-log responses, with pronounced basal and upper inflexions. Gamma response is typically 30–60&nbsp;API and sonic response 70–80&nbsp;µsft<sup>–1</sup>. Localized fluctuations in log response are attributable mainly to the presence of finer-grained strata towards the top of major depositional units.
+
Facies Cl is characterized by blocky and consistent gamma-ray and sonic-log responses, with pronounced basal and upper inflexions. Gamma response is typically 30–60API and sonic response 70–80µsft–1. Localized fluctuations in log response are attributable mainly to the presence of finer-grained strata towards the top of major depositional units.
  
 
=== Facies Ch – High-sinuosity channel deposits ===
 
=== Facies Ch – High-sinuosity channel deposits ===
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==== Geometry ====
 
==== Geometry ====
  
High-sinuosity channel deposits have been mapped in detail on shore in the UK by us at and around Waverly East opencast site, and previously by BP (Eagle Sandstone reservoir) in the Beckingham oilfield. These studies indicate a maximum thickness of 20–25&nbsp;m and lateral continuity for up to 1&nbsp;km perpendicular to palaeoflow and for several kilometres or tens of kilometres parallel to palaeoflow. Although produced by high-sinuosity channel activity, the deposits preserved consist of many point bar units and commonly form relatively straight channel belts.
+
High-sinuosity channel deposits have been mapped in detail on shore in the UK by us at and around Waverly East opencast site, and previously by BP (Eagle Sandstone reservoir) in the Beckingham oilfield. These studies indicate a maximum thickness of 20–25m and lateral continuity for up to 1km perpendicular to palaeoflow and for several kilometres or tens of kilometres parallel to palaeoflow. Although produced by high-sinuosity channel activity, the deposits preserved consist of many point bar units and commonly form relatively straight channel belts.
  
 
==== Wireline log characteristics ====
 
==== Wireline log characteristics ====
  
Facies Ch is invariably characterized by sharp basal gamma-ray and sonic-log inflexions. However, the upper contact may be either sharp or more gradational, depending upon the lithologies developed. The gamma log response is typically in the range 60–75&nbsp;API reflecting the argillaceous nature of the sandstones. It is typically accompanied by a sonic log response of 80–90&nbsp;µsft<sup>–1</sup>. The main variations in wireline log profiles occur in the upper part of sandstone units where gamma log response and δ''T ''values may increase markedly because of the presence of finer-grained units. δ''T ''values tend to decrease sharply at well cemented or concretionary beds.
+
Facies Ch is invariably characterized by sharp basal gamma-ray and sonic-log inflexions. However, the upper contact may be either sharp or more gradational, depending upon the lithologies developed. The gamma log response is typically in the range 60– 75API reflecting the argillaceous nature of the sandstones. It is typically accompanied by a sonic log response of 80–90µsft<sup>–1</sup>. The main variations in wireline log profiles occur in the upper part of sandstone units where gamma log response and δ''T ''values may increase markedly because of the presence of finer-grained units. δ''T ''values tend to decrease sharply at well cemented or concretionary beds.
  
 
=== Facies Cd – Distributary mouth bar ===
 
=== Facies Cd – Distributary mouth bar ===
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==== Description ====
 
==== Description ====
  
Distributary mouth-bar deposits coarsen upwards overall and comprise an upper sandstone unit overlying a lower mudstone– siltstone unit. The lower mudstone–siltstone unit is typically 10– 15&nbsp;m thick and may contain either a marine or non-marine fauna. Overlying sandstones are well sorted and fairly clean, and they generally increase in grain size upwards from very fine to fine; however, they may locally be coarse and may fine upwards. Mudstones and siltstones are mostly parallel laminated or locally homogenized by bioturbation, particularly at marine horizons. The sandstones tend to be ripple cross laminated near the base and trough cross bedded at higher levels. Distributary mouth-bar units are commonly underlain and overlain by coal seams or rooty horizons.
+
Distributary mouth-bar deposits coarsen upwards overall and comprise an upper sandstone unit overlying a lower mudstone– siltstone unit. The lower mudstone–siltstone unit is typically 10– 15m thick and may contain either a marine or non-marine fauna. Overlying sandstones are well sorted and fairly clean, and they generally increase in grain size upwards from very fine to fine; however, they may locally be coarse and may fine upwards. Mudstones and siltstones are mostly parallel laminated or locally homogenized by bioturbation, particularly at marine horizons. The sandstones tend to be ripple cross laminated near the base and trough cross bedded at higher levels. Distributary mouth-bar units are commonly underlain and overlain by coal seams or rooty horizons.
  
 
==== Interpretation ====
 
==== Interpretation ====
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==== Geometry ====
 
==== Geometry ====
  
Mapping suggests that distributary mouth-bar deposits have a lobate geometry and diameter of ''c. ''10–15&nbsp;km. Thicker, Namurian shallow-water delta-front deposits may, however, be of greater lateral extent.
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Mapping suggests that distributary mouth-bar deposits have a lobate geometry and diameter of ''c. ''10–15km. Thicker, Namurian shallow-water delta-front deposits may, however, be of greater lateral extent.
  
====Wireline log characteristics====
+
Wireline log characteristics
  
The gamma-log response of basal mudstones and siltstones is in the range 125–>150&nbsp;API and they have a sonic-log response of 90–100&nbsp;µsft<sup>–1</sup>. The gamma log response gradually decreases upwards to about 35–40&nbsp;API in clean distributary mouth-bar sandstones and to 60–70&nbsp;API in more argillaceous deposits. A corresponding upward decrease in δ''T ''values to an average of 80–90&nbsp;µsft<sup>–1</sup> is also observed. The gradational upward increase in gamma-log response from the basal mudstones–siltstones into sandstone and sharp upper inflexion are characteristic of distributary mouth-bar deposits.
+
The gamma-log response of basal mudstones and siltstones is in the range 125–>150API and they have a sonic-log response of 90–100µsft<sup>–1</sup>. The gamma log response gradually decreases upwards to about 35–40API in clean distributary mouth-bar sandstones and to 60–70API in more argillaceous deposits. A corresponding upward decrease in δ''T ''values to an average of 80–90µsft<sup>–1</sup> is also observed. The gradational upward increase in gamma-log response from the basal mudstones–siltstones into sandstone and sharp upper inflexion are characteristic of distributary mouth-bar deposits.
  
 
=== Facies Oc – Crevasse splay ===
 
=== Facies Oc – Crevasse splay ===
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==== Description ====
 
==== Description ====
  
Crevasse-splay deposits are widely developed in the Upper Carboniferous, and almost invariably occur overlying lake or marine-bay deposits (facies Ol) and underlying swamp or flood-plain deposits (facies Os). The predominant lithology is very fine to fine-grained well sorted argillaceous sandstone, typically 2–5&nbsp;m thick, which is closely associated with interbedded siltstones. Sandstones typically have a gradational base and coarsen upwards from parallel laminated mudstones and siltstones; however, they may occasionally be erosively based and fine upwards. Sedimentary structures include convolute and ripple cross lamination, discontinuous parallel lamination and small-scale trough cross bedding. Rootlets are invariably developed in the upper parts of crevasse-splay sequences.
+
Crevasse-splay deposits are widely developed in the Upper Carboniferous, and almost invariably occur overlying lake or marine-bay deposits (facies Ol) and underlying swamp or flood-plain deposits (facies Os). The predominant lithology is very fine to fine-grained well sorted argillaceous sandstone, typically 2–5m thick, which is closely associated with interbedded silt-stones. Sandstones typically have a gradational base and coarsen upwards from parallel laminated mudstones and siltstones; however, they may occasionally be erosively based and fine upwards. Sedimentary structures include convolute and ripple cross lamination, discontinuous parallel lamination and small-scale trough cross bedding. Rootlets are invariably developed in the upper parts of crevasse-splay sequences.
  
 
==== Interpretation ====
 
==== Interpretation ====
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==== Geometry ====
 
==== Geometry ====
  
Mapping of crevasse-splay deposits during this study and previous published studies (Fielding 1984, Guion 1984, Haszeldine 1984) indicates a lobate geometry with major lobes averaging a few metres in thickness and about 5&nbsp;km across. Within these major lobes, individual lobes measuring a few hundred metres across are often discernible in large-scale outcrops (cf. Fielding 1984). Crevasse-splay feeder channels are typically up to 5–6&nbsp;m thick, several hundred metres across, perpendicular to palaeoflow and several kilometres long parallel to palaeoflow.
+
Mapping of crevasse-splay deposits during this study and previous published studies (Fielding 1984, Guion 1984, Haszeldine 1984) indicates a lobate geometry with major lobes averaging a few metres in thickness and about 5km across. Within these major lobes, individual lobes measuring a few hundred metres across are often discernible in large-scale outcrops (cf. Fielding 1984). Crevasse-splay feeder channels are typically up to 5–6m thick, several hundred metres across, perpendicular to palaeoflow and several kilometres long parallel to palaeoflow.
  
 
==== Wireline log characteristics ====
 
==== Wireline log characteristics ====
  
Facies Oc is characterized by very erratic gamma-ray and sonic-log responses, reflecting the variable interbedding of lithologies. Gamma-log response increases from 120–>150&nbsp;API in the underlying facies Ol mudstones–siltstones to 75–105&nbsp;API in argillaceous very fine to fine-grain crevasse-splay sandstones. A corresponding change also occurs in sonic-log response from 90–100 to 80–90&nbsp;µsft<sup>–1</sup>. Crevasse-splay deposits are characterized by gradationally based upwards-decreasing log responses with sharp inflections at the upper contact with overlying coal seams or lake or marine mudstones.
+
Facies Oc is characterized by very erratic gamma-ray and sonic-log responses, reflecting the variable interbedding of lithologies. Gamma-log response increases from 120–>150API in the underlying facies Ol mudstones–siltstones to 75–105API in argillaceous very fine to fine-grain crevasse-splay sandstones. A corresponding change also occurs in sonic-log response from 90–100 to 80–90µsft<sup>–1</sup>. Crevasse-splay deposits are characterized by gradationally based upwards-decreasing log responses with sharp inflections at the upper contact with overlying coal seams or lake or marine mudstones.
  
 
=== Facies Ol – Lake and marine-bay deposits ===
 
=== Facies Ol – Lake and marine-bay deposits ===
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==== Description ====
 
==== Description ====
  
Lake and marine-bay deposits consist essentially of blocky, dark grey to black mudstones and siltstones, which may contain either freshwater or marine faunas. Thin very fine-grained sandstones and nodular horizons are interbedded locally in places. Deposits range in thickness from a few tens of centimetres to exceptionally 15–20&nbsp;m, with an average thickness in the range 2–10&nbsp;m. The main sedimentary structure is parallel lamination; however, convoluting and bioturbation may also occur in places. Lake and marine-bay deposits almost invariably overlie swamp or flood-plain sediments (facies Os) with a sharp contact, and are overlain gradationally by either crevasse splay (facies Oc) or distributary mouth-bar (facies Cd) sediments.
+
Lake and marine-bay deposits consist essentially of blocky, dark grey to black mudstones and siltstones, which may contain either freshwater or marine faunas. Thin very fine-grained sandstones and nodular horizons are interbedded locally in places. Deposits range in thickness from a few tens of centimetres to exceptionally 15–20m, with an average thickness in the range 2–10m. The main sedimentary structure is parallel lamination; however, convoluting and bioturbation may also occur in places. Lake and marine-bay deposits almost invariably overlie swamp or flood-plain sediments (facies Os) with a sharp contact, and are overlain gradationally by either crevasse splay (facies Oc) or distributary mouth-bar (facies Cd) sediments.
  
 
==== Interpretation ====
 
==== Interpretation ====
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==== Geometry ====
 
==== Geometry ====
  
Mapping of lake and marine-bay facies reveals an uninterrupted areal extent of several hundred square kilometres. Thickness variations suggest that the depth of lakes and bays ranged from less than 10&nbsp;m to over 30&nbsp;m in places.
+
Mapping of lake and marine-bay facies reveals an uninterrupted areal extent of several hundred square kilometres. Thickness variations suggest that the depth of lakes and bays ranged from less than 10m to over 30m in places.
  
 
==== Wireline log characteristics ====
 
==== Wireline log characteristics ====
  
Lake and marine-bay mudstones and siltstones are characterized by high gamma-log responses of 120–>150&nbsp;API and δ''T ''values of 90–105&nbsp;µsft<sup>–1</sup>. Marine horizons commonly display particularly high gamma responses, often in excess of 150&nbsp;API. The presence of nodular beds causes a marked reduction in gamma-log response and δ''T ''values.
+
Lake and marine-bay mudstones and siltstones are characterized by high gamma-log responses of 120–>150API and δ''T ''values of 90–105µsft<sup>–1</sup>. Marine horizons commonly display particularly high gamma responses, often in excess of 150 API. The presence of nodular beds causes a marked reduction in gamma-log response and δ''T ''values.
  
 
=== Facies Os – swamp/floodplain deposits ===
 
=== Facies Os – swamp/floodplain deposits ===
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==== Description ====
 
==== Description ====
  
This facies comprises coal seams and associated rootlet-disrupted mudstones, siltstones and fine-grained sandstones. Coal seams are up to 2–3&nbsp;m thick and are typically banded, whereas the associated clastic strata, also on average 2–3&nbsp;m thick, are invariably structureless, because of disturbance by rootlets. Nodules of pyrite or siderite may be developed within all these lithologies. Swamp/floodplain sediments occur mainly at the top of cyclothems overlying channel (facies Cl and Ch), distributary mouth-bar (facies Cd), crevasse-splay (facies Cl) and locally lake or marine bay (facies Ol) deposits.
+
This facies comprises coal seams and associated rootlet-disrupted mudstones, siltstones and fine-grained sandstones. Coal seams are up to 2–3m thick and are typically banded, whereas the associated clastic strata, also on average 2–3m thick, are invariably structureless, because of disturbance by rootlets. Nodules of pyrite or siderite may be developed within all these lithologies. Swamp/floodplain sediments occur mainly at the top of cyclothems overlying channel (facies Cl and Ch), distributary mouth-bar (facies Cd), crevasse-splay (facies Cl) and locally lake or marine bay (facies Ol) deposits.
  
 
==== Interpretation ====
 
==== Interpretation ====
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==== Geometry ====
 
==== Geometry ====
  
Published studies (e.g. McCabe 1984) and mapping by us in the East Midlands indicate a laterally extensive sheet geometry extending over several hundred square kilometres. Total thickness is typically 2–4&nbsp;m, perhaps exceptionally reaching 6m or more in localized areas.
+
Published studies (e.g. McCabe 1984) and mapping by us in the East Midlands indicate a laterally extensive sheet geometry extending over several hundred square kilometres. Total thickness is typically 2–4m, perhaps exceptionally reaching 6m or more in localized areas.
  
 
==== Wireline log response ====
 
==== Wireline log response ====
  
Coal seams exhibit a very distinctive wireline log response characterized by relatively high gamma ray (''c. ''75&nbsp;API) and very high sonic-log values (>110&nbsp;µsft<sup>–1</sup>). Associated floodplain clastic strata typically give gamma log responses of 75–105&nbsp;API, reflecting the predominantly silty lithology and sonic responses of 80–90&nbsp;µsft<sup>–1</sup>. Compared with lithologically similar facies Ol deposits, the relatively low log values are probably attributable to the common occurrence of ironstone nodules in clastic floodplain sediments.
+
Coal seams exhibit a very distinctive wireline log response characterized by relatively high gamma ray (''c. ''75 API) and very high sonic-log values (>110µsft<sup>–1</sup>). Associated floodplain clastic strata typically give gamma log responses of 75–105API, reflecting the predominantly silty lithology and sonic responses of 80– 90µsft<sup>–1</sup>. Compared with lithologically similar facies Ol deposits, the relatively low log values are probably attributable to the common occurrence of ironstone nodules in clastic floodplain sediments.
 
 
[[category:Carboniferous hydrocarbon resources: the southern North Sea and surrounding onshore areas ]]
 

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