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

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==== 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.
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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–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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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.
 
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.
  
====Wireline log characteristics====
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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–>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.
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==== 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.
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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.
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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.
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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.
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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.
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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.
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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.
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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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