Editing Fluvial sandbody architecture, cyclicity and sequence stratigraphic setting – implications for hydrocarbon reservoirs: the Westphalian C and D of the Osnabrück–Ibbenbüren area, northwest Germany

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=== 2.1 Channel belts ===
 
=== 2.1 Channel belts ===
  
The term “channel belt” describes compound sandbodies comprising the deposits of separate channels that amalgamate to form multi-storey and multi-lateral units. Channel belts are of large lateral extent and thickness (tens of kilometres in width and up to 50 m in thickness) and rest on laterally extensive erosion surfaces. Geophysical log correlations demonstrate that these surfaces are correlatable for many kilometres, possibly even tens of kilometres parallel to flow ([[:File:YGS_CHR_05_FLUV_FIG_03.jpg|Figure 3]]). In the Piesberg quarry, four channel belts are identified, although only the lowermost few metres of the top one is exposed ([[:File:YGS_CHR_05_FLUV_FIG_04.jpg|Figure 4]]).
+
The term “channel belt” describes compound sandbodies comprising the deposits of separate channels that amalgamate to form multi-storey and multi-lateral units. Channel belts are of large lateral extent and thickness (tens of kilometres in width and up to 50m in thickness) and rest on laterally extensive erosion surfaces. Geophysical log correlations demonstrate that these surfaces are correlatable for many kilometres, possibly even tens of kilometres parallel to flow ([[:File:YGS_CHR_05_FLUV_FIG_03.jpg|Figure 3]]). In the Piesberg quarry, four channel belts are identified, although only the lowermost few metres of the top one is exposed ([[:File:YGS_CHR_05_FLUV_FIG_04.jpg|Figure 4]]).
  
 
Within each channel belt ([[:File:YGS_CHR_05_FLUV_FIG_05.jpg|Figure 5]]) it is possible to work out a chronology of channels based on the occurrence of cross-cutting erosion surfaces. The internal architecture of each channel sand-body can be described in terms of large-scale channel elements, of which barforms are the most important. Barforms are macro-forms that scale in width and depth to that of the channel in which they formed and they represent long-term products of river systems (American Society of Civil Engineers 1963, Bridge 1985). These are described in more detail in Tables 2 and 3.
 
Within each channel belt ([[:File:YGS_CHR_05_FLUV_FIG_05.jpg|Figure 5]]) it is possible to work out a chronology of channels based on the occurrence of cross-cutting erosion surfaces. The internal architecture of each channel sand-body can be described in terms of large-scale channel elements, of which barforms are the most important. Barforms are macro-forms that scale in width and depth to that of the channel in which they formed and they represent long-term products of river systems (American Society of Civil Engineers 1963, Bridge 1985). These are described in more detail in Tables 2 and 3.
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==== 2.2.1 Downstream accretion-dominated channel ====
 
==== 2.2.1 Downstream accretion-dominated channel ====
  
Channels dominated by downstream accretion form about 52 per cent of the succession and generally occurring in the lower parts of channel belts ([[:File:YGS_CHR_05_FLUV_FIG_04.jpg|Figure 4]], [[:File:YGS_CHR_05_FLUV_FIG_05.jpg|Figure 5]]). Channels typically vary from 10 m to 20 m in thickness, although complete channel fills are rarely preserved because of erosion by overlying channels. They have widths in excess of the working area of the quarries (i.e. hundreds of metres or more). Channel bases are highly irregular and erosive, scouring down in places by up to 10 m, and typically have an overlying pebbly lag conglomerate composed of intraformational mudstone, coal and extraformational vein quartz, with minor chert and quartzite clasts. Sandstones form up to 95 per cent of the channel fill. This varies from fine- to coarse-grained, and is pebbly in places. In addition, intraformational mudstone, coal and rare siderite clasts occur. Siltstone is present, forming thin, laterally impersistent beds and laminae.
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Channels dominated by downstream accretion form about 52 per cent of the succession and generally occurring in the lower parts of channel belts ([[:File:YGS_CHR_05_FLUV_FIG_04.jpg|Figure 4]], [[:File:YGS_CHR_05_FLUV_FIG_05.jpg|Figure 5]]). Channels typically vary from 10m to 20m in thickness, although complete channel fills are rarely preserved because of erosion by overlying channels. They have widths in excess of the working area of the quarries (i.e. hundreds of metres or more). Channel bases are highly irregular and erosive, scouring down in places by up to 10m, and typically have an overlying pebbly lag conglomerate composed of intraformational mudstone, coal and extraformational vein quartz, with minor chert and quartzite clasts. Sandstones form up to 95 per cent of the channel fill. This varies from fine- to coarse-grained, and is pebbly in places. In addition, intraformational mudstone, coal and rare siderite clasts occur. Siltstone is present, forming thin, laterally impersistent beds and laminae.
  
A variety of different bedforms and barforms occur. Both mid-channel and side-attached barforms are present, and simple and compound types can occur ([[:File:YGS_CHR_05_FLUV_TAB_02.jpg|Table 2]]). These barforms can be more than 12 m thick. Trough and planar-tabular cross bedding are common sedimentary structures ([[:File:YGS_CHR_05_FLUV_TAB_03.jpg|Table 3]]). Foreset and bounding-surface measurements generally show unidirectional trends (towards the northwest) and indicate that downstream accretion was the dominant process, although limited amounts of lateral accretion have been documented ([[:File:YGS_CHR_05_FLUV_FIG_05.jpg|Figure 5]]).
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A variety of different bedforms and barforms occur. Both mid-channel and side-attached barforms are present, and simple and compound types can occur ([[:File:YGS_CHR_05_FLUV_TAB_02.jpg|Table 2]]). These barforms can be more than 12m thick. Trough and planar-tabular cross bedding are common sedimentary structures ([[:File:YGS_CHR_05_FLUV_TAB_03.jpg|Table 3]]). Foreset and bounding-surface measurements generally show unidirectional trends (towards the northwest) and indicate that downstream accretion was the dominant process, although limited amounts of lateral accretion have been documented ([[:File:YGS_CHR_05_FLUV_FIG_05.jpg|Figure 5]]).
  
 
==== 2.2.2 Lateral accretion-dominated channel ====
 
==== 2.2.2 Lateral accretion-dominated channel ====
  
This type of channel forms up to approximately 30 per cent of the observed facies and is usually restricted to the upper third of an individual channel belt ([[:File:YGS_CHR_05_FLUV_FIG_04.jpg|Figure 4]]). Complete channel fills are commonly preserved and each channel may be interbedded with floodplain facies. These channels are smaller than the channels dominated by downstream accretion, 4–15&nbsp;m thick, and comprise sediment bodies 100–400&nbsp;m wide. Although sand-dominated, the fill of this type of channel is more varied. Siltstone forms an appreciable component, and claystone and coal are present in minor amounts (<3%), the latter two generally restricted to abandoned channel plugs.
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This type of channel forms up to approximately 30 per cent of the observed facies and is usually restricted to the upper third of an individual channel belt ([[:File:YGS_CHR_05_FLUV_FIG_04.jpg|Figure 4]]). Complete channel fills are commonly preserved and each channel may be interbedded with floodplain facies. These channels are smaller than the channels dominated by downstream accretion, 4–15m thick, and comprise sediment bodies 100–400m wide. Although sand-dominated, the fill of this type of channel is more varied. Siltstone forms an appreciable component, and claystone and coal are present in minor amounts (<3%), the latter two generally restricted to abandoned channel plugs.
  
 
Compound side-attached barforms tend to form important elements in these channels; mid-channel bars are absent. The former are gently inclined (<10°), asymptotic or sigmoidal in form and are heterolithic, generally showing a decrease in both grain size and scale of sedimentary structure up the length of individual beds ([[:File:YGS_CHR_05_FLUV_TAB_02.jpg|Table 2]]). Palaeocurrent measurements are usually oblique or normal to the dip of the accretion surfaces in these channels, indicating that lateral accretion was important ([[:File:YGS_CHR_05_FLUV_FIG_05.jpg|Figure 5]]). Adjacent to this type of barform a mud-filled lenticular channel plug is usually preserved ([[:File:YGS_CHR_05_FLUV_FIG_05.jpg|Figure 5]]).
 
Compound side-attached barforms tend to form important elements in these channels; mid-channel bars are absent. The former are gently inclined (<10°), asymptotic or sigmoidal in form and are heterolithic, generally showing a decrease in both grain size and scale of sedimentary structure up the length of individual beds ([[:File:YGS_CHR_05_FLUV_TAB_02.jpg|Table 2]]). Palaeocurrent measurements are usually oblique or normal to the dip of the accretion surfaces in these channels, indicating that lateral accretion was important ([[:File:YGS_CHR_05_FLUV_FIG_05.jpg|Figure 5]]). Adjacent to this type of barform a mud-filled lenticular channel plug is usually preserved ([[:File:YGS_CHR_05_FLUV_FIG_05.jpg|Figure 5]]).
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=== 2.3 Floodplain facies ===
 
=== 2.3 Floodplain facies ===
  
Non-channelized floodplain deposits are represented by three facies associations: lake, mire and poorly drained alluvial flood-plain, forming up to 18 per cent of the succession. These facies associations can be divided into eight facies ([[:File:YGS_CHR_05_FLUV_TAB_04.jpg|Table 4]]). They typically form the upper parts of gross upward-fining successions, where they form discrete layers, 2–10&nbsp;m thick, that separate the channel belts ([[:File:YGS_CHR_05_FLUV_FIG_04.jpg|Figure 4]], [[:File:YGS_CHR_05_FLUV_FIG_05.jpg|Figure 5]]).
+
Non-channelized floodplain deposits are represented by three facies associations: lake, mire and poorly drained alluvial flood-plain, forming up to 18 per cent of the succession. These facies associations can be divided into eight facies ([[:File:YGS_CHR_05_FLUV_TAB_04.jpg|Table 4]]). They typically form the upper parts of gross upward-fining successions, where they form discrete layers, 2–10m thick, that separate the channel belts ([[:File:YGS_CHR_05_FLUV_FIG_04.jpg|Figure 4]], [[:File:YGS_CHR_05_FLUV_FIG_05.jpg|Figure 5]]).
  
 
No particular vertical or lateral changes in lacustrine or palaeosol facies types were recorded at outcrop, although studies by the authors of subsurface successions in northern Germany reveal a systematic change in palaeosol types, with younger (late Westphalian D to Stephanian) parts of the succession showing evidence for progressively better-drained palaeosol types, including ferrosols and calcretes.
 
No particular vertical or lateral changes in lacustrine or palaeosol facies types were recorded at outcrop, although studies by the authors of subsurface successions in northern Germany reveal a systematic change in palaeosol types, with younger (late Westphalian D to Stephanian) parts of the succession showing evidence for progressively better-drained palaeosol types, including ferrosols and calcretes.
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The facies described suggest that deposition occurred on a large alluvial plain where laterally extensive fluvial channel systems formed the most important component. This is in general agreement with earlier models proposed for this part of the Westphalian in northwestern Germany (David 1990, Jankowski et al. 1993). Palaeocurrent measurements indicate that the dominant flow direction was towards the west-northwest ([[:File:YGS_CHR_05_FLUV_FIG_06.jpg|Figure 6]]). Hence, the Bolsovian to Westphalian D succession is believed to record a fluvial system fed from the highlands beyond the Variscan Front to the southeast of the district. A significant southwesterly palaeocurrent component and northeasterly increase in net-togross ratio was also recognized during this study ([[:File:YGS_CHR_05_FLUV_FIG_03.jpg|Figure 3]], [[:File:YGS_CHR_05_FLUV_FIG_07.jpg|Figure 7]]), and may indicate the existence, at times, of an axially fed fluvial system, derived from the northeast or east. The lack of marine facies suggests a setting far removed from the open sea.
 
The facies described suggest that deposition occurred on a large alluvial plain where laterally extensive fluvial channel systems formed the most important component. This is in general agreement with earlier models proposed for this part of the Westphalian in northwestern Germany (David 1990, Jankowski et al. 1993). Palaeocurrent measurements indicate that the dominant flow direction was towards the west-northwest ([[:File:YGS_CHR_05_FLUV_FIG_06.jpg|Figure 6]]). Hence, the Bolsovian to Westphalian D succession is believed to record a fluvial system fed from the highlands beyond the Variscan Front to the southeast of the district. A significant southwesterly palaeocurrent component and northeasterly increase in net-togross ratio was also recognized during this study ([[:File:YGS_CHR_05_FLUV_FIG_03.jpg|Figure 3]], [[:File:YGS_CHR_05_FLUV_FIG_07.jpg|Figure 7]]), and may indicate the existence, at times, of an axially fed fluvial system, derived from the northeast or east. The lack of marine facies suggests a setting far removed from the open sea.
  
The identification of two different types of channel sandbody indicates two distinct fluvial styles. The channels dominated by downstream accretion were bedload-dominated rivers, characterized by large barforms that migrated predominantly down stream. Channels were probably braided in form and produced fairly wide and extensive river tracts. The presence of different scales of channels, together with the evidence for large channel barforms, suggests that channel belts formed from multi-channel depositional systems. Their high degree of lateral mobility resulted in channel belts that are much wider than those of the active channels. Minimum channel depths must be greater than the preserved thickness of individual barforms, so deep rivers in excess of 15&nbsp;m are envisaged in some instances, although they probably ranged from 10&nbsp;m to 20&nbsp;m. These should be considered as minimum figures and are derived from the post-compactional thicknesses of the preserved sandbodies. The width of the overall braided river system is difficult to quantify but may have been a kilometre or more.
+
The identification of two different types of channel sandbody indicates two distinct fluvial styles. The channels dominated by downstream accretion were bedload-dominated rivers, characterized by large barforms that migrated predominantly down stream. Channels were probably braided in form and produced fairly wide and extensive river tracts. The presence of different scales of channels, together with the evidence for large channel barforms, suggests that channel belts formed from multi-channel depositional systems. Their high degree of lateral mobility resulted in channel belts that are much wider than those of the active channels. Minimum channel depths must be greater than the preserved thickness of individual barforms, so deep rivers in excess of 15m are envisaged in some instances, although they probably ranged from 10m to 20m. These should be considered as minimum figures and are derived from the post-compactional thicknesses of the preserved sandbodies. The width of the overall braided river system is difficult to quantify but may have been a kilometre or more.
  
The channels dominated by lateral accretion produce thinner, more heterolithic deposits. The absence of mid-channel bars suggests that the flow was generally confined within single channels. Palaeocurrent measurements indicate that flow was mainly across the barforms and that lateral accretion processes were important (cf. Smith 1987, Leeder 1999). Hence it is considered that these represent point bars and that the channel deposits were the result of highly sinuous rivers. In many instances the full channel thickness is preserved and a range of channel depths is indicated, varying from 4&nbsp;m to 15&nbsp;m. Again, these should be considered as minimum figures, derived from the post-compactional thicknesses of the preserved sandbodies. Active channels were narrower than the channels dominated by downstream accretion, and widths, based on preserved abandoned channel plugs and point bar length, probably ranged from 40&nbsp;m to 150&nbsp;m. The sandbodies produced by the process of lateral accretion are obviously larger than the original channel widths, and typically range from 100&nbsp;m to 400&nbsp;m across.
+
The channels dominated by lateral accretion produce thinner, more heterolithic deposits. The absence of mid-channel bars suggests that the flow was generally confined within single channels. Palaeocurrent measurements indicate that flow was mainly across the barforms and that lateral accretion processes were important (cf. Smith 1987, Leeder 1999). Hence it is considered that these represent point bars and that the channel deposits were the result of highly sinuous rivers. In many instances the full channel thickness is preserved and a range of channel depths is indicated, varying from 4m to 15m. Again, these should be considered as minimum figures, derived from the post-compactional thicknesses of the preserved sandbodies. Active channels were narrower than the channels dominated by downstream accretion, and widths, based on preserved abandoned channel plugs and point bar length, probably ranged from 40m to 150m. The sandbodies produced by the process of lateral accretion are obviously larger than the original channel widths, and typically range from 100m to 400m across.
  
 
The channels dominated by lateral accretion deposited a mixture of bedload and suspended sediment load. The finer grain size of some not all) of these channels indicates that they were probably muddier than those formed by downstream accretion. Flow fluctuations are indicated by mud-filled scours on bar tops. Channel abandonment was an important process, possibly linked to meander (neck or chute) cut-off. Following abandonment, these channels formed shallow oxbow lakes that filled with a variety of low-energy deposits, mainly muds, but including peat.
 
The channels dominated by lateral accretion deposited a mixture of bedload and suspended sediment load. The finer grain size of some not all) of these channels indicates that they were probably muddier than those formed by downstream accretion. Flow fluctuations are indicated by mud-filled scours on bar tops. Channel abandonment was an important process, possibly linked to meander (neck or chute) cut-off. Following abandonment, these channels formed shallow oxbow lakes that filled with a variety of low-energy deposits, mainly muds, but including peat.
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First-order cycles are typically hundreds of metres thick (i.e. generally within biostratigraphic control) and are beyond outcrop resolution. They commence with a widely developed sandstone complex ([[:File:YGS_CHR_05_FLUV_FIG_03.jpg|Figure 3]]), the base of which is typically coarse grained and conglomeratic. In relatively proximal locations (east and southeast) these thick multi-storey–multi-lateral complexes (up to several tens of metres thick) form packages of sandstone that are remarkable in their lateral extent; log correlation suggests that they cover areas of hundreds of square kilometres ([[:File:YGS_CHR_05_FLUV_FIG_03.jpg|Figure 3]]). In more distal settings (west and northwest), the percentage of sandstone decreases and the sands are typically finer grained, but still amalgamate at the bases of cycles ([[:File:YGS_CHR_05_FLUV_FIG_03.jpg|Figure 3]], [[:File:YGS_CHR_05_FLUV_FIG_07.jpg|Figure 7]]).
 
First-order cycles are typically hundreds of metres thick (i.e. generally within biostratigraphic control) and are beyond outcrop resolution. They commence with a widely developed sandstone complex ([[:File:YGS_CHR_05_FLUV_FIG_03.jpg|Figure 3]]), the base of which is typically coarse grained and conglomeratic. In relatively proximal locations (east and southeast) these thick multi-storey–multi-lateral complexes (up to several tens of metres thick) form packages of sandstone that are remarkable in their lateral extent; log correlation suggests that they cover areas of hundreds of square kilometres ([[:File:YGS_CHR_05_FLUV_FIG_03.jpg|Figure 3]]). In more distal settings (west and northwest), the percentage of sandstone decreases and the sands are typically finer grained, but still amalgamate at the bases of cycles ([[:File:YGS_CHR_05_FLUV_FIG_03.jpg|Figure 3]], [[:File:YGS_CHR_05_FLUV_FIG_07.jpg|Figure 7]]).
  
Each first-order cycle can usually be further divided into four or five smaller-scale second-order cycles ([[:File:YGS_CHR_05_FLUV_FIG_03.jpg|Figure 3]]). These are usually beyond outcrop resolution, although excellent exposures at Piesberg quarry enabled a near-complete second-order cycle to be examined ([[:File:YGS_CHR_05_FLUV_FIG_04.jpg|Figure 4]], [[:File:YGS_CHR_05_FLUV_FIG_05.jpg|Figure 5]]). A typical second-order cycle comprises a gross upward-fining succession typically 120–200&nbsp;m thick ([[:File:YGS_CHR_05_FLUV_TAB_05.jpg|Table 5]]). Coarse-grained, often conglomeratic, sandstones form thick sandbody complexes at the base of such cycles, passing upwards into floodplain mudstones and multiple palaeosols (including coal) at the top. These cycles can usually be correlated for tens of kilometres laterally ([[:File:YGS_CHR_05_FLUV_FIG_03.jpg|Figure 3]]).
+
Each first-order cycle can usually be further divided into four or five smaller-scale second-order cycles ([[:File:YGS_CHR_05_FLUV_FIG_03.jpg|Figure 3]]). These are usually beyond outcrop resolution, although excellent exposures at Piesberg quarry enabled a near-complete second-order cycle to be examined ([[:File:YGS_CHR_05_FLUV_FIG_04.jpg|Figure 4]], [[:File:YGS_CHR_05_FLUV_FIG_05.jpg|Figure 5]]). A typical second-order cycle comprises a gross upward-fining succession typically 120–200m thick ([[:File:YGS_CHR_05_FLUV_TAB_05.jpg|Table 5]]). Coarse-grained, often conglomeratic, sandstones form thick sandbody complexes at the base of such cycles, passing upwards into floodplain mudstones and multiple palaeosols (including coal) at the top. These cycles can usually be correlated for tens of kilometres laterally ([[:File:YGS_CHR_05_FLUV_FIG_03.jpg|Figure 3]]).
  
Third-order cycles produce fining-upwards successions up to about 60&nbsp;m thick ([[:File:YGS_CHR_05_FLUV_TAB_05.jpg|Table 5]], [[:File:YGS_CHR_05_FLUV_FIG_04.jpg|Figure 4]]). Outcrop studies allowed for the detailed examination of this order of cyclicity. The sedimentary facies show a predictable stacking pattern, characterized by channel-belt-dominated facies in the lower part (82%) and floodplain-dominated facies in the upper part (18%) ([[:File:YGS_CHR_05_FLUV_FIG_04.jpg|Figure 4]], [[:File:YGS_CHR_05_FLUV_FIG_05.jpg|Figure 5]]).
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Third-order cycles produce fining-upwards successions up to about 60m thick ([[:File:YGS_CHR_05_FLUV_TAB_05.jpg|Table 5]], [[:File:YGS_CHR_05_FLUV_FIG_04.jpg|Figure 4]]). Outcrop studies allowed for the detailed examination of this order of cyclicity. The sedimentary facies show a predictable stacking pattern, characterized by channel-belt-dominated facies in the lower part (82%) and floodplain-dominated facies in the upper part (18%) ([[:File:YGS_CHR_05_FLUV_FIG_04.jpg|Figure 4]], [[:File:YGS_CHR_05_FLUV_FIG_05.jpg|Figure 5]]).
  
 
'''[[:File:YGS_CHR_05_FLUV_TAB_04.jpg|Table 4]] Detailed descriptions of the poorly drained alluvial floodplain, lake and mire facies associations recognized from the Upper Carboniferous in the Osnabrück–Ibbenbüren area.'''
 
'''[[:File:YGS_CHR_05_FLUV_TAB_04.jpg|Table 4]] Detailed descriptions of the poorly drained alluvial floodplain, lake and mire facies associations recognized from the Upper Carboniferous in the Osnabrück–Ibbenbüren area.'''
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| Poorly drained alluvial floodplain || Alluvial palaeosol/overbank || Claystone and siltstone, reddish brown, purple grey, greenish grey, sandy lenses, rooted, mottles, listrics, brecciated, destratified, rare desiccation cracks, sphaerosiderite. In successions up to 6m thick, laterally continuous for hundreds of metres. || Suspension deposition on a vegetated alluvial floodplain. Shallow water becoming well-drained at times. Succession is pedoturbated and shows features attributed to periodic gleying and semi-gleying.
 
| Poorly drained alluvial floodplain || Alluvial palaeosol/overbank || Claystone and siltstone, reddish brown, purple grey, greenish grey, sandy lenses, rooted, mottles, listrics, brecciated, destratified, rare desiccation cracks, sphaerosiderite. In successions up to 6m thick, laterally continuous for hundreds of metres. || Suspension deposition on a vegetated alluvial floodplain. Shallow water becoming well-drained at times. Succession is pedoturbated and shows features attributed to periodic gleying and semi-gleying.
 
|-
 
|-
|  || Crevasse splay || Individual beds of fine to medium-grained sandstone. Generally less than 0.6m in thickness; form laterally continuous sheets a few hundred metres across. Interbedded with floodplain facies. Bed bases sharp or erosive, commonly fine upwards. Current ripple cross- lamination, climbing ripples, small-scale sets of cross- bedding, undulatory lamination, plane bedding. || Sand-laden, unconfined flood events. Breaching of main channel forms crevasse channel which introduces flood deposits into adjacent floodplain area. Unidirectional, lower flow regime currents dominant. Occasional upper flow regime conditions indicated by plane beds.
+
|  ||  || Crevasse splay Individual beds of fine to medium-grained sandstone. Generally less than 0.6m in thickness; form laterally continuous sheets a few hundred metres across. Interbedded with floodplain facies. Bed bases sharp or erosive, commonly fine upwards. Current ripple cross- lamination, climbing ripples, small-scale sets of cross- bedding, undulatory lamination, plane bedding. || Sand-laden, unconfined flood events. Breaching of main channel forms crevasse channel which introduces flood deposits into adjacent floodplain area. Unidirectional, lower flow regime currents dominant. Occasional upper flow regime conditions indicated by plane beds.
 
|-
 
|-
 
| Lake || Open lacustrine || Grey to dark grey and black siltstones and claystones, laminated, some plant material, carbonaceous, siderite beds. Up to 2m thick, > 100's metres wide || Deposition from suspension in the central parts of a perennial lake. Reducing conditions.
 
| Lake || Open lacustrine || Grey to dark grey and black siltstones and claystones, laminated, some plant material, carbonaceous, siderite beds. Up to 2m thick, > 100's metres wide || Deposition from suspension in the central parts of a perennial lake. Reducing conditions.
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The change in fluvial style upwards through a cycle represents a long-term change in channel form, and slope, discharge, sediment load and vegetation all affect channel patterns (Leopold et al. 1964, Schumm 1977). Channels adjust to an alteration of hydrological regime by changing channel widths, depths, gradient, meander wavelength, sinuosity, and width–depth ratios. It is known that channel meandering is favoured by relatively low slopes, a high suspended load to bedload ratio and cohesive bank sediments (Leopold & Wolman 1957). The possible cause of the cyclicity and the associated change in channel form are the subject of the following discussion.
 
The change in fluvial style upwards through a cycle represents a long-term change in channel form, and slope, discharge, sediment load and vegetation all affect channel patterns (Leopold et al. 1964, Schumm 1977). Channels adjust to an alteration of hydrological regime by changing channel widths, depths, gradient, meander wavelength, sinuosity, and width–depth ratios. It is known that channel meandering is favoured by relatively low slopes, a high suspended load to bedload ratio and cohesive bank sediments (Leopold & Wolman 1957). The possible cause of the cyclicity and the associated change in channel form are the subject of the following discussion.
 
  
 
'''[[:File:YGS_CHR_05_FLUV_TAB_05.jpg|Table 5]] Main characteristics of the different scales of cyclicity recognized in the Upper Carboniferous of northern Germany.'''
 
'''[[:File:YGS_CHR_05_FLUV_TAB_05.jpg|Table 5]] Main characteristics of the different scales of cyclicity recognized in the Upper Carboniferous of northern Germany.'''
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|| 1
 
|| 1
 
|| Hundreds of metres
 
|| Hundreds of metres
|| Large-scale upwards-fining succession, with coarse-grained multi-storey channel sands at base and mudstone prone tops.
+
|| Large-scale upwards- fining succession, with coarse-grained multi- storey channel sands at base and mudstone prone tops.
|| ?Orogenic/tectonic cycle in the hinterland – likely source in the east (?Baltica region) – a Caledonide zone formed by collision between Eastern Avalonia and Laurussia into Baltica.
+
|| ?Orogenic/tectonic
 +
 
 +
cycle in the hinterland – likely source in the east (?Baltica region) – a Caledonide zone formed by collision between Eastern Avalonia and Laurussia into Baltica.
 
|-  
 
|-  
 
|| 2
 
|| 2
|| Typically 120–200&nbsp;m
+
|| Typically 120–200m
|| Upwards-fining cycles, characterized by thick, stacked, coarse-grained channels at base and multiple coal-palaeosol horizons at the top. Typically contain 2–3 smaller-scale upwards-fining third-order cycles; each successive third-order cycle containing less sand.
+
|| Upwards-fining cycles, characterized by thick, stacked, coarse-grained channels at base and multiple coal-palaeosol horizons at the top. Typi- cally contain 2–3 smaller-scale upwards-fining third-order cycles; each successive third-order cycle containing less sand.
|| Hinterland tectonic pulses, e.g. movement along the Tornquist–Teisseyre or Rynkøbing Fyn High Zone. Produces large pulses of sediment flux.
+
|| Hinterland tectonic pulses, e.g. movement along the Tornquist– Teisseyre or Rynkøbing Fyn High Zone. Pro-duces large pulses of sediment flux.
 
|-  
 
|-  
 
|| 3
 
|| 3
|| 40–60&nbsp;m
+
|| 40–60 m
|| Upwards-fining successions, characterized by thick, stacked, coarse- grained low-sinuosity channels at base passing upwards into meandering channel systems, lacustrine muds and poorly drained palaeosols and coals.
+
|| Upwards-fining succes- sions, characterized by thick, stacked, coarse- grained low-sinuosity channels at base pass- ing upwards into mean- dering channel systems, lacustrine muds and poorly drained palaeosols and coals.
 
|| ?Tectonically driven source area climatic fluctuations resulting in variations in the amount of precipitation. This affects sediment flux and base level in the basin.
 
|| ?Tectonically driven source area climatic fluctuations resulting in variations in the amount of precipitation. This affects sediment flux and base level in the basin.
 
|-
 
|-
 
|}
 
|}
 
 
The principal controls on the stratigraphical architecture of sedimentary systems in basins are rates of accommodation-space generation and sediment supply (Shanley & McCabe 1994). Accommodation space is generated by the combined effects of thermal subsidence, eustasy, loading and compaction, and sediment supply determines how much of this accommodation space is filled up (Jervey 1988, Weltje et al. 1998). Recent advances in our understanding of the sedimentary response to relative sea-level variations means that the cyclicity recognized here could be fitted into a model whereby eustasy forms the dominant controlling mechanism. However, this is not thought to be the case, as there is good evidence that both Variscan tectonics and changes in climate were more important during late Carboniferous times (Besly 1987, Jankowski et al. 1993), and it is proposed that the cyclicity observed results from these controls.
 
The principal controls on the stratigraphical architecture of sedimentary systems in basins are rates of accommodation-space generation and sediment supply (Shanley & McCabe 1994). Accommodation space is generated by the combined effects of thermal subsidence, eustasy, loading and compaction, and sediment supply determines how much of this accommodation space is filled up (Jervey 1988, Weltje et al. 1998). Recent advances in our understanding of the sedimentary response to relative sea-level variations means that the cyclicity recognized here could be fitted into a model whereby eustasy forms the dominant controlling mechanism. However, this is not thought to be the case, as there is good evidence that both Variscan tectonics and changes in climate were more important during late Carboniferous times (Besly 1987, Jankowski et al. 1993), and it is proposed that the cyclicity observed results from these controls.
  
 
==== 3.2.1 Evidence for Variscan tectonic influence ====
 
==== 3.2.1 Evidence for Variscan tectonic influence ====
  
Towards the end of the Carboniferous ([[:File:YGS_CHR_05_FLUV_FIG_08.jpg|Figure 8]]), the Gondwanan plate collided with both the Laurentian plate to the west and the Iberian plate to the south, linked as part of the Asturian deformation phase of the Variscan orogeny (Warr 2000). The principal direction of Variscan convergence was probably towards the north and northwest as part of an oblique (dextral) collisional regime. By about Bolsovian–Westphalian D times the final Asturian phase of deformation began, which ultimately led to the formation of the Pangaean supercontinent ([[:File:YGS_CHR_05_FLUV_FIG_08.jpg|Figure 8]]). This closed off the Rheic Ocean such that all access to the sea was cut off in central Europe and the UK (Maynard et al. 1997). There is clear evidence that Variscan orogenic processes affected northern Germany during the Westphalian and Stephanian, and that the deformation front migrated northwards. This gave rise to a flexural foreland basin and the northwards migration of the depocentre, with the result that alluvial-plain sedimentation became dominant in Germany, with sediment being supplied from the uplifted Variscan mountain belt to the south and east. The evidence for this includes:
+
Towards the end of the Carboniferous ([[:File:YGS_CHR_05_FLUV_FIG_08.jpg|Figure 8]]), the Gondwanan plate collided with both the Laurentian plate to the west and the Iberian plate to the south, linked as part of the Asturian deformation phase of the Variscan orogeny (Warr 2000). The principal direction of Variscan convergence was probably towards the north and northwest as part of an oblique (dextral) collisional regime. By about Bolsovian–Westphalian D times the final Asturian phase of deformation began, which ultimately led to the formation of the Pangaean supercontinent ([[:File:YGS_CHR_05_FLUV_FIG_08.jpg|Figure 8]]). This closed
* Isopachs with a strongly asymmetrical pattern ([[:File:YGS_CHR_05_FLUV_FIG_09.jpg|Figure 9]]), with up to 4&nbsp;km of strata accumulating in northern Germany (Drozdzewski 1993). This can be explained by flexure of the crust in advance of loading by Variscan nappes.
+
 
 +
off the Rheic Ocean such that all access to the sea was cut off in central Europe and the UK (Maynard et al. 1997). There is clear evidence that Variscan orogenic processes affected northern Germany during the Westphalian and Stephanian, and that the deformation front migrated northwards. This gave rise to a flexural foreland basin and the northwards migration of the depocentre, with the result that alluvial-plain sedimentation became dominant in Germany, with sediment being supplied from the uplifted Variscan mountain belt to the south and east. The evidence for this includes:* Isopachs with a strongly asymmetrical pattern ([[:File:YGS_CHR_05_FLUV_FIG_09.jpg|Figure 9]]), with up to 4km of strata accumulating in northern Germany (Drozdzewski 1993). This can be explained by flexure of the crust in advance of loading by Variscan nappes.
 
* The recognition of a disconformity between Westphalian and Stephanian strata in northern Germany (Hedemann & Teichmüller 1971, Hedemann et al. 1984), linked to Variscan uplift.
 
* The recognition of a disconformity between Westphalian and Stephanian strata in northern Germany (Hedemann & Teichmüller 1971, Hedemann et al. 1984), linked to Variscan uplift.
 
* During Bolsovian and younger times, Variscan uplift led to a significant increase in the amount of sediment delivered into the basin, with some associated re-working of pre-existing basin material (Gayer et al. 1993, Jankowski et al. 1993). This high proportion of sandstones is particularly marked in the east, in the Hamwiede Schneverdingen and South Oldenburg areas, and there is a progressive decrease westwards and northwestwards towards the Ems area ([[:File:YGS_CHR_05_FLUV_FIG_03.jpg|Figure 3]], [[:File:YGS_CHR_05_FLUV_FIG_07.jpg|Figure 7]]). Palaeocurrent data for these channel sandbodies show strongly unidirectional flow directions towards the west and northwest, away from the rising Variscan mountains ([[:File:YGS_CHR_05_FLUV_FIG_06.jpg|Figure 6]]).
 
* During Bolsovian and younger times, Variscan uplift led to a significant increase in the amount of sediment delivered into the basin, with some associated re-working of pre-existing basin material (Gayer et al. 1993, Jankowski et al. 1993). This high proportion of sandstones is particularly marked in the east, in the Hamwiede Schneverdingen and South Oldenburg areas, and there is a progressive decrease westwards and northwestwards towards the Ems area ([[:File:YGS_CHR_05_FLUV_FIG_03.jpg|Figure 3]], [[:File:YGS_CHR_05_FLUV_FIG_07.jpg|Figure 7]]). Palaeocurrent data for these channel sandbodies show strongly unidirectional flow directions towards the west and northwest, away from the rising Variscan mountains ([[:File:YGS_CHR_05_FLUV_FIG_06.jpg|Figure 6]]).
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Although it is clear that a sea-level linkage could produce the cyclicity described, there is no direct evidence for such a control. Elsewhere in Europe and North America there is good support for Upper Carboniferous base-level changes being driven by glacio-eustatic sea-level rises (Holdsworth & Collinson 1988, Maynard & Leeder 1992, Flint et al. 1995). However, most of this evidence comes from Namurian and early Westphalian strata, and, as discussed previously, the structural setting of northern Germany in the Bolsovian suggests that the Rheic Ocean had closed and that there was no direct marine connection to this basin (Maynard et al. 1997). In an inland basin such as this it is often difficult to be sure of the role that sea level has on facies patterns. If sea level was controlling the local base level, then this must represent a subtle influence that is not represented in the preserved succession by marine facies. No marine or tidal facies were identified from upper Westphalian and Stephanian successions in this area.
 
Although it is clear that a sea-level linkage could produce the cyclicity described, there is no direct evidence for such a control. Elsewhere in Europe and North America there is good support for Upper Carboniferous base-level changes being driven by glacio-eustatic sea-level rises (Holdsworth & Collinson 1988, Maynard & Leeder 1992, Flint et al. 1995). However, most of this evidence comes from Namurian and early Westphalian strata, and, as discussed previously, the structural setting of northern Germany in the Bolsovian suggests that the Rheic Ocean had closed and that there was no direct marine connection to this basin (Maynard et al. 1997). In an inland basin such as this it is often difficult to be sure of the role that sea level has on facies patterns. If sea level was controlling the local base level, then this must represent a subtle influence that is not represented in the preserved succession by marine facies. No marine or tidal facies were identified from upper Westphalian and Stephanian successions in this area.
  
Alternatively, third-order cycles could be driven purely by tectonic processes, possibly linked to short-term episodes of crustal loading (see Miall 1996: 477). However, such a mechanism alone may not produce the high-frequency events required to drive the cyclicity at this scale. Although not well constrained, it is thought likely that these third-order cycles have a timespan comparable with fourth-order sea-level cycles (10<sup>5</sup> years).
+
Alternatively, third-order cycles could be driven purely by tectonic processes, possibly linked to short-term episodes of crustal loading (see Miall 1996: 477). However, such a mechanism alone may not produce the high-frequency events required to drive the cyclicity at this scale. Although not well constrained, it is thought likely that these third-order cycles have a timespan comparable with fourth-order sea-level cycles (105 years).
  
 
Hoffman & Grotzinger (1993) suggest that the climatic belt in which an orogen develops influences the tectonic style of the orogen and the architecture of the adjacent foreland basin. Monsoonal belts, such as those that would have characterized Upper Carboniferous times, would be typified by high rates of precipitation, leading to rapid erosional unroofing, deep erosion and the rapid filling of the foreland basin with sediment (Sinclair & Allen 1992, Hoffman & Grotzinger 1993). Drier periods would have less vegetation cover and hence would be characterized by increased erosion of bedrock, which would result in large volumes of material being available for transportation (Schumm 1968, Cecil 1990).
 
Hoffman & Grotzinger (1993) suggest that the climatic belt in which an orogen develops influences the tectonic style of the orogen and the architecture of the adjacent foreland basin. Monsoonal belts, such as those that would have characterized Upper Carboniferous times, would be typified by high rates of precipitation, leading to rapid erosional unroofing, deep erosion and the rapid filling of the foreland basin with sediment (Sinclair & Allen 1992, Hoffman & Grotzinger 1993). Drier periods would have less vegetation cover and hence would be characterized by increased erosion of bedrock, which would result in large volumes of material being available for transportation (Schumm 1968, Cecil 1990).
  
Olsen (1990) recognized two scales of cyclicity from Devonian meandering channel systems from east Greenland, one of the order of ''c''. 20&nbsp;m thick and a higher-order one at ''c. ''100&nbsp;m. He attributed these to climatic variations as a result of changes in Earth’s orbital parameters, reflecting 20000&nbsp;yr Milankovitch precession cycles with modulation by ''c. ''110000&nbsp;yr eccentricity cycles. Although it is possible that Milankovitch orbital forcing could account for the third-order cycles described here, the structural setting in northern Germany at this time makes it more likely that these cycles represent the product of changes in the balance between tectonically induced accommodation and climatically modulated sediment supply. Similar climatic controls on Carboniferous and Devonian non-marine successions have been described respectively by Glover & Powell (1996) and McKie & Garden (1996).
+
Olsen (1990) recognized two scales of cyclicity from Devonian meandering channel systems from east Greenland, one of the order of ''c''. 20m thick and a higher-order one at ''c. ''100m. He attributed these to climatic variations as a result of changes in Earth’s orbital parameters, reflecting 20000yr Milankovitch precession cycles with modulation by ''c. ''110000yr eccentricity cycles. Although it is possible that Milankovitch orbital forcing could account for the third-order cycles described here, the structural setting in northern Germany at this time makes it more likely that these cycles represent the product of changes in the balance between tectonically induced accommodation and climatically modulated sediment supply. Similar climatic controls on Carboniferous and Devonian non-marine successions have been described respectively by Glover & Powell (1996) and McKie & Garden (1996).
  
It is proposed that times of hinterland uplift resulted in greater orographic precipitation and hence the basal channel-belt-dominated parts of each third-order cycle reflect the rapid expansion of fluvial systems as the amount of clastic flux outstrips the accommodation space available. The change in fluvial style upwards through a cycle and the increased frequency of coals and other floodplain deposits indicates a decrease in the efficiency of the fluvial systems, a reduction in stream gradients and an elevation of the groundwater table. In a hydrographically enclosed system such as this, high rates of precipitation and any concomitant basinal subsidence would increase both the amount of sediment aggradation and raise the base level. Thus, relative base-level (effective lake-level) rise results in the progressive drowning of fluvial systems. The resulting decrease in the efficiency of the fluvial systems to transport the coarse clastic fraction is thought to be the main cause of the change from low- to relatively high-sinuosity fluvial systems and ultimately into floodplain conditions.
+
It is proposed that times of hinterland uplift resulted in greater orographic precipitation and hence the basal channel-belt-dominated parts of each third-order cycle reflect the rapid expansion of fluvial systems as the amount of clastic flux outstrips the accommodation space available. The change in fluvial style upwards through a cycle and the increased frequency of coals and other floodplain deposits indicates a decrease in the efficiency of the fluvial systems, a reduction in stream gradients and an elevation of the groundwater table. In a hydrographically enclosed system such as this, high rates of precipitation and any concomitant basinal subsidence would increase both the amount of sediment aggradation and raise the base level. Thus, relative base-level (effective lake-level) rise results in the progressive drowning of fluvial systems. The resulting decrease in the efficiency of the fluvial systems to transport the coarse clastic fraction is thought to be the main cause of the change from low-to relatively high-sinuosity fluvial systems and ultimately into floodplain conditions.
  
 
This hypothesis is difficult to prove conclusively, as evidence for intra-cycle variations in climate is lacking in these successions. However, climatically controlled facies changes have been proven from the Westphalian D and younger successions of northern Germany and the UK, and primary redbed facies, including calcretes and localized alluvial fans, all indicate deposition under increasingly drier and more arid conditions, linked to the growth of a rain shadow associated with uplift of the Variscan mountains (Besly 1987, 1988). It is also known that in younger Westphalian D and Stephanian successions, the effects of increased evapotranspiration are manifested by a change to better-drained palaeosols and an absence of coals upwards through a cycle. This may indirectly support the view that climatic controls operated during earlier times.
 
This hypothesis is difficult to prove conclusively, as evidence for intra-cycle variations in climate is lacking in these successions. However, climatically controlled facies changes have been proven from the Westphalian D and younger successions of northern Germany and the UK, and primary redbed facies, including calcretes and localized alluvial fans, all indicate deposition under increasingly drier and more arid conditions, linked to the growth of a rain shadow associated with uplift of the Variscan mountains (Besly 1987, 1988). It is also known that in younger Westphalian D and Stephanian successions, the effects of increased evapotranspiration are manifested by a change to better-drained palaeosols and an absence of coals upwards through a cycle. This may indirectly support the view that climatic controls operated during earlier times.
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=== 4.1 Exploration scale ===
 
=== 4.1 Exploration scale ===
  
Reservoir distribution is related to basin-scale processes that have interacted to produce cyclical sequences such that potential reservoirs are not randomly distributed through the stratigraphical succession, as would be predicted by a purely stochastic model. Correlations shows that thick stacked sandbodies with high net-to-gross ratios not only occur in the more proximal (southeastern) parts of the basin but can also be predicted to occur in the medial parts, where they will be preferentially stacked at the bases of first-order cycles on a repetitive vertical scale of approximately 160–200&nbsp;m ([[:File:YGS_CHR_05_FLUV_FIG_10.jpg|Figure 10]]). On a regional scale, second- and third-order cycles are more difficult to correlate, indicating some lateral variability.
+
Reservoir distribution is related to basin-scale processes that have interacted to produce cyclical sequences such that potential reservoirs are not randomly distributed through the stratigraphical succession, as would be predicted by a purely stochastic model. Correlations shows that thick stacked sandbodies with high net-to-gross ratios not only occur in the more proximal (southeastern) parts of the basin but can also be predicted to occur in the medial parts, where they will be preferentially stacked at the bases of first-order cycles on a repetitive vertical scale of approximately 160–200m ([[:File:YGS_CHR_05_FLUV_FIG_10.jpg|Figure 10]]). On a regional scale, second- and third-order cycles are more difficult to correlate, indicating some lateral variability.
  
This work has identified stratigraphical levels within the Bolsovian, which are more sand prone, providing a predictive framework for exploration. Typically, a cycle is 160–200&nbsp;m thick and commences with a widely developed sandstone complex, covering areas of tens to hundreds of square kilometres ([[:File:YGS_CHR_05_FLUV_FIG_10.jpg|Figure 10]]). Such sandstone-prone intervals locally form important gas reservoirs in northwest Germany and also in northeast Netherlands (e.g. the “Tubbergen Sandstone”). Although probably part of a different depositional system, similar sequences and regionally developed sandstone complexes have been identified in the Dutch–UK offshore (S. Kelly, personal communication, 2003).
+
This work has identified stratigraphical levels within the Bolsovian, which are more sand prone, providing a predictive framework for exploration. Typically, a cycle is 160–200m thick and commences with a widely developed sandstone complex, covering areas of tens to hundreds of square kilometres ([[:File:YGS_CHR_05_FLUV_FIG_10.jpg|Figure 10]]). Such sandstone-prone intervals locally form important gas reservoirs in northwest Germany and also in northeast Netherlands (e.g. the “Tubbergen Sandstone”). Although probably part of a different depositional system, similar sequences and regionally developed sandstone complexes have been identified in the Dutch–UK offshore (S. Kelly, personal communication, 2003).
  
 
=== 4.2 Field scale ===
 
=== 4.2 Field scale ===
  
The distribution of reservoir and reservoir-quality variations within a Carboniferous field is typically a complex problem, with reservoir volume, connectivity and productivity being particular issues. In the early phase of field life, there is little hard data; reservoirs are modelled either stochastically or objectively with a stochastic component to try to “capture” uncertainty. Volumetrics, well planning and production profiles rely on the accuracy of actual data and analogue input to these models. A robust correlation framework and an understanding of sandbody types and distribution are needed.
+
The distribution of reservoir and reservoir-quality variations within a Carboniferous field is typically a complex problem, with reservoir volume, connectivity and productivity being particular issues. In the early phase of field life, there is little hard data; reservoirs are modelled either stochastically or objectively with a stochastic component to try to “capture” uncertainty.
 +
 
 +
Volumetrics, well planning and production profiles rely on the accuracy of actual data and analogue input to these models. A robust correlation framework and an understanding of sandbody types and distribution are needed.
  
 
The work presented here predicts that high net-to-gross reservoirs should be field wide or greater in extent and possess a distinct layering, alternating with floodplain fines. These sandstone bodies are concentrated towards the bases of second-order cycles, with narrower, more ribbon-like, heterolithic sands located higher in the cycles. The reservoirs are clearly layered on a second-order scale, with the tops of cycles comprising mud-dominated floodplain and lacustrine associations that are laterally continuous on a field scale and hence form potential barriers to fluid flow (see [[:File:YGS_CHR_05_FLUV_FIG_03.jpg|Figure 3]]b). The recognition of second- and third-order cyclicity allows correlation of channel-belt scale sandbodies rather than individual channels.
 
The work presented here predicts that high net-to-gross reservoirs should be field wide or greater in extent and possess a distinct layering, alternating with floodplain fines. These sandstone bodies are concentrated towards the bases of second-order cycles, with narrower, more ribbon-like, heterolithic sands located higher in the cycles. The reservoirs are clearly layered on a second-order scale, with the tops of cycles comprising mud-dominated floodplain and lacustrine associations that are laterally continuous on a field scale and hence form potential barriers to fluid flow (see [[:File:YGS_CHR_05_FLUV_FIG_03.jpg|Figure 3]]b). The recognition of second- and third-order cyclicity allows correlation of channel-belt scale sandbodies rather than individual channels.
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The observed stacking patterns and cyclicity would fit into a sequence stratigraphical model, whereby base-level changes, controlled by glacio-eustatic sea-level oscillations, and there is good evidence for such processes occurring in the Upper Carboniferous. However, tectonic processes operating in both the basin and adjacent orogenic zones are thought more likely to have exerted the dominant control. Variscan compressional activities which took place farther to the south during the Bolsovian–Westphalian D, not only affected the amount and type of sediment delivered into the basin but also controlled the generation of accommodation space in the basin. This is good evidence that tectonic processes controlled the generation of cycles and stacking patterns within each cycle. It is also suggested that there is a distinct climatic signature controlling cyclicity, modulating the stacking patterns and ultimately impacting on fluvial style.
 
The observed stacking patterns and cyclicity would fit into a sequence stratigraphical model, whereby base-level changes, controlled by glacio-eustatic sea-level oscillations, and there is good evidence for such processes occurring in the Upper Carboniferous. However, tectonic processes operating in both the basin and adjacent orogenic zones are thought more likely to have exerted the dominant control. Variscan compressional activities which took place farther to the south during the Bolsovian–Westphalian D, not only affected the amount and type of sediment delivered into the basin but also controlled the generation of accommodation space in the basin. This is good evidence that tectonic processes controlled the generation of cycles and stacking patterns within each cycle. It is also suggested that there is a distinct climatic signature controlling cyclicity, modulating the stacking patterns and ultimately impacting on fluvial style.
  
A better understanding of fluvial style, architecture and stacking patterns leads to benefits in the understanding of hydrocarbon reservoirs. The deterministic nature of the facies is an important feature to bear in mind when modelling reservoirs. It is evident that these high net-to-gross Upper Carboniferous reservoirs are clearly layered and vertically compartmentalized. Thus, the implications for modelling is that such systems cannot be modelled simplistically as a “tank of sand” and detailed assessment and correlation is required in order to identify and distinguish intra-reservoir baffles, such as remnant overbanks and barform drapes.
+
A better understanding of fluvial style, architecture and stacking patterns leads to benefits in the understanding of hydrocarbon reservoirs. The deterministic nature of the facies is an important feature to bear in mind when modelling reservoirs. It is evident that these high net-to-gross Upper Carboniferous reservoirs are clearly layered and vertically compartmentalized.
 +
 
 +
Thus, the implications for modelling is that such systems cannot be modelled simplistically as a “tank of sand” and detailed assessment and correlation is required in order to identify and distinguish intra-reservoir baffles, such as remnant overbanks and barform drapes.
  
 
== Acknowledgements ==
 
== Acknowledgements ==
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Besly, B. M. 1987. Sedimentological evidence for Carboniferous and Early Permian palaeoclimates of Europe. ''Extrait des Annales de la Société Géologique du Nord ''151, 131–43.
 
Besly, B. M. 1987. Sedimentological evidence for Carboniferous and Early Permian palaeoclimates of Europe. ''Extrait des Annales de la Société Géologique du Nord ''151, 131–43.
  
Besly, B. M. 1988. Palaeogeographic implications of late Westphalian to early Permian redbeds, Central England. In ''Sedimentation in a synorogenic basin complex: the Upper Carboniferous of northwest Europe'', B. M. Besly & G. Kelling (eds), 200–221. Glasgow: Blackie.  
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Besly, B. M. 1988. Palaeogeographic implications of late Westphalian to early Permian redbeds, Central England. In ''Sedimentation in a synorogenic basin complex: the Upper Carboniferous of northwest Europe'', B. M. Besly & G. Kelling (eds), 200–221. Glasgow: Blackie. Bridge, J. S. 1985. Paleochannel patterns inferred from alluvial deposits: a critical evaluation. ''Journal of Sedimentary Petrology ''55, 579–89.
 
 
Bridge, J. S. 1985. Paleochannel patterns inferred from alluvial deposits: a critical evaluation. ''Journal of Sedimentary Petrology ''55, 579–89.
 
  
 
Bristow, C. S. 1987. Brahmaputra River: channel migration and deposition. In ''Recent developments in fluvial sedimentology'', F. G. Ethridge, R. M. Flores, M. D. Harvey (eds), 63–74. Special Publication 39, Society of Economic Paleontologists and Mineralogists, Tulsa, Oklahoma.
 
Bristow, C. S. 1987. Brahmaputra River: channel migration and deposition. In ''Recent developments in fluvial sedimentology'', F. G. Ethridge, R. M. Flores, M. D. Harvey (eds), 63–74. Special Publication 39, Society of Economic Paleontologists and Mineralogists, Tulsa, Oklahoma.
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Glover, B. W. & J. H. Powell 1996. Interaction of climate and tectonics upon alluvial architecture: Late Carboniferous–Early Permian sequences at the southern margin of the Pennine Basin. ''Palaeogeography, Palaeoclimatology, Palaeoecology ''121, 13–34.
 
Glover, B. W. & J. H. Powell 1996. Interaction of climate and tectonics upon alluvial architecture: Late Carboniferous–Early Permian sequences at the southern margin of the Pennine Basin. ''Palaeogeography, Palaeoclimatology, Palaeoecology ''121, 13–34.
  
Glover, B. W. & N. S. Jones 1997. Systematic distribution of coals within the Westphalian C–D succession of NW Germany and their implications for sequence stratigraphy. Coal Geology Workshop “Sequence Stratigraphy Applied to Coal-Bearing Strata: Quo Vadis?”, organized by VITO, March 1997, Hasselt, Belgium.  
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Glover, B. W. & N. S. Jones 1997. Systematic distribution of coals within the Westphalian C–D succession of NW Germany and their implications for sequence stratigraphy. Coal Geology Workshop “Sequence Stratigraphy Applied to Coal-Bearing Strata: Quo Vadis?”, organized by VITO, March 1997, Hasselt, Belgium. Hallsworth, C. R. & J. I. Chisholm 2000. Stratigraphic evolution of provenance characteristics in Westphalian sandstones of the
  
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.
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Yorkshire coalfield. ''Yorkshire Geological Society, Proceedings ''53, 43–72.
  
 
Hallsworth, C. R., A. C. Morton, J. Claoué-Long, C. M. Fanning 2000. Carboniferous sand provenance in the Pennine Basin, UK: constraints from heavy-mineral and detrital-zircon age data. ''Sedimentary Geology ''137, 147–85.
 
Hallsworth, C. R., A. C. Morton, J. Claoué-Long, C. M. Fanning 2000. Carboniferous sand provenance in the Pennine Basin, UK: constraints from heavy-mineral and detrital-zircon age data. ''Sedimentary Geology ''137, 147–85.
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Hoyer, P., J. Leisser, M. Teichmüller, R. Teichmüller 1971. Chapter 3. The Carboniferous of Ibbenbüren, the Hüggel and the Piesberg, (c): metamorphism of coal. The Carboniferous Deposits in the Federal Republic of Germany. ''Fortschritte in der Geologie von Rheinland und Westfalen ''19, 87–90.
 
Hoyer, P., J. Leisser, M. Teichmüller, R. Teichmüller 1971. Chapter 3. The Carboniferous of Ibbenbüren, the Hüggel and the Piesberg, (c): metamorphism of coal. The Carboniferous Deposits in the Federal Republic of Germany. ''Fortschritte in der Geologie von Rheinland und Westfalen ''19, 87–90.
  
Jackson, R. G. 1976. Depositional model of point bars in the lower Wabash River. ''Journal of Sedimentary Petrology ''46, 579–94.  
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Jackson, R. G. 1976. Depositional model of point bars in the lower Wabash River. ''Journal of Sedimentary Petrology ''46, 579–94. Jankowski, B., F. David, V. Selter 1993. Facies complexes of the Upper Carboniferous in northwest Germany and their structural implications. In ''Rhenohercynian and Subvariscan fold belts'', R. A. Gayer, R. O. Greilling, A. K. Vogel (eds), 139–58. Wiesbaden: Vieweg.
 
 
Jankowski, B., F. David, V. Selter 1993. Facies complexes of the Upper Carboniferous in northwest Germany and their structural implications. In ''Rhenohercynian and Subvariscan fold belts'', R. A. Gayer, R. O. Greilling, A. K. Vogel (eds), 139–58. Wiesbaden: Vieweg.
 
  
 
Jervey, M. T. 1988. Quantitative geological modeling of siliciclastic rock sequences and their seismic expression. In ''Sea-level changes: an integrated approach'', C. K. Wilgus, B. S. Hastings, C. G. St C. Kendall, H. W. Posamentier, C. A. Ross, J. C. Van Wagoner (eds), 47–69. Special Publication 42, Society of Economic Paleontologists and Mineralogists, Tulsa, Oklahoma.
 
Jervey, M. T. 1988. Quantitative geological modeling of siliciclastic rock sequences and their seismic expression. In ''Sea-level changes: an integrated approach'', C. K. Wilgus, B. S. Hastings, C. G. St C. Kendall, H. W. Posamentier, C. A. Ross, J. C. Van Wagoner (eds), 47–69. Special Publication 42, Society of Economic Paleontologists and Mineralogists, Tulsa, Oklahoma.
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Miall, A. D. 1996. ''The geology of fluvial deposits: sedimentary facies, basin analysis, and petroleum geology''. Berlin: Springer.
 
Miall, A. D. 1996. ''The geology of fluvial deposits: sedimentary facies, basin analysis, and petroleum geology''. Berlin: Springer.
  
Miall, A. D. 1997. ''The geology of stratigraphic sequences''. Berlin: Springer.  
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Miall, A. D. 1997. ''The geology of stratigraphic sequences''. Berlin: Springer. Morton, A. C., J. C. Claoué-Long, C. R. Hallsworth 2001. Zircon-age and heavy-mineral constraints on provenance of North Sea Carboniferous sandstones. ''Marine and Petroleum Geology ''18, 319–37. Morton, A. C., C. Hallsworth, A. Mosciarello 2005. Interplay between northern and southern sediment sources during Westphalian deposition in the Silverpit Basin, southern North Sea. This volume: 135– 146.
 
 
Morton, A. C., J. C. Claoué-Long, C. R. Hallsworth 2001. Zircon-age and heavy-mineral constraints on provenance of North Sea Carboniferous sandstones. ''Marine and Petroleum Geology ''18, 319–37.  
 
 
 
Morton, A. C., C. Hallsworth, A. Mosciarello 2005. Interplay between northern and southern sediment sources during Westphalian deposition in the Silverpit Basin, southern North Sea. This volume: 135– 146.
 
  
 
Nijman, W. & C. Puigdefabregas 1987. Coarse-grained pointbar structure in a molasse-type fluvial system, Eocene Castisent Sandstone Formation, South Pyrenean Basin. In ''Fluvial sedimentology'', A. D. Miall (ed.), 487–510. Memoir 5, Canadian Society of Petroleum Geologists, Calgary.
 
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