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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During Upper Carboniferous times, northern Germany formed part of the West European Carboniferous Basin, outcrops of which today stretch from Ireland into Poland (Maynard et al. 1997; [[:File:YGS_CHR_05_FLUV_FIG_01.jpg|Figure 1]]). Throughout much of the early Westphalian, coal-forming conditions interdigitated with lacustrine environments on a low-lying, low-gradient, poorly drained alluvial floodplain. By Bolsovian times, hinterland uplift, linked to Variscan orogenic processes in the south, led to the formation of major perennial sandy fluvial systems that transported large volumes of sediment into the basin. Although coal-forming environments persisted into Westphalian D times, the gradual change from a humid to a semi-arid climate, brought about by the rain-shadow effect of the rising Variscan mountain chain, meant that poorly drained conditions were gradually replaced by a better-drained redbed setting (Besly 1987). By late Carboniferous (Stephanian) times, sedimentation was dominated by redbed alluvial plain facies, including well drained calcrete palaeosols. Ultimately, in Stephanian to Autunian times, the foreland basin was partially inverted and its deposits cannibalized as the effects of the Variscan Orogeny spread northwards.
 
During Upper Carboniferous times, northern Germany formed part of the West European Carboniferous Basin, outcrops of which today stretch from Ireland into Poland (Maynard et al. 1997; [[:File:YGS_CHR_05_FLUV_FIG_01.jpg|Figure 1]]). Throughout much of the early Westphalian, coal-forming conditions interdigitated with lacustrine environments on a low-lying, low-gradient, poorly drained alluvial floodplain. By Bolsovian times, hinterland uplift, linked to Variscan orogenic processes in the south, led to the formation of major perennial sandy fluvial systems that transported large volumes of sediment into the basin. Although coal-forming environments persisted into Westphalian D times, the gradual change from a humid to a semi-arid climate, brought about by the rain-shadow effect of the rising Variscan mountain chain, meant that poorly drained conditions were gradually replaced by a better-drained redbed setting (Besly 1987). By late Carboniferous (Stephanian) times, sedimentation was dominated by redbed alluvial plain facies, including well drained calcrete palaeosols. Ultimately, in Stephanian to Autunian times, the foreland basin was partially inverted and its deposits cannibalized as the effects of the Variscan Orogeny spread northwards.
  
Outcrops and many coal exploration boreholes in the Osnabrück–Ibbenbüren area have proved strata of upper Bolsovian and Westphalian D age (Bässler et al. 1971). The entire Bolsovian succession is believed to be approximately 850 m in thickness and the Westphalian D is about 700 m thick (David 1990). Hydrocarbon wells have also confirmed younger (Stephanian) strata in the Ems area to the northwest (Schuster 1968, Hedemann et al. 1984, Josten et al. 1984). The outcrops in the Osnabrück–Ibbenbüren area form faulted inliers surrounded by younger Permian (Zechstein) and Triassic successions. They form part of the more extensive northwest-trending Nordwestfälisch–Lippische lineament on the northern margin of the Mesozoic Munster Basin (Bässler et al. 1971). This Upper Carboniferous succession has undergone major compressional deformational episodes, particularly linked to late Carboniferous (Variscan) and late Cretaceous inversion events (Drozdzewski 1985). The coals are typically bituminous to semi-anthracites and show an increase in rank towards Piesberg, where the coals attain anthracite rank (Hoyer et al. 1971, Stadler & Teichmüller 1971).
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Outcrops and many coal exploration boreholes in the Osnabrück–Ibbenbüren area have proved strata of upper Bolsovian and Westphalian D age (Bässler et al. 1971). The entire Bolsovian succession is believed to be approximately 850m in thickness and the Westphalian D is about 700m thick (David 1990). Hydrocarbon wells have also confirmed younger (Stephanian) strata in the Ems area to the northwest (Schuster 1968, Hedemann et al. 1984, Josten et al. 1984). The outcrops in the Osnabrück–Ibbenbüren area form faulted inliers surrounded by younger Permian (Zechstein) and Triassic successions. They form part of the more extensive northwest-trending Nordwestfälisch–Lippische lineament on the northern margin of the Mesozoic Munster Basin (Bässler et al. 1971). This Upper Carboniferous succession has undergone major compressional deformational episodes, particularly linked to late Carboniferous (Variscan) and late Cretaceous inversion events (Drozdzewski 1985). The coals are typically bituminous to semi-anthracites and show an increase in rank towards Piesberg, where the coals attain anthracite rank (Hoyer et al. 1971, Stadler & Teichmüller 1971).
  
 
Little previous detailed sedimentological work has been published on these successions, although the work of David (1987, 1990), Selter (1990), Jankowski et al. (1993) and Glover & Jones (1997) are notable exceptions. David (1990) recognized various depositional environments, of which braided rivers, overbank siltstones, swamps (coals), crevasse splays, lacustrine and brackish-water are common. Of these, major northward-flowing channel systems were the most significant (David 1987, 1990). The work presented here agrees broadly with the facies model suggested by David (1990), although no evidence for brackish-water deposits was identified from core or outcrop studies.
 
Little previous detailed sedimentological work has been published on these successions, although the work of David (1987, 1990), Selter (1990), Jankowski et al. (1993) and Glover & Jones (1997) are notable exceptions. David (1990) recognized various depositional environments, of which braided rivers, overbank siltstones, swamps (coals), crevasse splays, lacustrine and brackish-water are common. Of these, major northward-flowing channel systems were the most significant (David 1987, 1990). The work presented here agrees broadly with the facies model suggested by David (1990), although no evidence for brackish-water deposits was identified from core or outcrop studies.

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