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

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Subdivision of the offshore Coal Measures section using the onshore macrofaunal biozone scheme of marine-band faunas will always be partly subjective, when only cuttings samples are available and palynology is the only available biostratigraphical tool. Sequence stratigraphical subdivision off shore is also partly subjective and will be subject to changes and refinements in future studies. Nevertheless, the basinwide macrofaunal biozones provide a good starting point for recognizing systems tracts, as they are generally lithologically discrete. Offshore wells, particularly those penetrating a significant interval thickness and furnished with a good suite of wireline logs, permit large parts of the Coal Measures to be examined in terms of broad lithofacies trends at a scale often not available in the onshore section.
 
Subdivision of the offshore Coal Measures section using the onshore macrofaunal biozone scheme of marine-band faunas will always be partly subjective, when only cuttings samples are available and palynology is the only available biostratigraphical tool. Sequence stratigraphical subdivision off shore is also partly subjective and will be subject to changes and refinements in future studies. Nevertheless, the basinwide macrofaunal biozones provide a good starting point for recognizing systems tracts, as they are generally lithologically discrete. Offshore wells, particularly those penetrating a significant interval thickness and furnished with a good suite of wireline logs, permit large parts of the Coal Measures to be examined in terms of broad lithofacies trends at a scale often not available in the onshore section.
  
Sequence analysis is best achieved where there are good biostratigraphical data and well dated neighbouring offset wells. A combination of terrestrial plant miospore zonal markers and palynofacies influxes can provide reasonably refined dating of the Coal Measures section and give good pointers towards the probable correlation of a particular gamma-ray spike with a named marine band. If a full suite of spectral gamma-ray logs is available, selection of one particular gamma-ray spike may be enhanced using the method of Hollywood & Whorlow (1993). This has been successfully demonstrated by the authors on unpublished wells with spectral gamma-ray logs available in digital format, doing the necessary arithmetical manipulation of a large log file with a digital spreadsheet. Expected interval thickness and wireline-inferred sedimentary facies of particular parts of the section are key parts of the analysis. As a very generalized rule, transgressive deposits are characterized by abundant coals and marine bands. Highstand deposits (if preserved un-eroded by the succeeding lowstand deposits) have common marine bands. Lowstand sediments are characterized by thick, regional, stacked, alluvial channel sandstones (i.e. potential reservoirs).
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Sequence analysis is best achieved where there are good biostratigraphical data and well dated neighbouring offset wells. A combination of terrestrial plant miospore zonal markers and palynofacies influxes can provide reasonably refined dating of the Coal Measures section and give good pointers towards the probable correlation of a particular gamma-ray spike with a named marine band. If a full suite of spectral gamma-ray logs is available, selection of one particular gamma-ray spike may be enhanced using the method of Hollywood & Whorlow (1993). This has been successfully demonstrated by the authors on unpublished wells with spectral gamma-ray logs available in digital format, doing the necessary arithmetical manipulation of a large log file with a digital spreadsheet. Expected interval thickness and wireline-inferred sedimentary facies of particular parts of the section are key parts of the analysis. As a very generalized rule, transgressive deposits are characterized by abundant coals and marine bands. Highstand deposits (if preserved
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un-eroded by the succeeding lowstand deposits) have common marine bands. Lowstand sediments are characterized by thick, regional, stacked, alluvial channel sandstones (i.e. potential reservoirs).
  
 
Released wells from The Netherlands and UK sectors permit the study of lateral changes within systems tracts of the Coal Measures over a significant part of the southern North Sea. A north–south transect of the western side of the southern North Sea ([[:File:YGS_CHR_06_ASEQ_FIG_01.jpg|Figure 1]], [[:File:YGS_CHR_06_ASEQ_FIG_08.jpg|Figure 8]], [[:File:YGS_CHR_06_ASEQ_FIG_09.jpg|Figure 9]]) based on the correlation of eight wells selected from UK quadrants 44, 49 and 53, and Dutch quadrants K and P, has been compiled. The correlations demonstrate a significant degree of continuity of interval thicknesses and show lateral facies changes within individual systems tracts.
 
Released wells from The Netherlands and UK sectors permit the study of lateral changes within systems tracts of the Coal Measures over a significant part of the southern North Sea. A north–south transect of the western side of the southern North Sea ([[:File:YGS_CHR_06_ASEQ_FIG_01.jpg|Figure 1]], [[:File:YGS_CHR_06_ASEQ_FIG_08.jpg|Figure 8]], [[:File:YGS_CHR_06_ASEQ_FIG_09.jpg|Figure 9]]) based on the correlation of eight wells selected from UK quadrants 44, 49 and 53, and Dutch quadrants K and P, has been compiled. The correlations demonstrate a significant degree of continuity of interval thicknesses and show lateral facies changes within individual systems tracts.

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