Seismic interpretation and generation of depth surfaces for Late Palaeozoic strata in the Irish Sea Region

摘要

This report describes the methodology and results of a regional seismic interpretation of theudbasins of the Irish Sea. It does not review the basins of the Celtic Sea. The aim of theudinterpretation was to map the distribution of Palaeozoic basins and highs, interpreting the keyudDevonian-Carboniferous surfaces and main structural elements of the area. About 40,000 kmudof 2D seismic reflection data have been interpreted and tied to key released wells in theudproject area. The seismic and well data were augmented by donated reports from sponsorudcompanies.udA set of 8 depth structure maps of key horizons have been produced for the pre-Permianudsuccession. These maps provide a key dataset to aid assessment of the petroleum systems ofudthe Palaeozoic strata within the study area. The surfaces, supplied digitally at a grid spacingudof 5000 m, give a regional view of the topography of the horizons, and comprise:ud ‘UVAR’ (Variscan Unconformity) beneath Permian and Triassic strataud Base Warwickshire Group (late Westphalian - ?Stephanian)ud Top Namurian (Base Pennine Coal Measures Group)ud Top Intra-Namurian (Top Bowland Shale in south, Base Millstone Grit elsewhere)ud Top Visean (Lower Carboniferous)ud Intra-Visean (amalgamated with Top Middle Border Group in north)ud Base Carboniferous (amalgamated with Base Clyde Plateau lavas in the NorthudChannel to South-West Arran Sub-Basin)ud ‘UCAL’ Acadian (Caledonian) Unconformity)udIt is important to note that the variable data quality and sparsity of deep wells leads to audseismic interpretation which is strongly driven by regional geological models, themselvesudheavily dependent on inference from the onshore area. This is particularly the case with theuddeeper Carboniferous horizons which are not penetrated by any well and which may be onlyudweakly reflective. In such cases, picks from better quality data may be interpolated throughudareas with poor quality data, as a modelled surface, to ensure a continuous surface forudgridding.udThe well dataset has been re-interpreted (Wakefield et al., 2016) before integration with theudseismic interpretation.udThe following general observations are made:udThe present study has confirmed the Permian-Mesozoic structural framework for the regionudestablished by Jackson and Mulholland (1993) and Jackson et al. (1995, 1996, 1997).ud The basin recognised in Quadrant 109 by Jackson and co-workers (op. cit.) isudreinterpreted as a major Carboniferous half-graben structure controlled by audsyndepositional fault on its NW side. It continues beneath thin Permo-Triassic coverudinto the Eubonia Basin and Ogham Platform, preserving a thick Westphalianudsuccession, including inferred Warwickshire Group strata. It is inferred to haveudcontinued eastward into the Lagman Basin prior to its tectonic dissection by audcombination of Variscan inversion and Permo-Mesozoic graben development alongudthe Keys Fault.ud A belt of Variscan fold/thrust inversion structures on the Godred Croven Platform isudcorrelated with structures on the Formby Platform and Ribblesdale Foldbelt onshore.udCR/16/041 Last modified: 2016/05/30 09:37ud10ud The area of Carboniferous (undivided) subcrop depicted on mapping by BGS (1994)udto north and west of the Isle of Man has been reclassified into Visean and Namurianudelements.ud The presence of significant thicknesses of Carboniferous strata in the southern part ofudthe North Channel is regarded as doubtful, but farther north, in the Larne, Rathlin andudSouth-West Arran Sub-basins, greater thicknesses may be present.udA detailed description of tectono-stratigraphic development based on the seismicudinterpretation is given in Pharaoh et al. (2016b), integrated with the petroleum systemudanalysis.