This report describes the methodology and results of a regional seismic interpretation of the
basins of the Irish Sea. It does not review the basins of the Celtic Sea. The aim of the
interpretation was to map the distribution of Palaeozoic basins and highs, interpreting the key
Devonian-Carboniferous surfaces and main structural elements of the area. About 40,000 km
of 2D seismic reflection data have been interpreted and tied to key released wells in the
project area. The seismic and well data were augmented by donated reports from sponsor
companies.
A set of 8 depth structure maps of key horizons have been produced for the pre-Permian
succession. These maps provide a key dataset to aid assessment of the petroleum systems of
the Palaeozoic strata within the study area. The surfaces, supplied digitally at a grid spacing
of 5000 m, give a regional view of the topography of the horizons, and comprise:
‘UVAR’ (Variscan Unconformity) beneath Permian and Triassic strata
Base Warwickshire Group (late Westphalian - ?Stephanian)
Top Namurian (Base Pennine Coal Measures Group)
Top Intra-Namurian (Top Bowland Shale in south, Base Millstone Grit elsewhere)
Top Visean (Lower Carboniferous)
Intra-Visean (amalgamated with Top Middle Border Group in north)
Base Carboniferous (amalgamated with Base Clyde Plateau lavas in the North
Channel to South-West Arran Sub-Basin)
‘UCAL’ Acadian (Caledonian) Unconformity)
It is important to note that the variable data quality and sparsity of deep wells leads to a
seismic interpretation which is strongly driven by regional geological models, themselves
heavily dependent on inference from the onshore area. This is particularly the case with the
deeper Carboniferous horizons which are not penetrated by any well and which may be only
weakly reflective. In such cases, picks from better quality data may be interpolated through
areas with poor quality data, as a modelled surface, to ensure a continuous surface for
gridding.
The well dataset has been re-interpreted (Wakefield et al., 2016) before integration with the
seismic interpretation.
The following general observations are made:
The present study has confirmed the Permian-Mesozoic structural framework for the region
established by Jackson and Mulholland (1993) and Jackson et al. (1995, 1996, 1997).
The basin recognised in Quadrant 109 by Jackson and co-workers (op. cit.) is
reinterpreted as a major Carboniferous half-graben structure controlled by a
syndepositional fault on its NW side. It continues beneath thin Permo-Triassic cover
into the Eubonia Basin and Ogham Platform, preserving a thick Westphalian
succession, including inferred Warwickshire Group strata. It is inferred to have
continued eastward into the Lagman Basin prior to its tectonic dissection by a
combination of Variscan inversion and Permo-Mesozoic graben development along
the Keys Fault.
A belt of Variscan fold/thrust inversion structures on the Godred Croven Platform is
correlated with structures on the Formby Platform and Ribblesdale Foldbelt onshore.
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The area of Carboniferous (undivided) subcrop depicted on mapping by BGS (1994)
to north and west of the Isle of Man has been reclassified into Visean and Namurian
elements.
The presence of significant thicknesses of Carboniferous strata in the southern part of
the North Channel is regarded as doubtful, but farther north, in the Larne, Rathlin and
South-West Arran Sub-basins, greater thicknesses may be present.
A detailed description of tectono-stratigraphic development based on the seismic
interpretation is given in Pharaoh et al. (2016b), integrated with the petroleum system
analysis
