The Variscan Front and Bristol Channel-Bray Fault (BCBF) appear to be key controls on the hydrocarbon potential of the Variscan basement. The former defines the northern limit of penetrative, Variscan deformation, whilst the latter is inferred to have been a dextral strikeslip fault, which was active during and after the Silesian and separates the Culm Basin from the South Wales Coalfield. Isotopic and trace element data indicate that the Culm Basin and South Wales Coalfield had distinct source areas during the Westphalian, consistent with significant separation of the two basins across the BCBF at this time. The Culm Basin contains Namurian-Westphalian sandstones, black shales and thin coals. Vitrinite reflectance data from North Devon suggest that the shales and coals locally retain gas generation potential. Gravity data, recently compiled by Getech as part of the OGA’s 21st Century Exploration Roadmap Project, suggest that the Culm Basin extends westwards towards the South Celtic Sea Basin. Here, Irish and UK wells 58/3-1 and 93/6-1 penetrate possible Westphalian strata. Both wells displayed an increase in background gas (predominantly methane) within the Carboniferous section; gas was also noted in the overlying Triassic (Mercia Mudstone?) within93/6-1.
The main risks associated with any Carboniferous-sourced gas play in the South Celtic Sea region are: 1) the uncertain thickness and extent of Westphalian black shales and coals; 2) the challenge locating basement domains with limited Variscan metamorphic overprint; and 3) the critical timing between post-Varsican thermal maturation and regional reservoir trap and seal development. Intra- Carboniferous reservoirs may exist, but are likely to be highly deformed and structurally complex. By contrast, Westphalian strata to the north of the BCBF and Variscan Front are likely to be significantly less deformed, and may share affinities with classic “Coal Measures” facies in South Wales and northern England, and gas prone sequences encountered in the East Irish Sea. In conclusion, the hydrocarbon prospectivity of potential Carboniferous, or Carboniferous-sourced, plays is likely to be greater to the north of the Variscan Front and BCBF than in the region to the south.