SUMMARY Available data suggest that, subsequent to Triassic basin development, syn-depositional normal faulting continued in Jurassic and early Cretaceous times with subsidence of the East Irish Sea and Solway basins relative to the Lake District Block. Cessation of extension in the mid-Cretaceous led to regional subsidence with little distinction between blocks and basins. Palaeogene times heralded a period of regional uplift and erosion, associated with development of the Icelandic Plume and emplacement of the Scottish Tertiary igneous province. Superimposed upon this, Alpine crustal compression produced minor oblique-reversal of the basin-margin faults and weak basin inversion. Depth of burial estimates from well-log data agree with uplift values computed from thermal modelling of apatite fission-track palaeotemperatures. They indicate that post-Cretaceous uplift ranged from less than 1750 m over the Lake District Block, to 2000 m at Sellafield and more than 2200 m over the surrounding basins.

The Lough Allen Basin contains approximately 2.9 km of Dinantian (Courccyan) to Silesian (Arnsbergian) limestone, mudstone and sandstone, mostly of marine origin. The basin was initiated during the Courceyan Stage as a result of movement of the basin margin fault complex along the southeast side of the Ox Mountains inlier. The main rifting phase, during the late Courceyan, resulted in the deposition of a southeasterly thinning, initially non-marine, elastic wedge. The intrabasinal Dowra-Macncan High and Slisgarrow Trough developed during the Chadian Stage. Movement on the Curlew fault, which bounds the basin to the south, may have begun in the Courccyan but culminated in the Arundian. There is little direct evidence of fault-controlled sedimentation in the Holkerian and much of the Asbian, although the distribution of carbonate mudmounds of Asbian age in part coincides with earlier structural elements. During the late Asbian, there appears to have been renewed tectonic activity, which led to substantial regional differences of thickness of the lower formations of the Leitrim Group. Thcrc is little evidence of tectonic control on sedimentation from the Brigantian to the Arnsbcrgian.Vitrinite reflectance determinations show that the Carboniferous rocks in the basin arc supra-mature for the generation of oil but are mature with respect to the generation of gas. The maturation values for the youngest preserved rocks suggest that they have been buried under 3-5 km of Upper Palaeozoic cover.

Only in the last few years has there been a significant increase in exploration for coalbed methane (CBM) in Great Britain. There are several geological controls on British CBM prospectivity which combine with socio-political constraints in limiting the direct transfer of experience and technology from the USA where, until recently, CBM development has been concentrated. In many parts of the world, the depth of burial and rank of coals may be used as approximate indicators of CBM potential. However, in most British coal-bearing basins, one of the most important factors controlling the amount of preserved adsorbed methane in coals appears to be the degree of syn-and post-depositional basin inversion. Although enough methane to saturate the coals was probably generated during the formation of the Late Carboniferous basins, extensive degasification took place during the end-Carboniferous Variscan orogeny. Subsequent Permo-Triassic and later reburial of coals seems to have been insufficient to replenish adsorbed methane over much of the CBM target areas of Great Britain. Those British coal-bearing basins which can be identified as having been originally deeply buried, only mildly inverted during the Variscan orogeny and which have remained relatively deeply buried beneath Mesozoic cover until early Cenozoic times are likely to contain the most preserved adsorbed methane and consequently prove the best prospects. Identification of a consumer and an adequate infrastructure further limit the potential of CBM prospects. In addition, at present, geological factors such as in situ stress, Coal Measures sedimentology, coal cleat orientation, hydrogeology and hydrology, and planning and environmental issues are considered in more detail only at the well-siting stage. As a knowledge of British CBM grows, however, these factors will become more important in initial licence acquisition.