Controls of coalbed methane prospectivity in Great Britain

Abstract

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.