Repurposing Britain’s Coal Measures: insights from seismic and borehole-based mapping, and geothermal modelling

As a consequence of 19^th and 20^th century coal mining, a vast quantity of subsurface data has been accumulated on Britain’s late Carboniferous Coal Measures and the basins in which they have accumulated. Here we discuss current understanding of the geological evolution of the British Isles during this period, as well as how these data can be repurposed as the UK seeks to reduce its greenhouse gas emissions.

It is widely assumed that Britain’s Coal Measures accumulated during a period of tectonically quiescent, thermally induced post-rift subsidence and that the lateral extent of the Variscan foreland in southern England and Wales was restricted. Burial history curves constructed from stratigraphic successions across the UK suggest, however, that during the late Carboniferous the UK was characterised by accelerated subsidence rates as far north as northern England and Scotland, beyond any perceivable flexure-induced foredeep. On local scales, seismic and borehole-based mapping shows that many individual depocentres were strongly influenced by syn-depositional faulting, folding and positive inversion. This influence is illustrated by repeated local unconformities within the late Carboniferous succession across England and Scotland as well as variable isochore thickness trends. We propose that this succession was influenced by a combination of both local tectonic controls and regional controls such as supralithospheric orogenic loading and sublithospheric dynamic loading. In this sense, we believe that the British Variscan foreland system, as the British Isles should be referred to as during the late Carboniferous, resembled a ‘broken’ foreland system such as that of Patagonia, South America.

Understanding the nature of Britain’s Coal Measures has gained renewed importance given the need to reduce carbon emissions and seek alternative sources of energy. Across the UK, there are a number of active projects aiming to harness latent heat from abandoned underground coalmines. In addition, coupled CO₂ sequestration and enhanced coal bed methane recovery offers a further, if riskier, low carbon subsurface energy prospect. To aid deep geothermal exploration, subsurface data from northern England is being compiled in order to construct regional 3D geothermal models. Our models highlight hot areas within the subsurface more realistically than equivalent maps based on contouring around individual borehole temperature measurements that are skewed by sparsely distributed data points and, potentially, inaccurate measurements. Deep heat-producing granite bodies and the variable thickness of thermally resistive rock units, such as the Pennine Coal Measures Group, are highlighted as dominant controls on the distribution of deep geothermal energy in northern England.