Structural Geothermal Plays in the Lower Carboniferous Limestone, East Midlands, UK: Insights from Legacy Geophysical Data

Mohamed Gouiza , Dave Healy, Nick D. Shaw

Lower Carboniferous Limestone (LCL) is a promising geothermal target in Britain, but uncertainty in subsurface structure and permeability limits resource assessment. Here, we use the Welton area of the East Midlands, where legacy hydrocarbon exploration data provide detailed subsurface coverage, to investigate the controls on LCL geothermal prospectivity. We integrate 3D seismic reflection, well, gravity and aeromagnetic data with gravity forward and inverse modelling to characterise the geometry and structural evolution of the LCL. Seismic interpretation reveals a heterogeneous reservoir partitioned into a western rift-bounding fault corridor, an eastern relay system and a northern fault-intersection zone, together with fault-tip and segment-linkage structures. Seismic-stratigraphic relationships record late Carboniferous reactivation of this inherited extensional architecture, expressed by localised LCL doming and erosion, Namurian absence and onlap, and onlap within the lowermost Westphalian. These relationships indicate variable inversion and possible weathering or palaeo-karst development on uplifted LCL highs. Gravity and magnetic lineaments reproduce and extend seismic-scale structural trends, while source-depth estimates indicate that parts of the fault network were localised above deeper inherited fabrics. Gravity modelling shows that plausible LCL density variations alone cannot explain the observed gravity field, requiring deeper or unmodelled density heterogeneity. The integrated results define three conceptual structural geothermal plays: a fault-damage-zone play, a relay/linkage play and a fault-intersection play. These settings may favour secondary permeability through fracturing, augmented by inversion-related deformation and possible palaeokarst development, although present-day transmissivity remains unconstrained. This study demonstrates how legacy seismic and potential-field datasets can support geothermal targeting in structurally complex carbonate reservoirs.