The Importance of Tectonic Inheritance and Reactivation in Geothermal Energy Exploration for EGS Resources in SW England

Abstract

Exploration for Enhanced Geothermal System (EGS) resources has long sought natural deep fractured reservoirs. The Upper Rhine Graben is the most studied area for these targets including the successful geothermal plants at Soultz-sous-Forêts, Bruchsal, Insheim, Landau and Rittershoffen (Genter et al., 2010; Vidal et al., 2017). Previous geothermal projects, such as the Hot Dry Rock project at Rosemanowes (1977-1991) in Cornwall (SW England), targeted areas that lacked major faults and fractures (Barker et al., 2000). The Upper Rhine Graben was incepted during Eocene-Oligocene rifting in a NNE-SSW orientation that underwent Miocene sinistral reactivation (Schumacher, 2002). However, structural inheritance from dextrally reactivated Variscan ENE-WSW faults during the Miocene may also contribute to the reservoir (Bertrand et al., 2017). Geothermal energy exploration is currently experiencing a revival in SW England, targeting EGS resources within deep granite-hosted fault zones. Two deep wells have been drilled at the United Downs Deep Geothermal Power Project to depths of approximately 5.0 and 2.5 km near Redruth and Eden-EGS Energy have advanced plans for deep geothermal wells near St Austell. The geology of SW England comprises several E-W-trending Devonian-Carboniferous sedimentary basins that were deformed during the Variscan orogeny (Carboniferous) and intruded by the Cornubian Batholith (Early Permian). The region is characterized by a complex fault network including major NW-SE fault zones (first-order) that have traces lengths more than 10 km and usually comprise multiple faults across a broad zone of deformation. These are prospective EGS targets due to their inferred down-dip persistence, Cenozoic reactivation (Holloway & Chadwick, 1986; Cooper et al., 2012; Anderson et al., 2018), near parallelism with contemporary σH (Batchelor & Pine, 1986) and are known for low magnitude (