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 granitehosted 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 >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 (<M4) seismicity persisting to the present day. Second-order faults, defined based on trace length rather than their kinematic relationships, typically have trace lengths <10 km and exhibit a range of orientations, may be an important consideration for EGS resources in the region. The Cornubian Batholith has the highest measured heat flow in the United Kingdom and most prospective for EGS resources (Busby & Terrington, 2017). Based on airborne radiometric data, the granites are high heat-producing with 13.3% of the granite outcrop (1239 km2 ) showing values >6 µW-3 (Beamish & Busby, 2016). Granite emplacement post-dates the inception of both first- and second-order faults within the host rocks, but the granites demonstrate inheritance of both fault sets. Intra-granite structures indicate episodic fault reactivation from the Permian through to the Cenozoic (Holloway & Chadwick, 1986; Shail & Alexander, 1997). Furthermore, a persistent hydrothermal mineralization record in the region demonstrates that both first- and second-order faults have previously been episodically open for fluid flow and geothermal activity. Herein, the fault network is assessed across different plutons to demonstrate the ubiquity of first-order NW-SE faults and investigate the interaction of second-order faults. By analyzing data from Yeomans et al. (2019), different age plutons are shown to have variable lineament populations. Whilst initial research regarding tectonic inheritance requires further work, the available data appear to indicate an evolving stress regime in the Early Permian. We also present a proposed workflow for generating new EGS targets as part of a staged modelling process. A key aspect of this process is the incorporation of data from regional and outcrop scale modelling to determine if the regional structural framework is representative of the target reservoir. This will allow modelling to be adapted from regional to local scales to best identify and subsequently refine geothermal energy targets.