Geological Overview of the United Downs Deep Geothermal Power Project, Cornwall, UK

Abstract

The United Downs Deep Geothermal Power (UDDGP) project is the first geothermal power project in the United Kingdom. It aims to develop the geothermal resources in the heat-producing granites that lie beneath Cornwall in SW England. Financial support has come from the European Regional Development Fund and the local authority (Cornwall Council) who, together, have provided £13m of the £18m project budget. The project aims to produce 1 to 3 MW electricity from two deep wells drilled into early Permian granite which underlies much of SW England (the Cornubian Batholith). The batholith is formed of high heat production fractured granite which offers great potential for exploitation of geothermal resources due to the elevated temperature gradients. Fluid will be circulated between an injection well at 2.5km depth and a production well situated 2km directly below it. Both wells are planned to intersect a permeable fault structure known to be associated with episodic fluid transport since the Permian, as well as contemporary groundwater circulation to several km depth. Temperatures at the production depth of ~4.5km are in the region of 1900 C. The target fault structure is a more than 15km long NNW-SSE oriented complex strike-slip fault zone several hundred metres wide as mapped at surface and in shallow mine workings. Along with many sub-parallel structures in SW England it acted as a transfer fault during Variscan thrust faulting (Carboniferous), although inheritance from pre-orogenic Devonian rifting is possible. In the early Permian these structures also acted as transfer faults during NNW-SSE regional extension, during which the Cornubian Batholith was emplaced. Microgranite dykes and magmatic-hydrothermal mineralisation (W-Sn-Cu-Zn-AS metal lodes) are associated with the batholith and occur in and around the United Downs site. The area basically represents a ‘fossilised’ major Early Permian geothermal complex whose rich mineral deposits became the focus of a world-leading mining industry. From surface the wells prove marine low-grade metasediments of Upper Devonian age containing a range of intrusive bodies, overlying granite with many intersections of strike-slip faults. The configuration, physical and flow properties of these structures at depth, within the target fault zone, will be crucial for the success of the project. Pre-drill uncertainties associated with the geological conditions at depth will be reviewed and the latest understanding of the geological conditions presented. This paper summarises the geological setting of the project and the new geological understanding provided by the wells which will explore at depths never before achieved in onshore UK. A wealth of new information will be acquired.