Geological controls on upper crustal heat flow for deep geothermal energy in Cornwall

SW England has locally high surface heat flows associated with elevated levels of radioactive elements (U, Th and K) within the granites of the Cornubian Batholith. As such, it is one of the most prospective areas in the UK for deep geothermal energy. Previous investigations into the regional potential for high temperature geothermal energy production included the 42 million pound geothermal Hot Dry Rock research programme, undertaken by Camborne School of Mines (1975-1991). There has been renewed strategic impetus to ensure that at least one of the two sites in Cornwall with planning permission for deep geothermal is funded. The £18M United Downs Deep Geothermal Power project, near Redruth in Cornwall, will commence drilling two wells in October 2018, one 2.5 km deep and the other 4.5 km deep; this activity, which will result in the UK's deepest onshore geothermal borehole, provides the framework for the PhD. The purpose of the project is to address uncertainties regarding heat production and conduction models in the Cornish crust which include: (i) radioactive elements U, Th and K are not present in sufficiently high quantities within the previously investigated upper parts of the granite to account for observed heat flow, (ii) He-4 production from historical deep geothermal wells is higher than anticipated, (iii) geophysical modelling has progressively reduced the interpreted volume of the Cornubian granites. These inconsistences imply substantive heat source(s) may occur within, or below, the deeper parts of the batholith.

The project will develop further understanding of the controls on upper crustal temperatures and deep geothermal energy exploitation in Cornwall and how this might relate to: (i) heterogeneous radiogenic heat production within the composite Cornubian Batholith (presently unsampled below 2.6 km), (ii) heat contributions from mid/lower crustal and mantle sources, and (iii) modification by fracture-controlled deep groundwater circulation. Drilling within the Carnmenellis Granite will occur over a depth range of approximately 1.5-4.5 km. The arisings will permit mineralogical and whole rock geochemical characterisation of the granites, including the U, Th, K budget and their mineralogical hosts and evidence for high-temperature alteration and leaching. Analytical techniques include optical microscopy, QEMSCAN (automated SEM), electron microprobe and XRF/ICP-MS). The use of QEMSCAN is novel as it permits rapid mineralogical analysis and host rock interpretation from fine-grained drilling arisings. These data will be complemented by those obtained during downhole logging (gamma ray, formation fluid composition / temperature). Radiogenic heat production will be modelled and compared with measured heat flow data. Discrepancies will be evaluated in terms of the potential role of upper crustal convective fluid flow and/or mid / lower crustal and mantle heat contributions using existing deep geophysical data and an understanding of admissible scenarios during the post-Variscan tectonic evolution of SW England. Confirmation of deeper heat sources would have profound implications for regional geothermal and crustal models and deep geothermal projects in granitic terrains globally. The student will receive training at Camborne School of Mines in: (i) mineralogical and geochemical analytical and interpretation skills, (ii) petrological and heat flow modelling skills, and (iii) regional crustal evolution. Training in downhole log interpretation and fracture-controlled fluid flow models will be provided by GeoScience Limited.