| dc.description.abstract | The United Downs Deep Geothermal Project (UDDGP) is situated near Redruth in Cornwall and is the first deep (5 km) geothermal power project to commence in the UK. Two deviated geothermal wells, one of which is the deepest UK onshore borehole, have been completed to measured depths of 2393 m and 5275 m (2214 m and 5054 m true vertical depth). These intersect the NNW-SSE-trending Porthtowan Fault Zone (PTFZ), within the Early Permian Cornubian Batholith. High heat flow values in SW England, double the UK background, make the region a favourable geothermal target, with elevated geothermal gradients of 36 °C/km compared to the UK average of 26 °C/km. The high surface heat flow is principally caused by the elevated U, Th and K within the crust.
The Cornubian Batholith is composite and can be divided, near surface, into five different granite types (G1-G5) that were formed through variable degrees of source rock partial melting at different temperatures, and fractional crystallisation processes. As a consequence, the granites have heterogeneous U, Th and K contents that control heat generation and heat flow. Previous high-resolution airborne gamma-ray data has demonstrated the spatial variation in near-surface granite heat production, and the Rosemanowes Hot Dry Rock (HDR) Project has provided U, Th and K distributions to depths of 2652 m in the Carnmenellis Granite. However, challenges remain for modelling the high surface heat flow due to uncertainties relating to the radioelement concentrations at greater depths, the volume and distribution of the granites and petrogenetic controls on the U and Th distribution.
To address these issues, the research had three aims: 1) to evaluate potential biases involved in the sampling and analysis of drill cuttings and to develop a robust analytical procedure; 2) to reassess genetic models for the Cornubian Batholith, specifically the Carnmenellis granite, from U and Th data; 3) determine the heat flow at United Downs and place the heat production profile and petrogenesis of the granites within the context of the 1D heat budget model for SW England lithosphere.
Drill cutting biases occur due to wellbore processes (e.g. cutting depth constraints, contamination), surface sampling (e.g. loss of fines) and laboratory analysis (e.g. measurement volumes in a given size fraction). The effects of these issues on the representivity of the cuttings are evaluated from modelling cutting depth constraints during well transport, quantifying compositional variations between the coarse (> 2 mm) and bulk (> 63 um) cutting fractions, and statistical analysis on the measurement volumes. The evaluation of the UDDGP cuttings illustrated that they have a depth resolution of c. 10 m. Soft, friable and liberated accessory minerals concentrate within the fines and are preferentially lost within the bulk cutting fraction (> 63 μm) compared to the coarse fraction (>2 mm). Despite these biases, reliable interpretation of the geochemical and mineralogical data is still possible as the natural granite variability is greater than the biases produced during drilling.
A multi-proxy approach, using mineralogy, whole rock geochemistry, mineral chemistry and U-Pb zircon geochronology was used to understand the granite petrogenesis and emplacement mechanisms. The proxies identified four granite facies, derived from discrete batch melting events over a period of c. 4.3 Ma. Two granite sub-groups exhibit internal fractionation trends. The dominant compositional control are heterogeneities in the source rock and melt temperature, contrary to previous petrogenetic interpretations that invoked fractional crystallisation (Chappell & Hine, 2006; Simons et al., 2016). Spectral gamma (U, Th) forms a reliable proxy for separating non-cogenetic and cogenetic melts, and thus can be used for future petrogenetic studies, well correlation and understanding the origins of the U and Th distributions with depth.
The United Downs temperature field was evaluated using the equilibrium temperature log, high temperature (170 °C) thermal conductivity measurements, spectral gamma (U, Th, K) wireline log and a 1D heat balance model. The heat flow of United Downs was determined to be 114 mWm-2 (± 10%), similar to previous estimates from the Rosemanowes Hot Dry Rock Project. There is a substantial increase in Th below 3000 m that indicates the deeper parts of the batholith contribute substantially to overall heat production. Despite the high heat production at depth, it is still not enough to reconcile surface high heat flows with the current granite thickness models, therefore deeper heat sources must be invoked.
Overall this thesis has characterised the geology and temperature field to a depth of 5,054 m from the United Downs Deep Geothermal Project (UD-1). The research has contributed to the understanding of the temperature gradients and anomalously high heat flows that are characteristic of SW England. Granite heat production has been linked in a petrogenetic context for the first time within SW England. | en_GB |
