IAP-25-050

Assessing the geothermal potential of Scottish granites by establishing their long-term thermal history

The development of a sustainable and economically viable geothermal energy system is predicated on having a comprehensive understanding of geological, thermogeological and hydrogeological conditions. This informs the identification of target geothermal resources, drilling costs, well engineering design, and the anticipated thermal energy output of the system, amongst other critical aspects.
One method of achieving this is through exploratory drilling, which is often prohibitive due to high up-front capital cost. In lieu of this, utilising existing data from legacy geophysical surveys or existing boreholes, e.g., for mineral or hydrocarbon exploration, can be of benefit when gaining an understanding of subsurface conditions. However, utilisation of such datasets may not be possible, particularly in geological settings with limited or no mineral or hydrocarbon exploration. In such locations, alternative methods of identification and quantification of geothermal energy resources must be employed. An example of this is the Caledonian terrane of Scotland, which hosts potential geothermal energy resources in high heat producing granites.

The focus of this study is to investigate the differences in radiogenic heat production and, with that, differences in geothermal potential of Scottish granites. Most Scottish granites are of Ordovician-Early Devonian age, but their heat production values vary from 5 µW/m^3 [1]. Suggested explanations range from differences in formation ages and geochemical evolution to varying levels of erosion and rock exhumation. Precise assessment of existing and new geochronology and low-temperature thermochronology data linked with geologic and thermo-physical rock property data will enable us to resolve these issues.
Low-temperature thermochronology and geochronology data from rock samples, integrated with thermal and thermo-kinematic (and potentially landscape evolution) modelling , provide insights into the thermal evolution of the Earth’s upper crust and rock exhumation and erosion histories. When combined with bulk rock geochemistry and radiogenic heat properties, these data can elucidate potential systematic relationships between formation age, geochemistry, and magnitude and history of exhumation and the geothermal potential of the plutons.

This study presents a novel approach to i) bridge the gap between long-term (geologic) thermal evolution of the Earth’s crust and present steady state heat flow and geothermal potential, and ii) explore the use of low-temperature thermochronology to appraise geothermal energy resources in the Highlands of Scotland. Findings of this project will demonstrate the applicability of such techniques in similar geological settings in the UK, and worldwide.

The candidate will work with available granite samples from around Scotland to produce new geo- and thermochronological data (apatite and/or zircon U-Pb, fission-track, and (U-Th)/He), work with new bulk rock geochemistry data, and compile relevant existing geologic, geochemical, geophysical, and thermo-physical rock property data. They will conduct a limited amount of fieldwork (~1 week) to close gaps in field measurements of radiogenic heat production (e.g., [1]). The candidate will use thermal and thermo-kinematic modelling tools (e.g., HeFTy, Pecube, GLIDE ) to integrate new and available data to evaluate the differences in Scottish granites’ heat production values and simulate the present crustal steady state heat flow for different thermo-physical and radiogenic heat properties (e.g., FEFLOW, or new bespoke solutions in MATLAB).

The ideal candidate for this project is comfortable working in an analytical laboratory and is interested in using modelling tools to interpret new and existing data. While the overall project objectives are as described above, the choice of modelling tools and how far different aspects of the study are explored will depend on the interests of the candidate and project progression. There is also potential for using applied case studies to illustrate practical scenarios in which geothermal resources are developed.

Year 1

The candidate will conduct a thorough literature review, familiarising themselves with Scottish geology, its geothermal energy potential, and geo- and thermochronology methods. They will compile relevant existing data and prepare and start analysing mineral separates from available granite samples for geo- and thermochronology at the University of Glasgow. Fieldwork will be planned and conducted by the student together with one of the supervisory team (including gamma ray spectrometry training).

Year 2

Geo- and thermochronologic data production and evaluation will be completed within year 2. In year 2, the candidate will familiarise themselves with computational tools and develop a strategy for integrating data with modelling. This could be using numerical and/or analytical methods. They will receive relevant training at the University of Glasgow and BGS. First results will be presented at the EGU General Assembly (Vienna, Austria) and prepared for publication in a first manuscript.

Year 3

Conduct and finalise regional (or applied) modelling, interpretations, and data synthesis. Regional modelling will focus on developing models using the skills acquired in year 2 that will simulate crustal heat flow across the study area. It is anticipated that a second manuscript will be prepared on the modelling results for publication. The candidate will attend the UK Geothermal Symposium (London) to present the key findings of their thesis.

Year 3.5

Finalising results, manuscripts for publication, and thesis.

– Laboratory skills: geo- and thermochronologic analyses, data reduction, data interpretation, microscopy and imaging
– Field measurements of radiogenic heat production (gamma ray spectrometry)
– Working with a diverse range of datasets and databases from geologic maps to geochemical, geophysical and thermo-physical data, and modelling outputs
– Critical assessment of data quality and analyses
– Interpretation and integration of observational and modelling data
– Analytical and numerical modelling using existing software or bespoke solutions; exposure to programming environments (e.g., MATLAB, Python) and computational thinking
– Understanding of geothermal energy systems
– Multi-disciplinary work
– Soft skills: project management, scientific writing, science communication, written and oral reporting, taking part in research-related and social activities of the institutes involved, planning of field campaigns