Low temperature thermochronology as an investigation tool for deep geothermal energy: insights from the Midland Valley sedimentary basin (Scotland).

Abstract

Deep (> 500 m below ground) geothermal energy is generated by heat sources within the Earth, including unusually high  lithospheric basal heat flow and/or intrusive bodies rich in radioactive isotopes, that heat the surrounding rocks and aquifers. This warm water can then be used for electricity production or to provide heat for buildings. These relatively high geothermal gradients can be found at depth in sedimentary basins where aquifers are surrounded by rocks with low thermal conductivity. Investigating the suitability of a basin for deep geothermal energy exploration requires, therefore, a thorough geological investigation of its spatially variable structure, stratigraphy and evolution. Low temperature thermochronology, namely apatite fission track and (U-Th-Sm)/He methods, are able to reconstruct the thermal structure of the shallow crust through time and, when data are available from boreholes, to quantify the evolution of the geothermal gradient, providing insights on the most promising areas where aquifers could be unusually warm. We have applied low temperature thermochronology to the study of the Midland Valley (MV) Basin, an extensive sedimentary basin onshore Scotland, hosting many potential energy consumers in the cities of Glasgow and Edinburgh. The Midland Valley mainly consists of alternating succession of sandstone and siltstone with mudstone, limestone and coal, predominantly of Carboniferous and Devonian age. The MV also experienced folding and faulting throughout its geological history; therefore, the succession is spatially highly variable, difficult to reconstruct by simply using the sparse borehole-derived stratigraphic constraints. Apatite fission track data from across the eastern sector of the basin and the UK Geoenergy Observatories borehole in Glasgow indicate a 1) rapid burial in the Carboniferous-Permian; 2) Permian-Mesozoic cooling and a 3) a relatively rapid early Cenozoic cooling, an event that is asynchronous across the basin. Using a combination of forward and inverse modelling techniques, we constrain the palaeo-geothermal gradients and highlight areas where the thermal structure of the shallow crust could still be relatively hot for aquifer geothermal energy.