How useful are outcrop samples to constrain subsurface thermal properties for geothermal exploration? Case study from the Chester formation, UK

• Outcrop samples provide a cost-effective option for early HSA exploration. • Surface weathering affects mineralogical properties of outcrop samples. • Core samples exhibit lower porosity and permeability than outcrop samples. • Thermal conductivity is higher in core samples and varies between lithotypes. Hot sedimentary aquifers (HSAs) offer a sustainable option for geothermal energy, but exploration risks remain a major challenge. Drilling is capital intensive, especially for geothermal projects, and early in project appraisal it is often uncertain whether a formation will provide sufficient thermal performance to justify investment. Outcrop samples have therefore gained attention as analogues for subsurface reservoir properties, providing low-cost preliminary data before committing to drilling. While this approach is well established in the hydrocarbon industry, its application to geothermal settings has been limited. Existing geothermal studies have largely focused on structural, petrophysical, or mineralogical characterisation, while systematic comparisons of these properties and thermal properties between outcrop and core samples remain rare. This is a critical gap, as thermal properties govern the long-term performance of a geothermal system. This study compares seventeen outcrop and twenty-one core samples (taken from depths of 22–96 m) from the Triassic Chester Formation in the Cheshire Basin, UK, to assess the transferability of outcrop-derived data. The samples were analysed for mineralogy, petrophysical, and thermal properties. Results show that while outcrop samples exhibit some variability due to weathering at the Earth’s surface, they generally have similar mineralogical and petrophysical properties to core samples. Thermal conductivity is higher in cores (2.15 ± 0.01 W m⁻¹ K⁻¹) compared to outcrops (1.74 ± 0.01 W m⁻¹ K⁻¹). Variations are also observed between different lithotypes, with pebble-rich samples generally exhibiting higher thermal conductivity than massive, layered, or cross-bedded sandstones. These findings highlight the potential of outcrop samples for early-stage geothermal exploration. They provide a cost-effective alternative to drilling for constraining rock properties, particularly thermal conductivity, at depth, while offering insights into depositional environment and lithology. This integrated approach could improve subsurface predictions, reduce exploration costs, and support the growth of the geothermal sector by de-risking investment decisions.