Mapping and Assessment of Geothermal Plays in Deep Carbonate Rocks – Cross-domain Implications and Impacts (Hotlime)

Hydrothermal systems in deep carbonate bedrock are among the most promising low-enthalpy geothermal plays across Europe. Apart from a few areas where viability of hydrothermal heat and power generation has been proved, most deep carbonate bedrock has received relatively little attention, because such rocks are perceived as ‘tight’. Exploration and development of the deep subsurface is an acknowledged high-risk investment, particularly in low-enthalpy systems, where tapping suitable temperatures for geothermal energy commonly requires drilling to depths of more than 3 km. In order to de-risk these challenging geothermal plays, it is crucial to improve our understanding of geological conditions that determine the distribution and technical recoverability of their potential resources. The efficacy of carbonate bedrock geothermal plays is crucially dependent on groundwater yield controlled by fracture conduits and karstification. This project identified the generic structural controls in deep carbonate formations, through a comparison of geological situations and their structural inventory, as well as collation of deep borehole data and their petro- and hydro-physical characteristics. A consistent assessment and the sharing of knowledge – bringing all partners to a common high level – resulted in uniformly applicable best practice workflows for estimation, comparison and prospect-ranking of hydrothermal resources in deep carbonate bedrock. Applied in eleven target areas by means of 2D or 3D mapping and characterization, these spatial assessments helped in de-risking the setup or maturation of regional plays, reveal possible cross-domain implications, and support sustainable subsurface management.