Basin scale structural modelling for assessing geothermal potential of fractured carbonates and fault zones in Ireland's Carboniferous Basin.

The Carboniferous basins in Ireland are rich in fractured carbonate rocks, which present significant potential as geothermal reservoirs. The National Geothermal Database (NGD) project, supported by Geological Survey Ireland, aims to evaluate this potential by developing a comprehensive 3D geological model and assessing geothermal resources. This evaluation includes petrophysical modelling of heat in place (HIP) and heat recovery, with particular attention to permeability enhancements from secondary fracturing and dolomitization related to fault zones.

The 3D model integrates an extensive dataset, including over 50,000 boreholes, 250,000 well tops, 2D seismic reflection data, geophysical surveys, and geological maps. Covering an area of 200 × 200 km, the model incorporates over 180 faults and six stratigraphic horizons, constrained at a high grid resolution (250 × 250 m). Fault analysis, including detailed displacement analysis, reveals that the basins' regional structure is dominated by NE-SW-oriented normal faults, with displacements exceeding 500 m, forming north- and south-dipping half grabens. A polarity shift is observed across the region: in western Ireland, the half grabens are primarily associated with north-dipping faults, while south-dipping faults dominate in the east. This structural variation significantly influences the spatial distribution of geothermal reservoirs, with Lower Carboniferous limestone exceeding 1 km depth located in the hanging walls of these half grabens.

Heat in place (HIP) is calculated using upscaled petrophysical properties—such as density, thermal conductivity, porosity, and heat capacity—derived from measurements and literature for each modelled formation. Temperatures are estimated based on formation-specific thermal conductivities and a uniform heat flow of 70 W/m². Recoverable heat is determined as a fraction of HIP, calculated for various intervals of interest (deep geothermal reservoirs up to 2 km, shallow reservoirs up to 200 m, or specific reservoir units). A recovery factor of 5% is applied outside fault zones, with higher values assumed within fault zones to account for enhanced permeability due to increased fracturing and dolomitization, as observed in outcrops and core data. The volume of the damage zone, defining areas of enhanced recovery, is estimated using fault scaling properties linking displacement and fault zone thickness and a probabilistic approach to account for fault zone thickness variability. The calculated recoverable heat provides site-specific insights into geothermal energy potential and suggests that Carboniferous Limestone reservoirs could play a key role in meeting regional heat demands and make a substantial contribution to Ireland’s net-zero carbon emissions target by 2050. More broadly, this study underscores the importance of incorporating structural modelling to assess resource availability and provides a valuable template for integrating fault zone characteristics into geothermal initiatives.