How important are faults to form geothermal systems?

Geothermal systems emerge from the interplay of available heat and sufficient permeability that allows for heat and mass transfer in the Earth’s upper crust. While temperature increases with depth according to the geothermal gradient and may be locally elevated by magmatic intrusions, fluid flow pathways are often structurally controlled. Structures such as faults and fracture networks can locally increase permeability, leading to focused upflow of hot fluids and downflow of cold meteoric fluids. In volcanic provinces as well as amagmatic geothermal systems, complex structural settings including fault relay zones and fault intersections are commonly described as potential favourable locations to form economic geothermal resources. Here, we conduct 3D numerical fluid flow simulations to test the controls of high-permeability structures on intermediate- and high-temperature geothermal system formation in both magmatic and amagmatic settings.

The model domain extends over 40x40x8 km, includes four normal faults, and a relay zone between two overlapping fault terminations. We systematically vary the permeability of the fault zones and the relay zone to explore their effect on geothermal system formation, while the background permeability is depth- and temperature-dependent. In magmatic settings, a magma body with an initial temperature of 900 °C provides additional energy during magma crystallisation and cooling; its location relative to the fault planes is varied to test effects on geothermal resource formation.

Preliminary results suggest that amagmatic settings with a uniform heat source at the bottom boundary rely on high-permeability structures, as they can locally enhance heat and mass transfer, potentially leading to the formation of intermediate-temperature geothermal systems. Structurally complex settings with larger rock volumes of increased permeability (e.g., relay zones) are of particular interest due to the increased potential of enabling and hosting upflow of geothermal fluids. Additional heat from magmatic intrusions, however, can override the structural controls on geothermal reservoir formation. In such magmatic settings, faults may temporarily enhance the upflow of hot fluids if flow pathways from the intrusion are naturally directed into the fault plane, which requires the intrusion to be located underneath the fault. Overall, however, heat and heat source location are the key controlling parameters governing both the formation and spatial location of geothermal reservoirs in magmatic settings.