Potential candidates of supercritical geothermal reservoir

Geological models and the properties of supercritical geothermal reservoirs are described with respect to supercritical geofluids, mechanical behaviors of reservoir rocks, crustal permeability and tectonic settings. The critical point of multicomponent geothermal fluids is considered to be located between the critical point trajectories of H₂O-NaCl and H₂O-CO₂ solutions, and the supercritical state divided into 'liquid-like' and 'vapor-like' regions in terms of dissolution of rocks. Mechanical conditions at the elastic-plastic boundaries of granitic rocks indicate potentially exploitable supercritical geothermal resources. Tensile fracturing is possible even in ductile rocks, and some permeability-depth relations proposed for the continental crust show no drastic permeability reduction at the BDT (Brittle-Ductile Transition). The reservoir permeability (>10-16 m2) could be maintained even under supercritical conditions. The permeableimpermeable boundary of the crust and geothermal reservoirs were affected by water-rock interaction such as dissolution and precipitation of silica minerals. Geochemical processes are a significant influence on the creation of a permeable-impermeable boundary in the crust. Possible tectonic settings for supercritical geothermal resources are classified into subduction zones, ridges, rifts, and hot spots. NE Japan is representative of an Island-Arc setting in a subduction system, and it is highly possible that exploitable supercritical geothermal resources may be located on and around the present volcanic front, and fossil calderas in late Miocene and Pleistocene are alternative candidates for supercritical geothermal reservoirs.