Magmatic fluids within the crust may exist under supercritical conditions (e.g. >374°C and >22.1 MPa for pure water). Geothermal systems using such supercritical fluids have gained attention as unconventional geothermal resources because they can offer significantly more energy than conventional geothermal fluids with temperatures <350°C. Although an understanding of the spatial distribution and fluid fraction of supercritical geothermal reservoirs is necessary for their resource assessment, the spatial distribution and fluid fraction of supercritical geothermal reservoirs worldwide are poorly understood due to the limited number of geophysical observations. Here, the magnetotelluric (MT) method with electrical resistivity imaging was used in the Yuzawa geothermal field, northeastern Japan, to obtain information on the fluid fraction and spatial distribution of a supercritical geothermal reservoir. Our MT data revealed a potential supercritical geothermal reservoir (>400°C) with a horizontal dimension of 3 km (width) × 5 km (length) at a depth of 2.5–6 km. The estimated fluid fraction of the supercritical reservoir was 0.5–2% with a salinity of 5–10 wt%. The melt was imaged below a supercritical geothermal reservoir. Based on the resistivity model, we propose a mechanism for the evolution of a supercritical fluid reservoir, wherein upwelling supercritical fluids supplied from the melt are trapped under less permeable silica sealing. As a result, supercritical fluids accumulate under the silica sealing. This study is the first to present a detailed estimation of the spatial distribution and fluid fraction of a potential supercritical geothermal reservoir.
