Estimation of spatial distribution and fluid fraction of a potential
supercritical geothermal reservoir by magnetotelluric data: a case study
from Yuzawa geothermal field, NE Japan
Abstract
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.