Abstract
A considerable number of rock bodies with varying percentages of supercritical fluid exist around the brittle–ductile transition (BDT) zone at a depth of several kilometers from the surface of the Earth, in northeastern Japan. As the BDT zone in the granitic basement of the continental crust is estimated to occur at about 380 °C, the identification of the depth corresponding to 380 °C is important to utilize the thermal energy inside the “supercritical geothermal systems”. In this study, we focused on an estimation method to determine the depth of the isothermal layer corresponding to 380 °C, using the activity index (AI) obtained from the maximum-temperature data of the geothermal wells and hot springs. The thermal profiles of deep and hot exploration boreholes and the hypocentral distribution of natural earthquakes were used to evaluate the characteristics and accuracy of the deep thermal structure, using the activity index. The estimated depth corresponding to 380 °C tended to be higher than the actual depth, with a maximum possible estimation error of approximately 9.7 km. Distribution maps showing the depth of the isothermal layer corresponding to 380 °C were created for six major geothermal fields in northeastern Japan, using the results from this study.
Highlights
IntroductionIt has been established that supercritical geothermal systems, which are located at depths near or below the brittle–ductile transition (BDT) zone in the granitic basement and that contain supercritical fluid (i.e., at temperatures and pressures >374 ◦ C and >22.1 MPa for pure water and >406 ◦ C and >29.8 MPa for seawater, respectively), have the potential for a huge amount of power generation and subsequent reduction of CO2 emissions in many countries with high-temperature geothermal resources [1]
It has been established that supercritical geothermal systems, which are located at depths near or below the brittle–ductile transition (BDT) zone in the granitic basement and that contain supercritical fluid, have the potential for a huge amount of power generation and subsequent reduction of CO2 emissions in many countries with high-temperature geothermal resources [1]
In Japan, researchers initiated a project to investigate the feasibility of geothermal development around and beyond the BDT zone in 2010 (Japan Beyond-Brittle Project, JBBP) [2], and concluded that the development of supercritical geothermal resources can solve most of the problems impeding the development of hydrothermal systems and hot dry rock enhanced geothermal systems (EGS) in the country
Summary
It has been established that supercritical geothermal systems, which are located at depths near or below the brittle–ductile transition (BDT) zone in the granitic basement and that contain supercritical fluid (i.e., at temperatures and pressures >374 ◦ C and >22.1 MPa for pure water and >406 ◦ C and >29.8 MPa for seawater, respectively), have the potential for a huge amount of power generation and subsequent reduction of CO2 emissions in many countries with high-temperature geothermal resources [1]. Researchers in the supercritical geothermal project observed that a considerable number of magmatic intrusions containing a supercritical fluid (supercritical geothermal systems), which have their origins in the subduction of oceanic plates [3], exist in the BDT zone at shallower depths in northeastern Japan. This was done by interpreting resistivity images from magnetotelluric (MT) surveys and the hypocentral distribution of inland earthquakes [4].
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