Geologic structure and volcanic history of the Yanaizu-Nishiyama (Okuaizu) geothermal field, Northeast Japan

A Study of Production/Injection Data from Slim Holes and Large-Diameter Wells at the Okuaizu Geothermal Field, Tohoku, Japan

“…In order of increasing depth (see Figure 2.3), the regional stratigraphic column in the Okuaizu geothermal area is the following (adapted from Seki, 1991 andMizugaki, 2000): Basement Rocks: Pre-Tertiary (>65.0 Ma), Basement has not been identified on the surface in the Yanaizu Nishiyama area nor has it been encountered in the drilling cores. Nevertheless, the basement is believed to be granitic rocks because granite fragments occur in Neogene tuffs obtained from the bottom of some of the boreholes and granite outcrops 20 km north and northwest of the Okuaizu Geothermal Field.…”

“…It was assumed that there was a hydrothermal reservoir of hot water in the Sunagohara formation with high permeability, and that anomalous geothermal Figure 2.5 (adapted from Mizugaki, 2000 andSeki, 2000).…”

“…The Okuaizu geothermal reservoir thus appears to be controlled by the Miyashita mudstone as an impermeable formation and two fault systems, that is, the Chinoikezawa and Sarukurazawa fault zones which function as pathways for hydrothermal fluids from greater depths (Mizugaki, 2000). Tosha, et al (2000) observed a positive self-potential (SP) anomaly over high temperature zones, the amplitude of which exceeded 100 mV before the commencement of geothermal fluid production.…”

“…Large-Diameter Wells at the Okuaizu Geothermal Field, Tohoku, Japan (adapted from Mizugaki, 2000 andSeki, 2000).…”

“…Geothermal investigations since 1974 by MMS, OAG, and governmental researchers in the vicinity of the Okuaizu Geothermal Field have included geological, geophysical, and geochemical studies (e.g., Nitta, et al, 1988;Nonokuchi, 1988;Takenouchi, 1988;Seki, 1991;Mizugaki, 2000;Seki, 2000;Toshiyuki, et al, 2000). Surface geothermal manifestations include solfataras, hot springs ranging in temperature from 40°C to 90°C, H 2 S and CO 2 discharges, hydrothermal alteration, and abandoned sulfur and clay mines.…”