Geothermal geology and comprehensive temperature model based on surface and borehole geology in Sengan, Northeast Japan

Geological data were used to explore the geological structure and hydrothermal alteration distribution of the Sengan area, Northeast Japan. Acidic alteration and high-temperature hot springs surround several Quaternary volcanoes within the Sengan area. Quaternary intrusive granitic-dioritic rocks are distributed beneath the Kakkonda, Matsukawa, and Matsuo-Hachimantai geothermal fields. In particular, the Kakkonda granite, a heat source in the Kakkonda geothermal field, has a high temperature exceeding 500 degrees C inside the body. The temperature structure of the Sengan area was investigated using temperature logging data (WD-1a and 53 wells) collected as part of the temperature recovery test conducted in the geothermal wells. The temperature structure was estimated based on temperature extrapolation, a 2-D temperature distribution created with the thin-plate spline method, and a 3-D temperature distribution obtained by combining the horizontal 2-D temperature data. High-temperature zones above 200 degrees C are distributed at sea level in Nyuto (eastern Lake Tazawa), Kakkonda, Matsukawa, Matsuo-Hachimantai (Hachimantai), and Mt. Akita-Yakeyama, as predicted using the temperature structure estimation method. The granitic-dioritic magma penetrates the pre-Tertiary basement units and the Neogene beneath several fields to form a high-temperature zone in the shallow depth, contributing to the formation of geothermal systems in the Sengan area. However, the linear extrapolation of the subsurface temperature tended to over-estimate the temperature in the greater depth and was valid only at a depth shallower than 3 or 4 km below sea level. This study delineated several high-temperature zones exceeding 200 degrees C above sea level in the Sengan area.