Geothermal Systems in the Akinomiya Area, Northeast Japan

Abstract

The authors have been investigating the characteristics of geothermal systems in the Akinomiya geothermal area under the Geothermal Development Promotion Survey by New Energy and Industrial Technology Development (NEDO) since 1996. Five investigation wells (N8-AY-1, N8-AY-2, N9-AY-3, N9-AY-4 and N9-AY-5) have been drilled. NEDO (1997, 1998) has discussed the results of this project in detail. We have conducted a geological survey on the surface, the cores and cuttings observation, FMI (Fullbore Formation Microimager) logging, gravity and electromagnetic surveys. As results of our investigations, we can deduce the geology, the geologic structure and the fracture systems. Using temperature logging and thermoluminescence, we clarified the temperature structure and the thermal regime. We have used X-ray diffraction to clarify the distribution of hydrothermal alteration on the surface and underground. We have examined the geochemistry of the geothermal discharge fluids and hot spring waters, Hg content, pressure logging, reservoir pressure monitoring, and pressure transient tests. Based on these surveys, we can infer the thermal structure, geochemical features, reservoir pressure and permeability near the wells. In this paper, we discuss the geology, structure, fracture systems, heat structure, reservoir pressure, geochemistry, and geothermal fluid flow, and also discuss the conceptual model and characteristics of hydrothermal convection systems in the area.