第四紀花崗岩をターゲットにした葛根田地熱地域の深部貯留層開発について Development of deep-seated geothermal reservoir bringing the Quaternary Granite into focus in the Kakkonda geothermal field, Northeast Japan
The Kakkonda geothermal field supports a 50MWe power station operated by Japan Metals and Chemicals Co., Ltd. and Tohoku Electric Power Inc. The station has been in operation since 1978. Currently, an additional power plant (30MWe) is scheduled to start operation in March 1996.<BR>The Kakkonda hydrothermal system has two distinct reservoirs both in temperature and permeability. The shallow reservoir is very permeable and 230 to 260°C whereas the deep reservoir is less permeable and more than 300°C. However, lost circulation during drilling occurs around the boundary between the Quaternary Kakkonda Granite (the neogranitic pluton) and its host rocks in the deep reservoir of approximately 3 km in depth. Therefore, it is very important for a well target to investigate a shape of the granite. The isograds of contact metamorphic minerals by the granite are found in depths deeper than lkm. Vertical distances between the isograds and the granite are as follows: biotite; 0.7-1.2km, cordierite; 0.6-0.7km, anthophyllite; 0.4-0.6km and andalusite; 0.01-0.3km. The depths of the isograds are shallower in north-east and slightly deeper in southwest. Thus, isograds of metamorphic minerals are good indicators to estimate the shape of the granite, namely a deep-seated fracture zone.<BR>After the granitic magma intruded, the mineralization in the field is as follows. (1) Pre-tertiary formation, Kunimitoge F. (Neogene) and old intrusive rocks were metamorphosed by the magma intrusion and the contact metamorphic minerals such as biotite, cordierite, anthophyllite, cummingtonite, clinopyroxene, orthopyroxene, K-feldspar, andalusite, garnet, spinel and magnetite were generated. Axinite, tourmaline, fluorite and molybdenite distribute around the shallower part of the granite top. (2) An up-flow trending NNW-SSE direction in the northwestern part in this area and that around A-base, generated calc-silicate minerals such as prehnite, epidote and actinolite-tremolite. (3) The up-flow zone trending NNW-SSE direction and the metamorphic rocks got the retrograde metamorphism when the shallow reservoir was formed. Clay minerals (montmorillonite, sericite) and zeolites (laumontite, wairakite) indicate the state of temperature and the limit of the shallow reservoir. Hematite, albite, calcite, dolomite, epidote and actinolite-tremolite in the deep reservoir have a possibility as indicators of the permeable zones in the deep reservoir.
資源地質 45(3), 131-144, 1995-06-10
The Society of Resource Geology