Evolution History of the Kakkonda Magma-hydrothermal System, Japan, Estimated through Simplified-model Numerical Simulatlons

At the Kakkonb geothermal area, Japan, a large Quaternary granitic body is dstributed below ca 2.5 km ckpth from the surface, associating with thick high-temperature contact me&amorphic zones in the overlying formations, and a meteoric-water-cbminated type hydrothermal system is developedat present time. During the 'Deep-seated Geothermal Resources Survey', WD-la well was drilled to 3729 m depth,& a thermal conduction zone with a maximum temperature over 500°C was observed in the Quaternary granitic body. Evolution history of the Kakkonda geothermal system was analyzed by the numerical simulation method based on the simplified onedimensional transient thermal conhction models, in which high 'extended thermal conductivity' was assumed for geothermal reservoirs (Shigeno, 1999a). The time-space temperature distribution dataobtainedfrom the WD-lawell couldbe explained by the following dynamic environment model (KR1 model): (1) At first, the thick contact metamorphic mnes were produced through heat storage by thermal conduction from the magma chamber emplaced probably ca 0.20 Ma; (2) Later (e.g. 0.03 Ma), reservoir formation caused the present convective temperature profile with the high conductive temperature gradient in the cooling Quaternary granitic body. A modfied three-stage scenario (KM2 model) that assumed contribution of magmatic fluid convection in the granitic body for the first stage improved fitting of the observed and simulated temperature profiles for the contact metamorphism. INTRODUCTION Deep reservoirs at exploited geothermal areas have become very important targets for power generation ckvelopments in Japan, where promising geothermal areas for the exploitation have been very limited by distributions of numerous National Parks and hotspring resort areas (e.g. Shigeno (1995)). Hence, the 'Deep-seated Geothermal Resources Survey' was started by MITI and NED0 in 1992, and a deep exploration well, WD-la, was dilled at the Kakkonch geothermal area, Iwate Prefecture, Northeast Japan. As a result, it revealed one type of deep geothermal environments in and around a young igneous M y emplaced to shallow levels, reaching 500°C at 3729 m depth in 1995 (Uchida et al., 19%; Muraoka et ul., 1998; Doi et al., 1998). Cooperating with N E D 0 for the project, a geothermal group at the Geological Survey of Japan has been studying the deep geothermal systems for their modeling, exploration methocls, reservoir evaluations, and resources assessments. This paper shows a study result for evolution history of the Kakkonch magma (igneous)-hyhthermal system estimated through simplified modeling and numerical simulation methods using the loggingand analysisdataofthe WD-lawell. OUTLINE OF KAKKONDA GEOTHERMAL SYSTEM AND WD-1A WELL In the Kakkonda area, the Quaternary granitic body was emplaced in preNeogene formations overlain by Neogene volcanicand sedimentaryformations (Fig. I ) . The time of the magma emplacement has been controversial, though most K-Ar ages obtained from the m i n e d s in the granitic rocks were ca 0.20 Ma (Kanisawa et ul., 1994; Doi et ul., 1998). The above formations were thickly affected by high-temperature contact metamorphism of the granitic body. However, the two semicontinuous hot-water type geothermal reservoirs, whose temperatures range cu. 220" to 380°C from shallow to deeplevels, are presently developedin these formations and the uppermost part of the granitic body (e.g. Hanano (1998)). The Kakkonda I (50 W e ) and11 (30 MWe) geothermal power plants have been operating in this areasince 1978 and 1996, respectively (e.g. Takanohashi (1999)).