Environmental Engineering Reference
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Figure 12. Plot of log (X
CO2
/X
N2
) versus log (X
CO2
/X
CH4
) diagram for the inclusion fluids and the
present reservoir fluids at the initial stage of exploitation of the Mori geothermal field (Muramatsu et
al., 1997) Symbols are the same as for figure 11. The schematic vapor-loss curve is the same as for
figure 10.
Dotted lines in figure 11 join the present reservoir fluid from the main feed point of five
wells (table 4) and the inclusion fluid in the analyzed samples from the depth near the point.
These lines, especially of wells NT-306 and ND-6, are nearly parallel to the schematic
degassing curve, probably suggesting that the CO
2
degassing has proceeded from the quartz-
anhydrite mineralization stage to the present.
3.3.2. Matsukawa Geothermal Field
The Matsukawa geothermal field is the only vapor-dominated geothermal resource so far
developed in Japan. The country's first geothermal power plant and the world's fourth, a 9.5
MWe facility, was brought on line at Matsukawa in 1966, and upgraded to 23.5 MWe in
1993. The conceptual model of the Matsukawa geothermal system has been discussed by
several authors (e.g. Nakamura, 1967; Ide, 1985; Akazawa and Muramatsu, 1988; Hanano
and Matsuo, 1990; Ozeki et al., 2001). Hydrothermal alteration in the Matsukawa geothermal
field and the surrounding area has been described by Sumi (1966, 1968) and Kimbara (1983),
who defined two alteration periods: an early alkaline stage and a later acidic stage. The
widespread occurrence of pyrophyllite and diaspore within the reservoir rocks in the western
upflow zone of the field are indicative of extensive fluid-rock interactions with acidic fluids
(Hemley et al., 1980). According to Muramatsu et al. (2006), the textural relationships of the
hydrothermal minerals in cores from well M11 drilled in this zone suggest that the acidic
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