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The RT value seems to be a few ten degrees higher than the Tb value after a standing time of
120 hours at 600 to 870 m depths. This mean the well must belong to pattern (d).
In conclusion, geothermal activity had a large cooling after precipitation of the measured
mineral in the reservoir of most geothermal fields in Japan. The minimum Th value of fluid
inclusions in transparent mineral from the production well have been used to judge whether
reservoir temperature is suitable to use as a production well with real time at a drilling site, in
addition to evaluate broadly the possibility of development of the reservoir in most Japanese
geothermal fields. The transparent mineral suitable to use for microthermometry is not only
hydrothermal minerals such as quartz, calcite and anhydrite, but also primary quartz in
igneous rock and quartz fragment in pyroclastic rock because the present reservoir fluid were
trapped as a secondary inclusion in these quartz formed by different mechanism.
Figure 4. Distributions of Th values of liquid-rich inclusions in the analyzed samples with static
borehole temperature (Tb) after standing times of 96 and 120 hours for well N58-OA-6 in the Okuaizu
geothermal field (Nitta et al., 1987).
To arrange the thermal evolution of Japanese geothermal fields, table 1 shows
predominant Th pattern for each well drilled by the GDPS project, and for most geothermal
fields where the power plant was constructed. Patterns (a) and (b) occupy 73 per cent of all
wells in the GDPS fields. Single pattern is recognized at the Kuju and Fukuejima Seibu fields,
but plural patterns are recognized at the other fields except for Okiura, Hachimantai Tobu and
Inawashiro. It thus reveals that the reservoir temperature has decreased to the present days in
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