Environmental Engineering Reference
In-Depth Information
According to Sasada et al. (1992) and Sawaki et al. (1997), the CO
2
contents in the
inclusion fluids of hydrothermal quartz and anhydrite are generally about one order of
magnitude higher than those in the present discharged fluids of the Sengan, Yuzawa,
Kurikoma, Okuaizu, Hohi, Kirishima, Hishikari and Broadlands geothermal fields. It suggests
that early stage fluids were trapped in the fluid inclusions before considerable boiling and
vapor-loss. Based on the differences in CO
2
/CH
4
,
CO
2
/N
2
and N
2
/Ar ratios between the
inclusion fluids and the present reservoir fluids, the fluid evolution in the Kirishima and
Kurikoma fields could explained by degassing and dilution, respectively (Sasada et al.,1992;
Sawaki et al.,1997). The fluid must be evolved by both degassing and dilution in Sengan
(Sasada et al., 1992). On the contrary, the similarity in CO
2
/CH
4
and CO
2
/N
2
ratios between
the inclusion fluids and the present reservoir fluids in Broadlands might show a small time
interval between fluid trapping in the hydrothermal quartz and present geothermal activity
(Sasada et al., 1992).
The gas analytical results of fluid inclusion from the Mori, Matsukawa and Kakkonda
geothermal fields are discussed in section 3.3.
3.3. Case Studies of Fluid Inclusion from Geothermal Fields,
Northeastern Japan
The northeastern Japan is one of the most important regions for Japanese geothermal
exploitation, and produces about 60 per cent of the nation's total installed geothermal
power capacity. The results of microthermometric measurement and gas analysis on fluid
inclusions from the Mori and Kakkonda liquid-dominated and the Matsukawa vapor-
dominated geothermal fields (figure 1), northeastern Japan are discussed in this section.
The Mori geothermal field is located in the Nigorikawa Basin, about 20 km northwest of
Mt. Komagatake, an active volcano, southwestern Hokkaido. The Hachimantai volcanic
area located in northeastern Honshu is one of the important areas for Japanese geothermal
exploitation, contains the Matsukawa and Kakkonda geothermal fields. The Th and ice
melting temperature (Tm (ice)) of fluid inclusion were measured on a USGS-type gas flow
heating/freezing stage, and gas composition of liquid-rich inclusion was analyzed by the
QMS at the Institute of the Geological Survey of Japan (AIST).
3.3.1. Mori Geothermal Field
The Mori geothermal power plant (50MWe) has been in operation since 1982.
Microthermometry and gas analysis of fluid inclusion from the Mori geothermal field
were published by Muramatsu et al. (1997, 1999). The genesis of the inclusion fluid and
the gas evolution of the reservoir fluid based on the fluid inclusion studies are discussed
below.
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