FLUID INCLUSION MICROTHERMOMETRY AND GAS CHEMISTRY IN GEOTHERMAL SYSTEM, JAPAN AND ITS APPLICATION FOR THE STUDY OF FLUID EVOLUTION - Energy Research Developments: Tidal Energy, Energy Efficiency and Solar Energy - page 176

Borisenko,1977), the ice melting data strongly suggests that aqueous Ca comprises a
significant component, even though K or Fe may be contained as a minor component in these
liquid-rich and polyphase inclusions. The NaCl-CaCl 2 -H 2 O ternary model composition was
therefore determined by a graphical method (Vanko et al., 1988) based on Tm (ice) and Tm
(NaCl) for the type D inclusions. As a result, CaCl 2 /NaCl weight ratio ranges from 2.2 to 2.9,
and the ternary fluid compositions are 9 to 11 wt% NaCl and 24 to 26 wt% CaCl 2 , or a total
salinity of 33 to 35 wt% (table 4).

Table 5. Microthermometric data and ternary fluid compositions of type D inclusions in
anhydrite from the Mori and Kakkonda fields (Data from Muramatsu et al., 1999, 2000)

Fluid composition (wt%)

Inclusion
number

Tm(ice)
( ℃ )

Tm(halite)
( ℃ )

Th
( ℃ )

H 2 O NaCl CaCl 2

2000m depth of well ND-6 in the Mori field
1 ￿41.2 160 250 65 11 24

2 ￿43.6 152 263 65 9 26

3 ￿42.2 155 259 65 10 25

4 ￿40.1 132 265 67 9 24

5 ￿43.3 147 258 65 9 26

1875m depth of Well-19 in the Kakkonda field
6 ￿40.3 163 300 65 11 24

All Tm (ice) values were measured in the presence of hydrohalite.

According to Muramatsu et al. (1999), four types of the inclusion fluids have been
generated by the following process. Phase separation due to boiling of initial homogeneous
hot seawater at the deeper parts of the reservoir might have produced Ca-rich hypersaline
brines, and vapor- and CO 2 -rich low saline fluids at the earliest stage of fluid evolution in the
Mori geothermal system. Afterwards, the low saline aqueous fluids derived from the mixture
of seawater and meteoric water have been dominant in the reservoir since the recent natural
state stage before operation of the Mori geothermal power plant in 1982.

Gas Evolution in the Reservoir Fluid

Figure 11 illustrates variation of CO 2 content with temperature for the inclusion fluids
(table 3) and the present reservoir fluids at the initial stage of exploitation (table 4). The CO 2
contents in the inclusion fluids in quartz are roughly plotted along the schematic vapor-loss
curve by a single step separation, suggesting that the CO 2 contents in the inclusion fluids in
quartz are considerably dominated by CO 2 degassing. Figure 12 illustrates the correlation
between CO 2 /N 2 and CO 2 /CH 4 ratios for the inclusion fluids and the present reservoir fluids.
quartz are considerably controlled by degassing.
The CO 2 /N 2 ratios generally increase with increasing the CO 2 /CH 4 ratios for the inclusion
fluids in quartz. This trend is roughly plotted along the schematic vapor-loss curve by a single
step separation, suggesting that the CO 2 /N 2 and CO 2 /CH 4 ratios of the inclusion fluids in
quartz are considerably controlled by degassing.