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 171

were trapped simultaneously in a hydrothermal mineral from the reservoir fluid, we can use
the minimum Th values of all Th data because of the same as trapping temperature (Bodnar
et al., 1985). If a fluid had no boiled, we may use Th value with high frequency. In this
case, we must check whether inclusions are much the same in size, because the bulk
composition is influenced by fluid composition of a large inclusion. Degassing trend on a
plot of log (X CO2 /X N2 ) versus log (X CO2 /X CH4 ) of a liquid is illustrated in figure 10. The
schematic vapor-loss curve was calculated for a single-step separation with adiabatic
cooling by assuming the liquid contained 2.6 mol % CO 2 , 0.5 mol % N 2 and 0.03 mol %
CH 4 at 240 ºC (open star). As boiling proceeds, less soluble gases preferably fractionate to
vapor phase in the order of N 2 , Ar, CH 4 and CO 2 (Giggenbach,1980; Potter and
Clynne,1978). As a result, both the CO 2 /CH 4 and CO 2 /N 2 ratios increase in the residual
liquid phase with progressive degassing.
Dilution trends on a plot of log X CO2 versus temperature of liquid are also illustrated in
figure 9. The schematic dilution curve (1) and curve (2) are the mixing lines between air-
saturated groundwater at 20 ºC and the hypothetical initial liquid of 1.5 mol %CO 2 at 350 ºC
(open star), and between the groundwater and the liquid differentiated at 300 ºC by vapor-
loss, respectively.
3.2.3. Bulk Gas Composition of Fluid Inclusion from Geothermal
Fields in Japan
Table 3 lists the bulk gas composition of liquid-rich inclusions in quartz and anhydrite
from twelve geothermal fields in Japan and New Zealand, including Mori (Muramatsu et al.,
1997), Matsukawa (Muramatsu et al., 2000), Kakkonda (Sawaki et al., 1999; Muramatsu et
al., 2000), Sengan, Yuzawa, Kurikoma, Okuaizu, Hohi, Hishikari, Broadlands (Sasada et al.,
1992) and Kirishima (Sasada et al., 1992; Sawaki et al., 1997). The water contents range from
91.5 to 100.0 mol%. Carbon dioxide is the major non-condensable gas in all samples,
containing less than 2.6 mol%. Nitrogen and CH 4 contents are less than 5.8 mol% and less
than 0.063 mol%, respectively, and generally about one and two orders of magnitude lower
than CO 2 content, respectively. Argon is present at less than 0.002 mol%. Gas contents in
quartz from 2260 to 2265 m depths in well ND-1 in the Mori field are the highest of all
measured samples.
The gas compositions of the present reservoir fluids from the geothermal fields are shown
in table 4. They were calculated using the chemical compositions of steams discharged from
the geothermal wells and the quartz or the Na-K-Ca geothermometers. The CO 2 and CH 4
contents in the inclusion fluids are slightly to one order of magnitude higher than those in the
present reservoir fluids. In contrast, the N 2 contents are generally about one to three orders of
magnitude higher in the former than in the latter.