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 175

Figure 10. Schematic model to show general trend as a result of degassing process operating on an
initial reservoir liquid in terms of log (X CO2 /X N2 ) versus log (X CO2 /X CH4 ) of an initial reservoir liquid.
The schematic vapor-loss curve was drawn through calculation for a single-step separation with
adiabatic cooling by assuming the hypothetical initial reservoir liquid composition of 3.2 mol% CO 2 ,
0.6 mol% N 2 and 0.05 mol% CH 4 at 265 ºC (open star). Equations for the vapor-liquid fractionation are
taken from Giggenbach (1980). The schematic dilution curve was drawn by assuming the mixing of the
air-saturated groundwater at 20 ºC with the differential reservoir liquid at 260 ºC formed by boiling of
the hypothetical initial reservoir liquid.
Formation of Ca-Rich Hypersaline Brine and CO 2 -Rich Fluid
Analyzed samples were hydrothermal quartz and anhydrite (φ0.5mm thick) selected from
cuttings and a boring core from geothermal wells in the field. Fluid inclusions in the minerals
are divided into four types based on phase relations at room temperature and cooling
microthermometry. Type A is a liquid-rich inclusion with low- to moderate-salinity
homogenized to liquid phase. Type B is a vapor-rich inclusion homogenized to vapor phase.
Type C is a vapor- and CO 2 -rich inclusion with low-salinity less than 3.8 wt% NaCleq.,
homogenized to vapor phase. Type D is a Ca-rich polyphase inclusion homogenized to liquid
phase. In particular, Type D inclusion is associated with Types B and C inclusions.
Because the minimum Th of Type A inclusions is in general agreement with the static
borehole temperatures after a long standing time of 13 days as mentioned in section 2.1
(figure 3), the reservoir fluid formed by the recent geothermal activity must be trapped in
parts of the inclusions. Relations among Tm (ice), Tm (halite) and Th of type D inclusions in
anhydrite from 2000 m in depth of well ND-6 are shown in table 5. Tm (halite) and Th range
from 132 to 160 ºC and from 250 to 265 ºC, respectively. Tm (ice) of -40.1 to -43.6 ºC in the
presence of hydrohalite are lower than the eutectic temperature of the system NaCl-H 2 O (-
21.2 ºC; Hall et al., 1988). It indicates that the brine is not simply NaCl-H 2 O binary fluid, but
contains significant amounts of additional aqueous components. Considering the eutectic
temperatures of the ternary system NaCl-FeCl 2 -H 2 O (-37.0 ºC; Borisenko,1977), NaCl-KCl-
H 2 O ternary (-22.9 ºC; Sterner et al.,1988), NaCl-CaCl 2 -H 2 O ternary (-55.0 ºC;