Jökull - 01.12.1987, Page 46

between the C02- and the H2S-geothermometers is con-
sidered to be due to removal of H2S from the steam in
the upflow. This is indeed supported by the lower
C02/H2S ratios in the drillhole discharges as compared
with the fumaroles (Tables 1 and 2). Precipitation as
sulphide minerals seems most likely. Dissolution alone
in the steam heated water is not likely to be significant
(Fig. 9).
The dissolution of H2S in steam heated water was
obtained as follows: By combining equations (4) to (8)
and isolating Xs f the following expression is obtained:
Xs,((hs - hw)/(l - Zb) - hs + hf) = hf -hw (15)
and from the mass balance equation for H2S viz:
mH2St = mH2Sw (1 — Xs,) + mH2Ss • Xs, (16)
we obtain
mH2s/mH2Sw = Xs, (1/P • KH2S — 1) + 1 (17)
where the subscript t denotes total H2S, P is total pres-
sure and KH2S is the Henry’s law coefficient.
The difference between the C02 and the H2S gas
geothermometers indicates that some half of the initial
H2S is lost from the steam in the upflow for those sam-
ples containing the highest H2S.
H2-temperatures are on the average lower than those
of H2S and C02; by 18°C and 54°C, respectively. In
several Icelandic geothermal fields, except for Krísuvík,
Arnórsson and Gunnlaugsson (1985) found that H2-tem-
peratures are intermediate between those of CO, and
H2S. This they explained by lesser tendency for H2 to be
removed in the upflow by chemical reactions than H2S
but greater tendency than C02. The exceptional pattern
for Krísuvík could be due to partial degassing at elevated
temperature without subsequent re-equilibration for the
geothermometry gases. Partial degassing would cause
greater lowering of H, concentrations in the boiled wa-
ter as compared with C02 and H2S because H2 has the
lowest solubility and secondary steam derived from the
boiled water would give relatively low H2-temperatures.
The low N2/Ar ratios of some of the fumarole samples
support this. To obtain N2/Ar ratios of 20 in boiled water
initially at 280°C degassing produced by 0.66% steam
formation by weight is required. Such boiling would
cause the H2 concentration in the water to be lowered to
35% of its initial value (Fig. 10) and the water would be
cooled by about 2°C in the process. Correcting for this
amount of degassing would raise the H2-temperatures by
17°C for all samples, thus causing these geothermometry
Fig. 10 Lowering of aqueous H2 concentration during
adiabatic boiling of water initially at 280°C (left curve)
and 300°C (right curve). Y is steam fraction by weight.
The temperature values on one of the curves indicate the
cooling resulting from the steam formation. — Lcekkun
á styrk H2 í vatni við innrœna suðu á 280°C (ferill til
vinstri) og 300°C (ferill til hœgri) vatni. Y er gufuhluti
miðað við þunga. Hitagildin á öðrum ferlinum gefa til
kynna kœlingu sem verður við gufumyndunina.
temperatures to be on average the same as those of H2S
but lower than the C02-temperatures by 37°C.
The geothermometer which assumes Fischer-Tropsch
reaction equilibrium and the geothermometer of
DAmore and Panichi (1980) yield invariably higher
temperatures than the gas geothermometers so far dis-
cussed. The reason is considered to be that equilibrium
for the Fischer-Tropsch reaction is not closely ap-
proached in the reservoir as seems to be the case for
Icelandic geothermal systems in general as deduced
from drillhole data (Arnórsson and Gunnlaugsson,
1985). The geothermometers of Nehring and DAmore
(1984) also yield high temperatures compared with the
other geothermometers. These geothermometers are
based on work in Cerro Prieto, Mexico, and assume that
the minerals pyrite, magnetite and graphite are involved
in controlling the concentrations of the respective gases.
As previously discussed the mineral buffer controlling
H2S and H2 concentrations in the reservoir under Svei-
fluháls is, by comparison with drilled geothermal fields
in Iceland, considered to contain pyrite and pyrrhotite
and this is most likely the cause for the high temper-
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