Page 107 - Geothermal Energy Systems Exploration, Development, and Utilization

2.5 Geochemistry 83
multiple fluid origin, elevation of the infiltration basin, underground transit time,
behavior of fluid exploitation (Tester et al., 2006; Taylor, 2007).
A sound understanding of the fluid chemistry in the exploration phase facili-
tates the building of a conceptual model and approaches the future exploitation
conditions. For the past several decades, the water chemistry and gas chemistry
of geothermal fluids have proved very effective in evaluating subsurface tem-
peratures, determining water origin, identifying and eliminating mixing effects,
and predicting scaling and corrosion (Fournier, 1977; Rybach, and Muffler, 1981;
Arnorsson, Gunnlaugsson, and Svavarsson, 1983; Arnorsson, 2000; Giggenbach,
1988).

2.5.2
Fluids and Minerals as Indicators of Deep Circulation and Reservoirs

Geochemical and isotopic methods have been found to be very effective for
the geothermal exploration and potential assessment of geothermal field and
surrounding systems. On a regional scale, hydrothermal system fluids associated
with heat sources carry geochemical and isotopic signatures that provide insights
into the deeper crustal processes operating in the magmatotectonic areas from
which they originate (Bowen, 1989; Giggenbach, 1997a). On a smaller scale,
geochemical and isotopic signatures also provide valuable information about
physical reservoir processes in geothermal reservoirs, and location(s) of source
inflows into geothermal fields (Arnorsson, 2000; Goff and Janik, 2000). Such
information is of course invaluable for resource exploration and development
programs.
Several geochemical methods are based on the relationships between hydrother-
mal alteration minerals occurring in high temperature geothermal systems and
fluids circulating within these systems (Table 2.1). These methods include not
only the popular chemical geothermometers and other techniques focusing on
water–rock interaction (or mineral–solution equilibrium) but also the evalua-
tion of irreversible mass transfer taking place during water–rock interaction
(Giggenbach, 1984), which helps the reconstruction of the thermal history of
the system (Reed, 1997). Knowledge of hydrothermal alteration mineralogy de-
veloping in high temperature geothermal systems can outline the extent of the
reservoir and the temperature of alteration, and is of utmost importance for fluid
geochemistry.
Hydrothermal alteration minerals found in outcrops and the drill holes, which
can be sampled from the cuttings and the less frequent cores, have been the subject
of many investigations (Browne, 1970, 1982; Heald, Foley, and Hayba, 1987;
Stober and Bucher, 2000). Observations include mineralogy, and fluid inclusions
in alteration minerals can be made in the laboratory.
The increasingly sensitive instruments and techniques available to investigate
fluid inclusions have proven useful in describing the evolution of geothermal
systems. Increasing proficiency in analysis of the gases and solids in fluid inclusions
has contributed valuable details about the genesis of fluids. The case histories