Page 118 - Geothermal Energy Systems Exploration, Development, and Utilization

94 2 Exploration Methods
2.5.6
Isotopic Characteristics of Fluids

An integrated isotopic approach can provide needed information to geothermal
prospecting regarding

• the sources of geothermal fluids and heat;
• the spatial distribution of fluid types;
• subsurface fluid flow directions and paths;
• the physiochemical processes affecting fluid composition, for example, water–
rock reaction paths and rates;
• the temporal evolution of geothermal systems.

The isotopic compositions of elements in geothermal fluids provide a quantitative
measure of material balance and can be applied to fluid samples from production
wells, hydrothermal and nonthermal springs, fumaroles, and so on; fluid inclusions
in rocks and minerals; and host rocks and minerals themselves (Tonani, 1970).
The isotopic compositions of elements in a fluid moving through the crust
will be modified in space and time in response to varying chemical and physical
parameters and/or by mixing with other fluids (Giggenbach, 1992a; O’Neil, 1986).
During this process, elements will either be conserved, thus preserving isotopic
information related to initial conditions and sources, or modified in a fashion that
is diagnostic of chemical reactions along a flow path.
The noble gases (He, Ne, Ar, Kr, and Xe) are excellent natural tracers for heat
and fluid sources, fluid origins, and reservoir processes. They are chemically inert
and, therefore, conserved in water–rock systems. Because noble gases have very
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low solubility in fresh water, particularly at high temperatures (T > 150 C), phase
separation (e.g., boiling) will generate a residual liquid that is strongly depleted in
noble gases and the relative noble gas composition will be fractionated relative to
the original fluid composition. The concentrations of noble gases in the residual
brines will be extremely depleted relative to the original reservoir fluid (Arnorsson,
2000). For instance, with a steam fraction of only 2.5%, the residual liquid is
depleted in 36 Ar by a factor of ∼20.
Although N 2 is reactive, in general its contents are not perturbed by chemical
reactions since it is by far the predominant nitrogen species. Considering a large
number of gas analyses coming from different tectonic settings, Giggenbach
(1991a) showed that the relative concentrations of He, Ar, and N 2 delineate the
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major source components. In principle, the He/ He ratios couldbe usedto
discriminate the contributions of mantle and crustal gases.
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As fluid containing mantle helium flows through the crust, the elevated He/ He
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ratio (elevated because of the mantle He contribution) will become diluted with
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the accumulating radiogenic He produced locally in the host rocks. The resulting
gradient in helium isotopic composition along the fluid flow path will be a function
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of the fluid velocity and He-production rate. Geothermal systems hosted in
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continental crust exhibit a wide range in He/ He ratios. There is evidence that