Geothermal exploration is often been divided into several
phases due to the 
many uncertainties involved.  This is done in order to
minimize cost and 
maximize information and makes it possible to decide at the
end of each phase 
whether to continue into the next phase or not.  Geochemical
techniques are 
extensively used in all phases.
In the exploratory phase the main purpose of geochemical
surveys are to 
predict subsurface temperatures and to obtain knowledge of
the origin and 
flow directions of the geothermal fluid.  The basic
philosophy behind this type of 
prospecting is that geothermal fluids on the surface reflect
physico-chemical 
and thermal conditions at depth.  Subsurface temperatures
are estimated with 
different geothermometers which are probably the most
important geochemical 
tool used in geothermal exploration and development.  They
are often classified 
into three groups: Water geothermometers; steam or gas
geothermometers; 
isotope geothermometers.  Isotopic techniques are used to
identify the origin 
of the geothermal fluids and flow directions.
In the exploration drilling phase geochemical methods play a
role in providing 
information on e.g. the ratio of water to steam in the
reservoir; the quality of 
water and steam in relation to the intended use; scaling and
corrosion 
tendencies.  This information is relevant to the preliminary
power plant design.
Geothermal reservoirs are dynamic in nature and will respond
to production 
and change with time from there initial conditions.  The
changes are basically 
associated with pressure drop which will increase boiling
and recharge of new 
water to the reservoir.  Therefore geochemical monitoring
focus largely on 
changes in well fluid composition in order to e.g. identify
recharge, changes in 
steam and water quality, scaling tendencies and to revise
conceptual models of 
the reservoir.  Geochemical methods are applied to problems
which may be 
linked to injection of geothermal fluids into the reservoir
to minimize the 
environmental impact of geothermal utilization and to slow
down the pressure 
drop.  This will increase the lifetime of individual wells
and the reservoir as a 
whole.

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