Page 106 - Geothermal Energy Systems Exploration, Development, and Utilization

82 2 Exploration Methods
Table 2.1 Geochemical tools and interpretation.

Geochemical tool Selected parameters Information on the EGS
reservoir

Secondary minerals Mineral assemblages Qualitative estimation of
(silica, silicates, carbonates, reservoir temperatures, res-
sulfides, zeolites, clays) ervoir heterogeneity
Fluid and gas chemical Major and minor species Rock type, chemical equilib-
composition rium, fluids mixing processes
Fluid and gas isotopic Hydrogen, carbon, sulfur, Elevation of the infiltration
composition strontium isotopes zone, underground residence
time, origin of the miner-
als, fluids mixing processes,
crustal/mantle origin
Gas composition N 2, CO 2 ,H 2 S, H 2 ,He, Ar, Fracture monitoring during
Rn drilling
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Water-stable isotopes D/ O Oxygen shift enrichment
Solute geothermometers SiO 2 ,Li,Na, K,Ca, Mg, Reservoir temperature, fluid
F, SO 4 uprising processes, mixing
processes
Isotope geothermometers 18 O(H 2 O), 18 O(SO 4 ) –
Gas geothermometers CO 2 ,H 2 S, H 2 ,CH 4 ,Ar –
Mud and fluid logging Major species, CO 2 , He Fracture monitoring during
drilling

oil, and geothermal exploration boreholes) can reveal important features on deep
fluid flow and water–rock interactions.
The natural surface manifestations of an EGS reservoir are by definition less
active and visible than for hydrothermal high temperature systems. In rift or in
mountainous conditions, convective fluids from great depths can rise toward the
surface and discharge in hot springs or in shallow porous aquifers.
Geochemical methods, applied during the various stages of exploration and
evaluation as well as exploitation, should be used simultaneously to geological and
hydrogeological appraisals and are particularly important because of information
they supply at costs that are relatively low compared to geophysical methods and
drilling.
Geochemical methods are applied to waters, gases, and secondary minerals
(Table 2.1). The parameters to be measured and analyzed concern the physic-
ochemical parameters, the dissolved chemical species (major, minor, and trace
elements), as well as the isotopes (stable and radioactive).
Interpretation of the geochemical data gives some input on the following
characteristics of a deep EGS reservoir: reservoir temperature (or reservoir depth if
the geothermal gradient is already known), type of reservoir rock, mixing processes,