Page 109 - Geothermal Energy Systems Exploration, Development, and Utilization

2.5 Geochemistry 85
Several hydrothermal minerals (e.g., epidote and chlorites) form solid solutions
that can adapt to some extent to changes in rock composition by changing their
composition, thus increasing their stability range. Further complications are due to
the development of mixed-layer minerals, involving clays and chlorites. In spite of
these complications, the geothermal systems explored through deep drilling have
shown a thermal zoning of the hydrothermal alteration mineralogy, which has led
to the identification of four hydrothermal alteration zones. The shallowest zone
is the argillic zone, which is characterized by the presence of montmorillonite,
eventually accompanied by illite, chlorites, and low temperature zeolites (e.g.,
◦
heulandite, stilbite). This zone develops up to temperatures of 150–160 C, above
which montmorillonite becomes unstable. The strong increase in chlorite and illite
contents and the appearance of mixed-layer clays characterize the transition to the
phyllitic zone, also termed illite–chlorite zone, which develops up to temperatures
◦
close to 200–250 C. The zeolite mineral typical of this zone is laumontite. The
following zone, called propylitic zone or zone of Ca–Al–silicates, is characterized by
the presence of secondary minerals, which are close to equilibrium with neutral,
sodium–chloride aqueous solution. This zone develops up to temperatures of
◦
300 C. Epidote, the most typical mineral, can start to form in small amounts
within the phyllitic zone, but it becomes abundant in the propylitic zone. Epidote
is usually accompanied by abundant adularia, albite, and sulfide minerals (e.g.,
pyrite, pyrrhotite, and sphalerite). The zeolite mineral typical of this zone is
wairakite. Chlorite and illite are also stable within this zone, but are less abundant
than in the phyllitic zone. The deepest zone is the thermometamorphic zone,
which is characterized by remarkable textural reorganizations of the original
lithotypes and by the appearance of high temperature mineral phases, such as
amphiboles (e.g., actinolite and tremolite), pyroxenes (e.g., diopside), biotite, and
garnets.
The rocks affected by argillic and phyllitic alterations are characterized by
extremely low permeability. In fact, the minerals typical of these two zones behave
plastically under mechanic stress, acting as a reservoir cap rock. The hydrothermal
minerals of the propylitic and thermometamorphic zones exhibit instead brittle
behavior, permitting the development of fractures that act as high permeability
pathways for geothermal fluids. Therefore, these two hydrothermal alteration zones
indicate the existence of geothermal reservoirs.
The petrographic logs, which are generally carried out during deep geother-
mal drillings, are based on this thermal zoning of the hydrothermal alteration
mineralogy.
If the geology is poorly known, it will be possible to predict the reservoir host
rock from the fluid chemistry.

2.5.3
Mud and Fluid Logging while Drilling

In the rotary drilling of geothermal wells, a drilling mud is used both to transport
the cuttings up to the surface and to impose hydrostatic pressure on the walls of