Page 110 - Geothermal Energy Systems Exploration, Development, and Utilization

86 2 Exploration Methods
the borehole. For these functions the mud must, for example, have an acceptable
viscosity and density. It is therefore important to monitor the characteristics of the
mud, e.g. by monitoring its ionic composition and to keep them within certain
limits. The techniques involve separation of the mud into liquid and solid portions,
and analysis of both.
The presence of geothermal reservoirs can be detected while drilling in geother-
mal formations by maintaining a chemical log of selected parameters in the return
drilling fluid or by tracing a well drilling mud. The geochemical monitoring of
drilling fluids appears an interesting method to characterize the rocks or waters
of drilled formations and to forecast and quantify fluid circulation depths. The
geochemistry of the formations is reflected in the composition of the drilling fluid
(Aquilina and Brach, 1995). Anomalies in the gas content of the fluids could be
measured when fractures are intersected. From the variations of gas composition
along the drilling depth, we are able to detect some possible fluid-producing zones
in different depths, which can correlate well with the logging data and drilling cores
(Vuataz et al., 1990).

2.5.4
Hydrothermal Reactions

Possible sources contributing major and trace elements to the discharges
(Figure 2.15) include the host rocks, the magma, and the fluids circulating in the
subsurface. Craig (1963) found that the deuterium content of geothermal waters
was always close to that of local meteoric waters, indicating that by far the major

Figure 2.15 Borehole discharge showing evidence of
hydrothermal reactions. (Photo F.-D. Vuataz).