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Exploring for Geothermal Systems 115

On the basis of these results, a larger, deep test well (Figure 6.17) was drilled in the Hg anomaly
and near the previous well that was drilled to 1367 m. This larger well terminated at 2119 m. A
temperature of 191°C was measured during evaluation of the well, providing clear evidence that a
substantial geothermal resource was present and accessible.
In order to induce flow, the hole was stimulated with liquid nitrogen to fracture the rock through
thermal stress and pressure. This effort was successful, resulting in a flow rate of up to 40 kg/s
of brine.
This effort to find a resource was successful and provides several important lessons for explora-
tion efforts. These include the following:

• Previously collected data were used to delineate a target area that would be explored in
detail. Note that the geological setting was propitious.
• Geothermometers were used whenever data were available for analysis. This can provide a
good basis for developing working hypotheses.
• The absence of recorded seismic activity could be an indication that the region was tec-
tonically inactive and, hence, not likely to possess a suitable resource. However, in this
instance, geological experience and expertise on the part of the investigators allowed
them to consider alternative interpretations. The bulk of the other evidence was strongly
suggestive of the presence of a resource, thus making it wise to develop multiple working
hypotheses.
• The use of soil analyses for trace metals provided an important and useful guide to where
a potential resource might be located. Drilling of the deep test well, in fact, was centered
in one such Hg anomaly.
• Other geophysical data (magnetic and gravity) provided some indication of a geological
anomaly, but the data were not sufficient to delineate a target. This is often the case with
such data. It can be useful, but needs additional techniques to provide a rigorous subsur-
face model.
• The absence of flowing water in a well is not necessarily an indication of an absence of
water in the system. In this case, formation or enhancement of fracture permeability suc-
cessfully allowed fluid flow to occur at a reasonable rate.

synposIs
Exploration for geothermal resources requires employing numerous techniques in order to identify
a resource. The initial phase must always rely on available geological information, from which
refinement of an exploration strategy can proceed. Important approaches include using geochemical
evidence to establish possible reservoir temperatures through the use of geothermometers, isotopic
analyses to identify anomalies that may indicate interaction with a thermal source. Geophysical tools
can be very useful in identifying target areas, depth to a resource and other parameters. However,
they can provide ambiguous results that require judicious analysis. It is wise to develop multiple
working hypotheses and test them repeatedly as new data are acquired. Thermal gradient wells are
particularly important for resolving uncertainties, but they generally follow after all other analytical
approaches have been used, since they are expensive. Remote sensing capabilities are rapidly evolv-
ing and may provide the ability, in the near future, to quickly identify hidden resources, potentially
avoiding months of labor-intensive field work.
Problems

6.1 List three surface features that are important indicators of near surface geothermal
resources, and discuss what they indicate.
6.2 Why might a warm spring occur at the surface but there would be no geothermal
resource in the subsurface directly below it?