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Resource Assessments 123

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much as ca. 70,000 m . This hypothetical example provides insight into the importance of having
data that provide constraints on the extent of the temperature distribution in the subsurface. In the
absence of well-defined constraints, it is important to understand what the limitations of a dataset
may be and to consider the implications for the accuracy of the assessments.
In many cases, there may be little or no subsurface data available from a drilling and exploration
program although evidence may exist that a geothermal reservoir is present. The presence of a flow-
ing hot spring is an example of such an occurrence. Strategies for dealing with instances in which
subsurface information is inadequate have been developed for some geological settings that allow
conservative estimates to be made of reservoir volume. One example of such a strategy comes from
the current resource assessment being developed by the United States Geological Survey (Williams
et al. 2008a). In the Great Basin of the western United States there occur hot springs that emanate
along so-called range-front faults. Range-front faults occur where a large basin kilometers in width
forms by subsidence of a block of continental crust. The block of subsiding crust is bounded by
faults that dip at steep angles. The faults are exposed at the surface of the Earth along the base
of ranges that develop on either side of the subsiding basin. It is because of this topographical
arrangement that these are called range-front faults. It is relatively common for springs to form
along these faults.
The springs are fed by water that has circulated deep into the Earth and emerged along the fault.
The faults are high permeability pathways because repeated movement along the fault crushes the
rock into a permeable fault zone. Fluids circulating deep in the basins escape upward through these
high permeability zones. Often the springs are warm or hot because their waters have circulated to
relatively deep levels where temperatures in excess of 200°C occur.
Williams et al. (2008a) note that such fault zones have high permeability regions (damage
zones) that may be between 100 and 500 m wide. Using the fluid chemistry, as described in
Chapter 6, it is possible to compute a water temperature that should approximate the temperature
of the hydrothermal reservoir. Using that temperature and the measured water temperature of the
spring, a range for the maximum circulation depth of the spring can be determined if the local
geothermal gradient is approximately known. Since the geothermal gradients has been measured
in a wide range of settings within the Basin and Range province, usually for academic research
efforts, it is possible to compute various possible depths for the reservoir, thus giving bounds
on the vertical dimension of the reservoir. The horizontal dimension is less well constrained.
In their assessment, Williams et al. (2008a) note that geological evidence and comparison with
other geothermal systems that have been developed in this type of setting suggest the horizontal
extent, running more or less parallel to the range-front fault, is between 1 and 5 km, with a most
likely value of about 2 km. The resulting dimensions allow a range of reservoir volumes to be
computed. Statistical analyses or Monte Carlo simulations then allow most likely reservoir vol-
umes to be computed.
Many instances of geothermal springs, however, do not occur in regions where there exists
sufficient geological information to allow such an “argument by analogy” approach. For those
instances, an assessment can only be based on knowledge and experience of geologists that has
been developed through years of studying such systems. Generally, such ambiguous occurrences
are treated as systems deserving of further study and are not directly incorporated into rigorous
assessments.

esTablIshInG The reserVoIr heaT conTenT

As is evident from Figure 7.2, the heat content and the reservoir volume are intimately related.
For example, the reservoir volume computed in the Tiwi example above was approximately
3
42,000 m within the region existing at temperatures above 250°C. The reservoir volume, how-
ever, would more than double if the lower temperature limit for the reservoir were selected to
be 150°C.