Page 32 - Geothermal Energy Systems Exploration, Development, and Utilization

8 1 Reservoir Definition
1.1.5
Two-dimensional Effects of Crustal Heterogeneities on Temperature Profiles

1.1.5.1 Steady-state Heat Refraction
The two-dimensional heterogeneity of the upper crust is outlined by geological
maps, where, for example, each rock composition is assigned one color. However,
it must be emphasized that thermal properties are not necessarily correlated with
rock composition, except for extreme cases (Clauser and Huenges, 1995). On one
hand, one may record similar temperature profiles through distinct areas where
small-scale lithological differences are observed, because the averaging effect of
heterogeneities smoothes out small-scale variations. On the other hand, when
large bodies with significantly distinct thermal properties are present, temperature
profiles may differ by several tens of degrees at shallow depths. In other words,
the horizontal geometry of anomalous bodies shall play a significant role in the
establishment of temperature differences at depth.
As far as surface heat flow is concerned, small-scale lithological contrast may
create large differences. For example, subvertical mineralized bodies can be rich
in highly conducting minerals (e.g., volcanic massive sulfides deposits, Mweni-
fumbo, 1993), which may result in large surface heat flow variations, whereas

Large aspect-ratio Small aspect-ratio
insulating body conducting body

(e.g., sedimentary basin, (e.g., Quartzites, volcanic
ash-flow caldeira) massive sulphide deposit)

Surface heat flow

– Heat flow variations focused – Heat flow variations above the
at boundaries anomaly
– Strong temperature variations – No temperature variations
Isotherms

Figure 1.4 Heat refraction in two dimensions, leading to
opposite effects according to the conductivity contrast or the
anomaly geometry.