During exploration drilling in Icelandic geothermal systems drill cuttings are
sampled at every two meters. The cuttings are analyzed on site during drilling
operations using a binocular microscope. These analyses reveal the lithology and the
occurrence of identifiable alteration minerals in the well. After the well has been
drilled a number of samples are selected for more detailed study, including XRD-
analysis and optical microscopy investigation with a petrographical microscope.
During drilling operations in high-temperature geothermal areas it is necessary to
keep the well temperature below a certain temperature in order to secure the
integrity of the drill string and prevent well discharge during drilling. This is
done by continuously pumping cold fluids into the well during drilling. As a result,
it is impossible to measure aquifer temperature directly at the time of drilling.
Traditionally all exploration geothermal wells in high-temperature areas in Iceland
are designed and drilled as production wells. This implies that all aquifers colder
than the desired production temperature have to be cased off. An empirical
relationship between formation temperature and the occurrence of specific alteration
minerals is used to determine a proper depth for the production casing. This method
is currently best estimation of aquifer production temperature that can be made
during drilling. Therefore aquifers that are too cold for production can be excluded
from the production part of a well based on the absence or presence of certain index
minerals. The relationship between formation temperature and alteration mineral
assemblages is based on empirical observations in Icelandic geothermal systems from
1970 up to the present (Kristmannsdottir 1979, Franzson 1998).
Alteration mineral growth can represent the temperature history of the geothermal
system. Many mineral generations can be found in cavities and fractures in the
geothermal system tell the story of its development. In the case of progressive
heating many of the low temperature minerals become unstable, dissolve and
disappear. Other alteration minerals are stable over a wide temperature range and
their temperature history can be revealed by fluid inclusion studies. Cooling on the
other hand is observed as overprinting of low temperature minerals covering minerals
characteristic of higher temperatures.