the rocks and geothermal fluids (hot water, steam and gas) interacts,
chemical reaction take place between the rock forming minerals
and the geothermal fluids. The rock forming minerals are subject
to changes (alteration minerals). These changes will depend
upon the temperature, pressure and chemical composition of geothermal
fluids. Upon the knowledge of these dependences, utilizing the
description of alteration minerals determined using an X-ray
diffractometer (Fig.2-1,2-2), the temperature and pH of geothermal
fluids that caused the identified alteration can be estimated.
Moreover, based upon the classification of alteration minerals
done according to fixed standards, knowing the actual distribution
altered materials as identified through the analysis of core
and cuttings samples, the location and extent of the cap rock
or that of the reservoir (Fig.2-3) can also be estimated.
West JEC we use the so-called Quartz Index (QI, Table 2-1, Hayashi,
1979) as a quantitative indication of each alteration mineral
and to decide on the alteration degree and depth of the geothermal
veins developing and to assess the permeability zones.
West JEC we have gain skill and have accumulated much experience
in the application of these techniques to geothermal fields
all over the world.