When 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.

In 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.

In West JEC we have gain skill and have accumulated much experience in the application of these techniques to geothermal fields all over the world.

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