Various methods to describe quantitatively the cores and the cuttings from geothermal wells are proposed as summarized below. (1) Location of a well should be shown by the latitude, the longitude and altitude together with its place name. (2) Color of samples can be expressed by the Rock-Color Chart published by the Geological Society of America. (3) Hardness of cores is classified into the following five steps ; very soft below 1 kg/cm
2, soft 1-10kg/cm
2, medium hard 10-100kg/cm
2, hard 100-500kg/cm
2, and very hard above 500kg/cm
2 in uniaxial compression strength. (4) Crack Index proposed here is shown by the following equation.where d is the crack density coefficient (For every 10 cm interval crack, d is 10.), ΣLcr is the total length of crack zones, and Lob is the length of core observed. (5) The results of modal analysis of the representative rocks from a formation should be shown by the statistical method, that is, the mean value and the standard deviation of each constituent mineral. (6) The “alteration ratio” is classified into the five stages ; wholly altered, mostly altered, half-altered, partially altered, and unaltered. (7) The “alteration degree” is classified into five essential types (I-V) by the crystal chemical characteristics of alteration minerals, Type I Silica mineral type, Type II Hexagonal sulfate type, Type III Aluminum silicate type, Type N Sheet aluminosilicate type, Type V Framework silicate type, in addition to two subordinate types (VI and VII), Type VI Partially altered type, and Type VII Unaltered type. (8) The results of X-ray diffraction of altered rocks are shown by the Quartz Index (QI) as follows;where Im is the strongest X-ray intensity of a mineral in the examined sample and Iq is that of pure quartz. (9) The mass color of zircon in volcanic rocks changes with its formation age, and the representative colors by the Mansell Color System are shown in Table 3. (10) The homogenization temperature of liquid inclusions in alteration minerals is very useful for the temperature estimation of geothermal fluid and also for the understanding of the thermal history of a geothermal system.
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