Four major lithostratigraphic units of the Miocene Shizukuishi Group, the Kakkonda Group, the Tamagawa Welded Tuffs and younger volcanic rocks are recognized in the Takinoue (Kakkonda) area. Occurrence of geothermal fluids is controlled by highly permeable zones provided by fault and fracture system associated with folding in the Shizukuishi Group. A syncline was probable formed by buckling associated with flexural slip, whereas an anticline was made by the combined mechanism of bending and flexural flow. The genesis of folds and faults can be ascribed to as they were formed by the movement of tilting blocks of the pre-Tertiary basement rocks and the east-west trending lateral compressive stresses. Therefore, favorable local traps for steam accumulation are confined to major faults, extension fractures in the trough of a syncline, and slip plane (bedding plane) of argillaceous rocks and others with exception of an anticline.
Out of geothermal fields, mercury in a gaseous form uprises in proportion to the underground geothermal activity. Such volatile mercury is captured at a number of survey points scattered systematically on the ground surface and following quantitative mercury measurements provide its horizontal distribution map, which could be an effective method for geothermal prospecting. In order to capture volatile mercury selectively, a gold needle, 1 mm in diameter 10 cm in length, is hung in a hole 1 m deep in the ground and is left for several successive days, and then is collected for determination. Mercury adsorbed on the gold needle is reliesed by heating and determined using an atomic absorption spectrophotometer whose detection limit is satisfactorily low, 0.1 ng. The rate of mercury adsorption more than 1 ng/day indicates the existence of promising geothermal resources beneath. In the process of estimating the degree of underground geothermal acitvity, it becomes necessary to know not only the absolute amount of adsorbed mercury but its distribution patterns such as localization, extension and direction. Several survey attempts by this method have successfully been accomplished in some geothermal and non-geothermal fields in Japan. It is noteworthy that this prospecting method is applicable even in less active geothermal areas without any visual geothermal manifestations. According to some laboratory experiments, momentary slight mercury adsorption precedes regular adsorption process. Regular adsorption of volatile mercury to a gold needle occures in accordance with the first order reaction in which the adsorbed amount increases in proportion to the surrounding concentration and/or the lapse of standing time.
The homogenization temperature of fluid inclusions in hydrothermal minerals from the Hatchobaru and the Kirishima geothermal fields in Kyushu has been measured. As a result it has revealed that the homo genization temperatures of the inclusions formed at reservoirs in the very active Hatchobaru geothermal field concentrate on a temperature near the boiling point curve of water, and agree well with the present reservoir temperature. Moreover, the geothermal fluid in the field seems to consist of the saturated or nearly saturated water, so that the pressure correction for the homogenization temperatue is unnecessary. On the other hand, in the less active Kirishima geothermal field the inclusions have a wide range of the homogenization temperature. the minimum values of which are quite close to the present underground temperature. This may indicate that the geothermal field has cooled down to some extent, forming the fluid inclusions at various stages of the cooling process. Some consideration has revealed that even the secondary inclusion has a homogenization temperature that is the same as that of the fluid at the time of necking down or refilling, if it traps only the saturated water. According, both primary and secondary inclusions are useful for knowing the thermal structure and the thermal history of geothermal fields.
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