The morphology of zircon crystals contained in borecores from Well NE-4, drilled in the Takigami field, Oita Prefecture, is described. As expected, zircon is common in dacitic rocks, but rare in andesitic ones. The prism indices for andesite are large while those for dacite are small, suggesting a higher temperature of andesitic magma. The elongation indices are large in rocks containing pumice. In lava flows, the values of the two indices for each sample are concentrated, whereas they are scattered in volcanoclastics. According to these morphological data, the stratigraphic boundaries of this well have been revised as follows : the Ajibaru Formation above 975m, the Takigami Formation from 975m to 2008m, and the Usa Group, below 2008m in depth. The comparison of zircon morphology between these borecores and the rocks from surrounding areas will lead to a better geological correlation.

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San Kamphaeng geothermal field is one of the most promising and extensively studied areas in Thailand. Geological studies at scale 1:10, 000 indicate that the area is composed of the oldest Mae Tha Carboniferous Formation, the Middle Permian Kiu Lom Formation, Triassic granites and andesites, and alluvial deposits. The area is a horst-and-graben type bordered by normal faults, the Huai Pong fault and the Huai Mae Koen fault, both trending in a NNW-SSE direction. Otcropping chemical alteration zones indicate that the hydrothermal solution causing the alteration is possibly an intermediate-to-acidic liquid of less than 200°Cemperature. Chemical geother-mometer calcuations using the SiO₂ and the Na-K-Ca method yield reservoir temperatures of about 160°Cnd 192°Co 207°Cespectively, as confirmed by a 30 m depth temperature survey that reached a geothermal fluid portion of 139°CSeismic studies indicate that the faults along the hot spring area and adjacent area still active and the geothermal fluids circulate and flow up to the surface along these conduits. In conclusion, the potential geothermal area in the San Kamphaeng geothermal system is confined to the Ban Pong Nok District, where actual manifestations are found on the surface and N-S and NW-SE trending faults are present.

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For a model of a lumped parameter reservoir combined with production wells, changes of production flow rates were calculated. From the results, followings can be said ; 1. the flow rate steeply decreases just after the start of production until the reservoir reaches to the saturated condition, and then, 2. it keeps nearly constant for a while and abruptly declines in non-reinjection case, though it gradually decreases in the case of reinjecting low temperature water, and 3. if the fluid influx from heat source is large enough for the reservoir to keep unsaturated condition, it keeps constant for a long time, over 1×10⁵ hours.

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1986年,1987年,1988年に,松川地熱発電所のM-7井において実施した圧力ビルドアップテストの結果について報告する。坑口で観測した圧力ビルドアップ挙動は,いずれもMDHプロット上で明瞭な直線を示し,坑井周囲の流動が通常よく用いられる放射状流で近似できることを示した。この圧カビルドアップ挙動の後半部において,坑井内または坑井近傍の地層中における蒸気の凝縮によると考えられる挙動が再現性よく観測された。この現象から,坑井周囲を流動する蒸気の圧力と温度は,調査を実施した1986年10月から1988年2月にかけてはほぼ一定であったものと推定された。また,圧力ビルドアップ前半部の挙動から,坑井のごく近傍において,この間に,蒸気流動の障害となる小規模なダメージの程度が細かく変化していた可能性のあることが推定された。

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