Geology, Mineralogy and Fluid Geochemistry of Wells Discharging Acid Fluids in Hatchobaru Geothermal Field, Kyushu, Japan
Saki TSUTSUMI, Tri WULANINGSIH, Yoshio SOEDA, Yumi KIYOTA, Koji MATSUDA, Syogo OSHIMA, Sachihiro TAGUCHI, Yasumoto TSUKADA
[West Japan Engineering Consultants, Inc., Japan]
The Hatchobaru field is located in Kyushu, Japan. Hatchobaru geothermal power plant has been operating since 1977 by Kyushu Electric Power Co., Inc. (KEPCO). The geothermal reservoir with fluid temperature ranging from 240 to 300oC, is water-dominated. Neutral-chloride fluid in the reservoir is dominant. However, some wells discharge acidic fluid. In Hatchobaru, acidic alteration zone is developed at shallower depth, which infers the acidic fluid is stored mainly at the shallower depth. On the other hand, feed zones of wells discharging acidic fluid are located at deeper depth where acidic minerals are not observed. Considering these occurrence, there still remains uncertainties of distribution and origin of acidic fluid at subsurface. We have conducted well geological and mineralogical studies by petrographic observation and XRD analysis to characterize alteration minerals. The results are; 1) acidic alteration minerals dominate at shallower depths (1,200 to 400 m asl.) 2) regardless of pH of discharged fluid, illite, chlorite and interstratified clay minerals occur below the acidic alteration zone, 3) acidic alteration minerals such as kaolinite and alunite are not identified at the deeper feed zones of the wells which discharge acidic fluid. In the results of XRD analysis, differences of mineral assemblage and spatial distribution of alteration minerals are not found among the wells discharging acidic fluid and neutral-alkaline fluid. These results suggest some possible mechanisms of discharging acidic fluid; 1) migration of acidic fluid from the surrounding areas, 2) fluid acidity at subsurface is not so low for the formation of acidic minerals. We discuss the origin of acidic fluid from the production area based on the results of the alteration mineral paragenesis, its distribution, and geochemistry of discharged fluids. Acknowledgements: We are grateful to JOGMEC and KEPCO for their permission of presentation. This study has conducted in the project financed by JOGMEC.
|        Topic: Geology||Paper Number: 12046|