Prehnite as an indicator of productive fractures in the shallow reservoir, Kakkonda geothermal system, northeast Japan

Abstract

Large amounts of chips of hydrothermal minerals were ejected with steam and hot water at the initial opening of a shallow production well (Well-4; total depth of 1448 m) after well completion at the Kakkonda geothermal field, northeast Japan. The ejectas yield important information to specify an indicative hydrothermal mineral of the productive fractures as an essential factor on development of the geothermal energy resource.The ejectas consist mainly of euhedral quartz, epidote (pistacite mole fraction of 0.22 to 0.25), prehnite and wairakite. The quartz and calc-silicate minerals precipitated as drusy minerals in a productive fracture where is connected to a feed point of the well. On the microscopic observation of the ejectas, cavities in the fracture are seen to have been partly self-sealed with precipitation of these minerals.Thermodynamic consideration shows that the fossil reservoir fluids are in equilibrium with epidote or prehnite-wairakite as well as quartz and calcite, whereas the present reservoir fluids are in equilibrium with prehnite as well as quartz and calcite. The decreasing of temperature and f_CO2 in the reservoir fluids has shifted the calc-silicate mineral stabilities from epidote or prehnite-wairakite to prehnite. The process may result from the degassing of CO₂-rich reservoir fluid. Prehnite appears to be a good indicator of the productive fractures in the shallow reservoir of the Kakkonda geothermal field.