Fluid evolution in the Kakkonda shallow geothermal reservoir, Iwate Prefecture, Northeastern Japan: a fluid inclusion study

Abstract

The Kakkonda geothermal system, Iwate Prefecture, Northeastern Japan, consists of two reservoirs with different temperature and permeability: shallow and deep reservoirs. The shallow reservoir which is located at shallower than about 1, 500 m in depth, was developed for the Kakkonda geothermal power plant unit 1 (50 MWe) operated since 1978. Fluid inclusions in quartz, calcite, anhydrite and gypsum veinlets, and in quartz fragments of dacitic tuff from the shallow reservoir have been studied using microthermometric and crushing stages to clarify the fluid evolution in a natural-state shallow reservoir. Salinities of fluid inclusions were determined after correction of melting point of ice for CO₂ content estimated semiquantitatively from the bubble behavior on the crushing stage.
     Four stages with different thermal and chemical properties of evolutional fluids with time was recognized based on the fluid inclusion data, combined with chemical analyses of the production fluids from the shallow reservoir: (1) stage I, polyphase inclusion-forming fluid; 280 to 350°C temperature, 31 to 35 wt.% salinity, (2) stage II, primary liquid inclusion-forming fluid; 220 to 310°C temperature, 0.0 wt.% and 0.4 to 2.0 wt.% CO₂ contents, 0.0 to 1.2 wt.% (average 0.5 wt.%) salinity (NaCl equivalent), (3) stage III, secondary liquid inclusion-forming fluid; 220 to 250°C, 0.2 to 0.3 wt.%, 0.0 to 0.7wt.% (average 0.1 wt.%), (4) stage IV, geothermal fluid after the operation of the Kakkonda power plant unit 1; 215 to 261°C, 0.01 to 0.04 wt.%, 0.06 to 0.08 wt.%.
     Magmatic fluid at stage I probably caused by igneous activity related to Kakkonda granite intrusion (slightly younger than 0.7-1.0 Ma; Kanisawa et al., 1994), is assumed to represent the first step in the geothermal evolution of the Kakkonda system. The subsequent geothermal fluids at the shallow reservoir evolved generally to lower values of temperature, CO₂ content and salinity with time. The precipitation of hydrothermal minerals such as quartz, anhydrite and K-feldspar (0.2±0.1 Ma; Koshiya et al., 1993) is related to stage II. Considerably decreasing CO₂ content from stage III to IV indicate that degassing was caused by a production test at the early stages after the end of drilling.
     Distribution of temperature and polyphase inclusion in and around the field suggests that high temperature fluid ascends at the northwestern area in the field through all stages. Open fracture had gradually selfsealed with the hydrothermal minerals of stage II, but geothermal fluid of stage IV still flows through the mineral-unfilled open fractures.