Estimating Deep Environments of Japanese Hydrothermal Systems Based on Geochemical Data from Geothermal Power Plants

Abstract

Slow progress of geothermal power generation development in Japan (the total capacity will be by 2000) has been caused fundamentally by restrictions to the exploitation at the national parks and hot-spring resort areas, in which most promising geothermal areas are located. Deep levels (deeper than about of the shallow hydrothermal systems under exploitation are ex cted to be forthcoming targets to develop for the next Two kinds of geochemical methods were applied to characterize and to evaluate the deep environments. (1) Static method: Chemical and isotopic composition data of non-reactive components in production fluids were analyzed in terms of dominant heating mechanisms, fluid origins and reservoir rock types for the above hydrothermal systems. Based on the results, the Group 1 areas (Onuma, Sumikawa and Ogiri) were estimated to be most promising with favorable fluid chemistry and large-scale reservoir development at the depth for the conventionaltype power generation. (2) Dynamic method: Chemical history data, especially long-term tracer monitoring data, from the Onuma geothermal power plant were systematically analyzed through singlebox- model numerical simulations, as a test case. The results indicated that the exploited shallow reservoir, with a very limited amount of circulating fluid, is supplied with high-temperature waters from the depth, supporting the estimations by the static method.