The structure of volcanic geothermal systems

Abstract

Abstract Geothermal manifestations are common on andesite volcanoes of the world's volcanic arcs and represent a potentially large energy source. Characteristically these volcanoes have fumaroles and acid-sulfate springs near their summits and neutral-chloride springs low on the flank of the volcano, but until recently, little was known about the nature of the geothermal system from which they issue. The Suretimeat Geothermal system in Vanuatu (New Hebrides) has a particularly well-developed acid-sulfate geothermal system and can be used to illustrate the structure of these systems. The acid-sulfate waters contain substantial amounts of dissolved solids and the pH is usually less than 2.0. Alteration of reservoir rocks is intense. Acid, low-salinity springs found around fumaroles are interpreted as condensate from steam and H2S-bearing volcanic gas which is both a heat source and, by oxidation of H2S, a major source of acidity. Oxygen isotope studies show that the acid-sulfate waters are of meteoric origin. The high flux of gas and steam through the fumaroles indicate a vapor-filled reservoir beneath the acid-sulfate reservoir. Drilling at Kawah Kamojang (Indonesia) has revealed a substantial low-density fluid reservoir, and evidence for a similar feature has been found in other volcanic systems. The existence of the strongly altered cap rock within the layered structure of the volcano reduces permeability and thereby stops quenching of the low-density reservoir. Springs of neutral-chloride waters around the base of the volcano represent leakage from a large hot-water reservoir. Volcanic gas streaming through the reservoir probably carries some of this water into the low-density phase above. These hot, neutral-chloride water systems must contain enormous thermal energy, but the development of large low-density and acid-sulfate systems is variable. Suretimeat emits about 150 MW thermal from the acid-sulfate system. On other volcanoes such as Yatsugatake (Japan) the main heat flux is from the lower neutral-chloride system and the acid-sulfate system, including fumaroles, emits only a few megawatts.