The Evolution of the Darajat geothermal system, West Java - Indonesia

Abstract

The Darajat geothermal system is a dry-steam resource, associated with the Quaternary Guntur volcano. Its geology is dominated by andesitic rocks; however the youngest unit is an obsidian flow from Mt. Kiamis. It is a product from a dacitic-rhyolitic parasitic cone in the Kendang caldera having an age younger than 50,000 years. The geothermal system itself is characterized by a ~240°C highly permeable reservoir containing mainly steam with ~2 wt.% non-condensable gases. The occurrence of liquid-formed hydrothermal minerals, such as wairakite, epidote and garnet associated with chlorite, illite and smectite, indicates that the Darajat reservoir was once liquid dominated. Here, the change in reservoir behavior, i.e. from liquid to vapour, was recognized by comparing the characteristics of the hydrothermal minerals, including their compositions and results from fluid-inclusion geothermometry, with present conditions.