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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. The geothermal system started when meteoric derived fluids at temperatures between 350 and 400°C ascended. The fluids were low in CO2. However, they may have been more saline and enriched in CO2 and H2S gasses where magmatic fluids mixed with the meteoric waters. The thermal fluids then boiled at 250-300°C below about +600 m or 1000 m depth in the northern part of the system but were slightly cooler at 200-250°C, when they reached near sea level or about 2000 m depth in the southern part. Boiling and further water-rock interactions caused the fluids to become fully saturated with respect to some calc-silicates and quartz. Consequently, grossular-andradite, actinolite-tremolite, high pistacite epidote, Fe-bearing prehnite and wairakite in association with quartz formed at this stage. Boiling became more intense, especially in the northern area, as indicated by the occurrence of platy calcite. Vapour that separated from the boiling fluids ascended through fractures and started to fill a reservoir. The residual liquid moved more slowly or even descended and accumulated below this vapour reservoir. Above the reservoir, vapour condensed and formed acid condensate waters. The occurrences of calcite and anhydrite after calc-silicates record these downward percolating condensate waters. Deposition of calcite and anhydrite decreased the porosity and permeability of rocks above the reservoir. Pressure differences between the reservoir and the surrounding rocks caused hydraulic fracturing to occur, hence increasing the permeability of the reservoir rocks. This lowered the pressure triggering more intense boiling in the reservoir. These events occurred several times. Consequently, more vapour filled the reservoir, until a steam zone formed. In the northern part of the field, the steam zone is 600 m thick but in the southern part it is only 100 m thick. Shallow intrusion of rhyolitic rocks likely accelerated the evolution of the geothermal system since it probably promoted rapid boiling. It is likely that the once liquid-dominated Darajat geothermal system evolved to the vapour-dominated system about 50,000 years ago. |
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