The Dieng Geothermal Field, Indonesia has a 2-phase reservoir with a current installed capacity of 60 MW. The data used is fluid geochemical data from manifestations taken in the period 1977 - 1997 and well fluid data (brine, gas, and stable isotopes) during production tests in 1980 – 1999. The Dieng Geothermal System is divided into 2 reservoir areas, i.e., the Sileri and Sikidang areas. The Sileri area has a neutral fluid characteristic, even though it has magmatic fluid influx potential. The neutralization process occurs due to the intensive water-rock interaction process in the reservoir. The reservoir has a temperature of around 283-338oC with a Cl content of 9,600 – 19,000 mg/kg. The source of the fluid is heated meteoric water, mixed with fluids resulting from magmatic degassing, boiling, and experiencing water-rock interaction with sedimentary or meta-sedimentary rocks. Besides being characterized by a high Cl content, interactions with sedimentary rock formations are also supported by a high boron content. The hydrothermal fluid in the Sileri reservoir flows vertically towards the upflow zone in two areas, i.e., Pagerkandang Crater (Well-10 and Well-1) and Merdada Crater (Well-4 and Well-9). There is also a steam cap zone in the Sileri area, which is characterized by excess enthalpy, high NCG, and a temperature profile that exceeds the boiling point depth. The Sikidang reservoir has a more acidic fluid character due to the more dominant influence of magmatic fluid. The Sikidang reservoir has a two-phase fluid, with most wells penetrating the steam cap zone and several wells penetrating the liquid zone, it is estimated that there is a boiling zone between liquid and steam reservoirs. The fluid temperature in the reservoir ranges from 260 to 324oC with high NCG content (2 – 21 wt%). The fluid flow in the Sikidang Area comes from upflow around Pangonan Crater, Well-V, Sikidang, and Sibanteng manifestations. This upflow zone also correlates with the acid fluid zone, therefore there is the potential for acid fluid to be found deep in the reservoir.

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