Geothermal systems on the island of Bali, Indonesia

Abstract

This paper presents an overview of the geothermal systems on the island of Bali, Indonesia. Physicochemical data of hot springs and shallow geothermal wells were collected from four geothermal locations: Penebel, Batur, Banjar and Banyuwedang. The concentrations for the three main anions varied significantly indicating a different geothermal history. The values for Cl− ranged from 0.1 to 1000 mg/L, for HCO3− from 20 to 2200 mg/L and for SO42 − from 0.1 to 500 mg/L. Although the island of Bali is underlain by carbonate rocks, a carbonate host rock for the geothermal reservoirs could not be confirmed, because the (Ca2 + + Mg2 +)/HCO3− molar ratios were approximately 0.4, well below 1.0 and the K/Mg ratios were approaching those of a calc-alkaline rock reservoir. The HCO3− of the thermal waters correlated with Ca2 +, Mg2 +, Sr2 + and K+ indicating water–rock interaction in the presence of carbonic acid. Phase separation was inferred for the Bedugul and Banjar geothermal systems, because of relatively high B/Cl ratios. Boron isotopes were determined for selected samples with values ranging from δ11B of 1.3 to 22.5‰ (NBS 951). The heavy δ11B of + 22.5‰ together with a low B/Cl ratio indicated seawater input in the Banyuwedang geothermal system. The hydrogen and oxygen isotopic composition of the thermal water plotted along the global meteoric water line (GMWL) and close to the mean annual value for precipitation in Jakarta indicating a meteoric origin of the geothermal water. Comparison of the Si, Na/K, Na/K/Ca and Na/Li geothermometers with actual reservoir temperature measurements and physicochemical considerations led to the conclusion that the Na/Li thermometer provided most reliable results for the determination of geothermal reservoir temperatures on Bali. Using this thermometer, the following reservoir temperatures were calculated: (1) Penebel (Bedugul) from 235 to 254 °C, (2) Batur 240 °C and (3) Banjar 255 °C. Due to seawater input this thermometer could not be applied to the Banyuwedang geothermal system. There application of a SiO2 thermometer indicated a reservoir temperature below 100 °C.