The Sipoholon Geothermal Field and adjacent geothermal systems along the North-Central Sumatra Fault Belt, Indonesia: Reviews on geochemistry, tectonics, and natural heat loss

The Sipoholon system in the Tarutung Basin is one of five geothermal systems associated with a c. 100 km stretch of the Sumatra Fault System (SFS) in North-Central Sumatra, Indonesia, that exhibits parallel sets of dextral strike-slip faults associated with seismic activity. Characteristics of discharged thermal fluids differ significantly between the systems. The gas-rich Sipoholon field transfers only c. 19 MW heat by Ca-SO4/HCO3 springs with SiO2 fluid equilibria pointing to temperatures of 50–80 °C at intermediate depths. Most of its thermal springs (T  In contrast, all four thermal systems in the Sarulla Block concession to the southeast of the Sipoholon field are convective high temperature systems that transfer significant heat to the surface. Fumaroles, steaming ground, thermal springs, hot pools and thermal lakes discharge at least 50 MW heat at three of the four prospects where the temperatures of springs and fumaroles often reach boiling point. Many hot springs of the Sarulla Block systems discharge dominantly Na-HCO3/Cl type waters with partially equilibrated cation compositions, a result of fluid-rock equilibration and mixing processes. SiO2 –fluid equilibria indicate 130 to c. 200 °C with cation geothermometers pointing to a high reservoir temperature (200–260 °C) for all four systems. This was confirmed by the first deep exploratory wells drilled c. 20 years ago in three prospects that encountered T > 250 °C at bottom hole (>1.5 km depth). The structural setting of the five geothermal fields is complex. At Tarutung it is associated with the narrow Tarutung Basin, filled with volcanic debris and ash. The basin exhibits a partly exposed dextral strike-slip fault along its NE margin and can be interpreted in terms of a trans-tensional pull-apart basin. Two of the adjacent Sarulla Block systems are associated with large volcanic debris basins (Namora-I-Langit and Silangkitang) that exhibit ‘half- graben’ structures in profiles normal to two large active faults of the Sumatra Fault System (SFS). The other two systems (Donotasik and Sibualbuali) are hosted by an elongated graben structure parallel and within the SFS and by a stratovolcano respectively.