Exploration Risk Assessment of the Mt. Talang Geothermal System, Sumatra, Indonesia

As lower risk prospects in Indonesia are developed, the next phase of high temperature geothermal exploration will include an increasing focus on exploration for neutral reservoirs adjacent to vapor core volcanoes, like Mt. Talang in Western Sumatra. The existence of a neutral geothermal reservoir located below the flank Mt. Talang volcano, that very likely has a vapor/acid core, can be discriminated by the neutral and the vapor core systems using water and gas geochemistry from springs and fumaroles, magnetotelluric (MT) resistivity imaging, geology and structural data sets. Mt. Talang is a predominantly andesite volcano within a trans-tensional basin of the Great Sumatra Fault Zone. The volcanic cone is constructed on an ancestral volcano exposed to the south and collapsed to the north. A vapor core has been interpreted from the gas chemistry of summit fumaroles, Mt. Talang’s thirteen small magmatic eruptions and surface acid alteration. The preferred conceptual model for Mt. Talang is analogous to a relatively small Mt. Apo, with a neutral, mature, liquid system isolated from a magmatic vapor core by a low permeability shell of clay and anhydrite alteration. Although this strategy for exploring vapor core volcanoes avoids many pitfalls by building conceptual models and testing them against analogs in order to discriminate neutral and vapor core zones, the risk of targeting wells adjacent to vapor core systems can be further mitigated by using lower cost slim hole drilling.