| Description |
Geophysical signatures of volcano-hosted geothermal systems in Indonesia are com-piled and synthesized. Parameters include electrical resistivity, seismicity, downhole logging, and pressure/temperature data; temperature and pressure states of systems are simulated through numerical models. The systems are Sungai Penuh, Hululais, Lumutbalai, Ulubelu, Kamojang, Kotamobagu, Tompaso, and Lahendong. The general resistivity structure of the systems comprises a vertical conductor under the volcano peak (< 10 ohm.m), a lateral conductor under the volcano ank (< 10 ohm.m), and an intermediate resistivity zone under the lateral conductor. Background formations are generally resistive with a resistivity > 70 ohm.m. The vertical conductor is the expression of either an active or inactive volcanic neck / magmatic chimney. The lateral conductor is the claycap of the geothermal system containing an argillic alteration zone that keeps the hot Fluids and the heat inside the reservoir. The topography of the volcano dictates the hydrology and shapes the extent of the lateral conductor. The hydrothermal fluids themselves are kept within the intermediate resistivity region, 10-60 ohm.m for a liquid-dominated system, and up to 100 ohm.m for a vapor-dominated system, preferably with the temperature of 200-300 °C. On the margins, the reservoir may have either sharp or diff use contact with the background or the claycap; vapor-dominated systems have a sharp contact. A di ffuse contact is usually associated with a gradational change in temperature, with temperature reversal at depth. The caprock of a geothermal system has an aseismic character possibly due to its ductile behavior from the clay and from its low permeability structure. The low permeability property impedes liquid movement and ashing that could induce seismicity. The propylitic reservoir is seismogenic due to less clay content and its high permeability that promotes liquid movement and ashing into steam which induces seismicity. Modeling studies suggest that the most favorable permeability con guration for a vapor-dominated reservoir similar to Kamojang con-tains a caprock with the permeability of 3 x 10-16 m2 accompanied with a rechargeregion/hostrock having the permeability of 10-18 m2 to 10-17 m2. A heat ow of 8 MW/km2 for 9 kyr is the minimum combination to produce a thick vapor-dominated zone (> 1000 meter) in a 2.8 km thick reservoir with the permeability of 10-13 m2. Model simulations are produced that reach steady state at 10.7 kyr, with 38-40 bar and 250-252 °C reservoir yielding a heat loss of 90 MW and 35 kg/s of steam escaping from the reservoir. This study provided an unusual opportunity to characterize and model volcano-hosted geothermal systems. |
