Geochemical Modeling of Acid Fluid from Volcanic Geothermal Areas, Case Study of Indonesian Geothermal Prospects
Asnawir NASUTION and Mulia P. NASUTION
[Faculty of Earth Science and Technology, ITB, Indonesia]
The Indonesian geothermal fields are mostly located at high elevation of volcanic areas, which are situated at more than 1500 m high. They are indicated by release small amount of SO2, HCl, high H2S and other minor H2, O2+Ar, CH4, and NH3 gases. High subsurface temperatures (220oC-300oC) of exploration and exploitation drillings, combined with results of geochemical, geological and geophysical studies have been used to construct a geochemical model of the geothermal system of the area. The release of amount of gas (SO2) on the low part of volcanic crater rim are function of deep magma process, e.g. vapor melt separation during coming up generating magma. The existence SO2 indicates a high temperature magmatic component, rising from underlying magma. On the other hand, the flank of this volcanic fumaroles have no SO2 gas, high H2S and CO2. The existence H2S assumed a low temperature gas presenting at the reservoir. The H2, O2+Ar, CH4, and NH3 gases probably indicate a secondary hydrothermal component slowly rising from a two phase, saline brine vapor, and covering the magmatic system. The H2 to CH4 and NH3 shows a decreasing in equilibration for individual species, interpreting slow rate varieties (CH4 and NH3) are formed at depth within the hydrothermal zones. The rapidly rising magmatic component are controlled by the SO2-H2S bupper. The H2S and CH4 seems to be stable in low temperature rock conditions, particularly at the geothermal reservoir, outside of volcanic vents. Modeling of acidic conditions within the hydrothermal zones from geothermal prospects may be indicated by presenting of active volcanic gases, eq. the Sorik Marapi, Lahendong and Namora Ilangit volcanic vents. Volcanic vents indicate a high temperature fluid interaction toward surface, where SO2 and H2S reacts with rocks. However, deep drillings to the geothermal reservoir, which is located northern part of outside the Sorik Marapi volcano represent neutral chloride fluids.
|        Topic: Exploration||Paper Number: 11157|