Regional Assesment Using Graphical Techniques of Indonesian Non-Volcanic Geothermal System In Central Sulawesi, Indonesia: Based on Fluid Geochemistry

摘要

Indonesia has many geothermal prospects which spreading out in Sumatra, Java, Bali, NTT and Sulawesi Islands. Most of Indonesian geothermal system is located in Sumatra and Java associated with high temperature system hosted by volcanic rock. Sulawesi island is different than those. In part of Sulawesi has non-volcanic geothermal systems hosted by granitic and metamorphic rock. Central Sulawesi has eleven geothermal prospects reported as non-volcanic geothermal system. The geothermal prospects are still in an exploration stage to get to know their geothermal system properties with geological, geophysical, and geochemical surveys. Till now, no one of those prospects are developed for electricity or direct use utilization. Some of 3G survey had been conducted by Center of Geological Resources, Geological Agency, Ministry of Energy and Mineral Resources of Indonesia. Geochemical surveys are used to get characteristics of fluid geochemistry, either water or gas. Fluid geochemistry, only water, will be evaluated in this paper to know characteristic of manifestations, type of fluid, and geothermometers. Water geochemistry are used also to know equilibrium of fluid and understand fluid process in either deep or shallow levels. Reservoir temperature by geothermometers not just one objective in analysis of water geochemistry. Whereas, the important thing is deduce deep temperature with considering water-rock interaction or fluid-rock equilibration. In this paper, the application of Na-K-Mg ternary, Na-K/Mg-Ca and K-Mg/Quartz diagram are applied to compare temperature equilibration each other, to consider the equilibrium of fluid, and to analyze some processes in either deep or shallow level of seven non-volcanic geothermal systems in Central Sulawesi, Indonesia. The seven non-volcanic geothermal systems, namely Tambu, Ranang, Lompio, Marana, Bora, Pulu, and Kadidia. The methods have been applied in some volcanic geothermal systems in New Zealand such as Waiotapu and Rotorua and Alto Peak geothermal field in Philippines. The K-Mg/Quartz diagram is consist of two low temperature geothermometers. These geothermometers can eliminate invalidity of each geothermometer, which could be caused by dilution process, equilibration with amorphous silica, or some residual effect of an acid zone. The Na-K/Mg-Ca diagram also has two geothermometers, Na-K geothermometer with equilibration of the system Mg-Ca. Both of them usually use to know the influence of shallow and low temperature processes. Na-K-Mg ternary diagram consist of fast-responding K-Mg with slowly re-equilibrating Na-K geothermometers to evaluate degree of attainment of fluid-rock equilibration. This ternary plot is powerful tool to keep water distinct between water suitable and unsuitable for the application of ionic solute geothermometers, to assess deep equilibrium temperature, and evaluate re-equilibrium and mixing effect on large number water samples. The results of study show that almost half of thermal springs attaint to full equilibrium line, although some of them plotted in partial equilibrium. These water of thermal springs are in part came from reservoir and in part undergone dilution or mixing processes. Three graphical techniques were showing the shallow and deep temperature of each prospect with some explanation about equilibrium and processes of fluids rise from deep to surface. Overall, equilibrium temperatures are taken and proposed from three graphical techniques as above for the whole set of Tambu discharges with temperature of 140-150°C, Lompio range of temperature of 210-220°C, Ranang-Kasimbar of 130°C, Bora temperature average of 210°C, Pulu temperature of 230-240°C and Sapo-Kadidia temperature of 230°C. Only temperature of Marana is unreliable. Equilibrium temperatures as discussed for those prospects, probably just define shallow conditions before thermal discharges appear in surface. In shallow levels, temperature of equilibrium is 120-140°C for Tambu, 130°C for Ranang-