Fobex 13: “Tugu Geothermal Prospect: Exploration Assessment – Geochemical Approach”

Tugu Geothermal Prospect (TGP) provides promising exploration result. Field observation and geochemical analysis have been undergone to assess and estimate geothermal potential of the area. Based on geochemical assessment, TGP has potential reservoir temperature of 250 – 300oC. Tools applied in this assessment are geothermal fluid origin analysis using Cl-HCO3-SO4 ternary diagram, geothermometry analysis using silica, Na-K equation, and Na-K-Mg trilinear plot, and hydrologic-thermal-chemical modelling. Other geological constraints, such as geologic structures, lithologic characteristics, and mass wasting potential, have also to be acquired in future exploration efforts.


Geothermal Manifestations

            TGP has five geothermal manifestations with different styles. The manifestations generally are springs (PS, TS, KS, and WPS) and pools (PP).

PS is a hot spring approaching boiling point (T = 97oC). Fluid of this spring has cloudy appearance and white vapor. Reaction of rock and acidic fluid (pH = 2.5) is interpreted as the cause of cloudy appearance. On the other side, PP has ebullient manifestation of near boiling temperature (T = 85oC). Geothermal fluid of this manifestation also shows cloudy appearance as a result of rock-acidic fluid interaction (pH < 3). Outcrop of fine-grained, white lithology becomes the wall of the pool. Boiling manifestation also observed in TGP at TS. With temperature of 100oC, extensive vapor is produced in this manifestation. TS has different fluid characteristics that it has clear to bluish water with pH of 8.

Moving to another site, KS exists as collapse crater in TGP with effects of Sulphur on the lithology. Fluid with cloudy appearance in Kotare Spring has temperature of 86oC and pH of 5. The last manifestation of TGP is WPS. WPS has clear fluid with temperature of 76oC and pH of 7.6. Deposits surrounding WPS are interpreted as silica sinter and there is possibility of sulfur trace. Geochemistry of WPS fluid will answer the possibility.


Geothermal Fluid Origin Analysis

            Geothermal fluid origin in TGP will be performed using Cl-HCO3-SO4 of Ellis and Mahon (1977). Fluid origin is manifested in geochemical characteristics. The implication of geochemical analysis is determination of fluid suitability for geothermometry analysis. Suitable fluid for geothermometry must represent reservoir condition and has trivial signature of fluid-rock interaction. Graphical plots of the analysis are presented in following illustration.


Diagram 1. Cl-HCO3-SO4 Plots for Geothermal Fluid Origin


Diagram 1 represents that based on the analysis, TS and WPS fall to chloride fluid category, PP in acid sulfate fluid category, and PS and KS as steam-heated water. Chloride fluid is almost always related to high temperature reservoir. Therefore, chloride water manifestation on the surface represents permeable zones within the field (Nicholson, 1993). Acid sulfate fluid has different genesis. This fluid is derived from condensation of geothermal gases into near-surface, oxygenated groundwater. Steam-heated water comes to the surface as steam condensates.


Geothermometry Analysis

            Geothermometry analysis is performed to WPS and TS samples. The most reliable geothermal manifestation is hot, boiling spring with high chloride concentration and a good flow rate (> 1 L/s). Interaction with host rocks changes the chemistry of the fluid to be acidic. On the other hand, bicarbonate fluid has little use in geothermometry because it only representing shallow temperature.

Geothermometry analysis employs silica equation, Na-K equation, and Na-K-Mg trilinear plot. Silica equation involves consideration of surface manifestation temperature. Different equations are employed in no steam loss and maximum steam loss (boiling). Na-K equation uses of Giggenbach (1988) to get in line with Na-K-Mg trilinear plot. Calculations involving the equations are given in the following paragraphs.



TS Geothermometry

Silica Equation – No Steam Loss

ToC =  – 273

ToC =  – 273

ToC = 253.814oC


Na-K Equation

ToC =  – 273

ToC =  – 273

ToC = 281oC


Na-K-Mg Calculation

Na/1000 = 1.275

K/100 = 2.23

= 0.26

ToC ~ 300oC (fully equilibrated)

WPS Geothermometry

Silica Equation – Maximum Steam Loss

ToC =  – 273

ToC =  – 273

ToC = 195.829oC


Na-K Equation

ToC =  – 273

ToC =  – 273

ToC = 243.240oC


Na-K-Mg Calculation

Na/1000 = 0.403

K/100 = 0.46

= 0.2

ToC ~ 250oC (partially equilibrated)



Diagram 2. Fluid Equilibration of TS and WPS Samples


Geothermometry analysis brings result of possible 250 – 300oC reservoir temperature. Based on Axelsson and Gunnlaugsson (2000), TGP is classified as high enthalpy system. Fluid equilibration is the degree to which fluid-rock interaction proceeds. Sodium is employed in the analysis because it tends to substitute with potassium to form K-feldspar (Nicholson, 1993).

Difference in geothermometry analysis of TS and WPS is subjected to question. The difference is induced by manifestation proximity to upflow zone. The closer manifestation to the upflow zone, the higher temperature yielded. TGP has prospective resources regarding its temperature. Hydrologic structure will give deeper consideration to geothermometry analysis. Primary implication of geothermometry analysis and hydrologic structure is placing exploration well.


TGP Geothermal Model

TGP Geothermal Model is the summary of hydrologic, thermal, and chemical structures of the study area. TGP Geothermal Model integrates field observations, fluid origin analysis, and geothermometry analysis combined with boiling point depth curve. Construction of this model is based on low-relief terrain. Reference to Nichols (1993) is conducted to build reliable model.

Initial step in building TGP Geothermal Model is consideration of boiling point depth curve. Acquired temperature of 250oC at WPS and of 300oC at TS are plotted to the curve. As a result, WPS temperature yields depth of 460 meters while TS yields 1095 meters. Plotting of 200oC and 150oC follows the other two.


Diagram 3. Boiling Point Depth Curve of TGP


TGP Geothermal Model considers TS as upflow zone, while WPS as outflow zone. This determination is based on the lower value of Cl in WPS and also thermal structure geometry. Thermal structure of TGP enables the movement of water to form WPS. PP, PS, and KS act as acid-sulfate muds in between chloride springs. Acid-sulfate waters are manifestation of fluid-rock interaction in geothermal system.



Based on field observations and geochemical analysis, the study concludes that:

  • Geothermal manifestations in TGP include PS, PP, TS, KS, and WPS.
  • Chloride waters have surface manifestation as clear to bluish water.
  • Cloudy waters may indicate acid-sulfate waters based on the study.
  • Origin of geothermal fluids are chloride waters (TS and WPS), steam-heated waters (PS and KS), and acid-sulfate waters (PP).
  • TS and WPS are brought to geothermometry analysis. TS is equilibrated waters, while WPS is partly equilibrated waters.
  • Geothermometry analysis using silica, Na-K equation, and Na-K-Mg trilinear plot indicates reservoir temperature of 250o – 300o TGP is high enthalpy geothermal potential.
  • TS is considered as upflow zone and WPS as outflow zone in TGP Geothermal Model. PS, PP, and KS are acid-sulfate springs and mud pools near upflow zone and outflow zone.
  • TGP is potential and promising for geothermal exploration and exploitation.











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