Chemical and isotopic geothermometers

Determination of reservoir temperature using various geothermometers of equilibrium processes of chemical or geochemical such as water, gas and mineral (host rock) in a system will inform the geochemical reaction species at different depth.

This process will inform the kinetic of prime mineral dissolution and second mineral deposition with exchange of temperature and pressure from different original fluid. The result of it, various species such as ¹⁸O and ³⁴S isotopes as well as Na, K, Ca and Mg cations and gases in the fluid as a result of the process, can become a geoindicator for evaluating temperature in geothermal system at various depths.

7.3.1. Isotope geothermometers of T¹⁸OSO4-H2O and T³⁴SSO4-H2S

Determination of reservoir temperature at Sibayak geothermal using 2 different geothermometer methods, i.e. T¹⁸OSO4-H2O and T³⁴S_SO4-H2S, can be calculated using (3), and (4) equations of each equilibrium of SO₄-H₂O and SO₄-H₂S. The result of the calculation is shown in Table VII.

The calculation of reservoir temperature made from both geothermometer methods is very different. The average temperature calculated based on T¹⁸OSO4-H2O ±270^oC whereas based on T³⁴SSO4-H2S is ± 450^oC.

This difference does not show inconsistency of those geothermometers caused by isotopic equilibrium of geothermometers at different depth and condition.

Isotope equilibrium of ³⁴S in SO₄²-H₂S species takes place deeper and very slowly. Therefore, when the fluid is moving up to the surface accompanied by exchange of temperature at the next reservoir, it does not change the formerly obtained equilibrium. The equilibrium process of SO₄-H₂S generally takes place at primer neutralized area where SO₄ and H₂S are in liquid phase. This process is in the hydrothermal system being close to association of volcanic-magmatic.

To the contrary, reservoir temperature shown by T¹⁸O_SO4-H2O takes place at reservoir dominated by water (liquid) at a shallower area where isotope equilibrium exchange of ¹⁸O in SO₄ and H₂O will take place when brought into contact with water.

Figure 7 shows reaction processes of species interaction of SO₄²-H₂O at geothermal reservoir having concentration of ¹⁸O sulfate and being relatively enriched with isotopes concentration of ¹⁸O in H₂O, because in this condition ¹⁸O_H2O moves to sulfate compound. Based on the calculation of geothermometer T¹⁸O_SO4-H2O, the reservoir temperature of Sibayak is considered 260 to 270^oC. The temperature obtained is relatively close to actual reservoir temperature at the present.

7.3.2. Chemical geothermometers

The main cations Ca, K and Mg as the result of dissolution and interaction between mineral, water and gas are geo-indicators for determining reservoir temperature and geochemistry equilibrium. Using the

equation presented by Fournier and Giggenbach, T_Na-K, T_Na-K-Ca and T_Mg geothermometer of production wells SBY-3, 5 and 6. Table XIII shows calculation result of chemical geothermometer.

Figure 7. Evolution of δ34S in Sibayak geothermal field

The above table shows temperature based on TNa-K reaching 350 ^oC (290-350^oC), whereas TNa-K-Ca shows temperature of 250 –270^oC and T_Mg-K shows the lowest temperature i.e. 190 – 217^oC.

K in T_Na-K is the element distributing temperature increasing where K element formed at the beginning of mineral dissolution (primer neutralization) at equilibrium of albite-feldspar. T_Na-K-Ca especially Ca element is the element indicator decreasing temperature where Ca formed from Alumina-Silica-feldspar equilibrium. Temperature of T_Na-K-Ca is close to measurement of custer in geothermal reservoir.

Figure 8 shows geothermometers of 4 main elements in equilibrium conditions that are different in graphic of correlation between 10 Mg (10 Mg + Ca) Vs 10 K (10K + Na) presented by Giggenbach. The graphic shows that production wells SBY-3, 5 and 6 are close to equilibrium line at the temperature 290-300 ^oC. It looks more representative showing the equilibrium of K, Na, Mg and Ca elements with secondary mineral.

7.3.3. Gas Geothermometer

The analysis data of condensable and non-condensable gas (CO₂, H₂S, He, Ar, H₂ and CH₄) show an important geo-indicator to determine reservoir temperature. In high temperature of geothermal system (>200^oC), reservoir temperature depends on the equilibrium of gases or gases-mineral controlled by concentration of the gases. There are some applicable geothermometers such as T_CO2-H2S and T_H2-CO2 of NEHRING and D’AMORE (1984), T_H2S and T_H2 of Arnorsson and Gunlougasson (1985) and T_h2-Ar of Giggenbach (1989) are used to calculate the temperature of Sibayak geothermal field.

Figure 8.Chemical equilibrium based on Na, K, Ca and Mg for samples from wells, hot springs and fumaroles in Sibayak geothermal field

Table VII shows the calculation of temperature based on those geothermometers (see Appendix mathematics equation) that can be calculated from Table VIII in mmol/kg steam.

The above table shows that temperature of geothermometers T_CO2-H2S, T_H2-CO2 and T_H2 is relatively similar to those 3 production wells i.e. 275 – 300^OC, except geothermometer TH2S having a temperature that is higher for those wells, i.e. 330^oC, whereas T_H2-Ar shows 250 – 270^oC. T_H2-Ar geothermometer has a more realistic temperature of reservoir and it is close to actual temperature.

This is caused by the Ar element originated from meteoric that can be used to indicate lower temperature. Ar gas originated from meteoric interactions with H2 gas of deep fluid in the reservoir.

Hence, gas equilibrium of H2-Ar in fluid is similar to T¹⁸OSO4-H2O occurring in shallower area compared to equilibrium of other gases such as CO2-H2S and H2-CO2.

Figure 9 shows equilibrium of gas multi component CO2, H2 and Ar in fluid phase as an equilibrium graphic of components in various temperatures presented by GIGGENBACH. The graphic shows that reservoir temperature indicated by concentration LCA (log CCO2/XAr) and LHA(log XH2/XAr) for those three wells and the steam vent is ≅ 250^oC.

Those multi component geothermometers show the reservoir temperature of Sibayak geothermal field.

The table also shows that temperature of steam vent indicated by TCO2-H2S, TH2-CO2 and TH2S is 350^oC.

This higher temperature possibly shows equilibrium process of gas at high temperature probably affected by volcanic system.

Figure 9. Ar-H₂-CO₂ gas geothermometers for well samples.