Applicability of carbonate clumped isotope thermometry in the Tuchang-Jentse geothermal field

Temperature and oxygen isotope composition of fluids, which are critical for reconstructing thermal structures and characterizing hydrothermal fluids in a geothermal system, can be obtained using carbonate clumped isotope analysis. Ideally, carbonate clumped isotope compositions (Delta 47) recording the thermal fluid temperatures and compositions for fault-associated veins and rhombic calcite crystals inside open cracks demonstrate previous tectonic activities and present reservoir conditions, respectively. We studied the clumped isotope geochemistry of veins and travertines in outcrops and cuttings from exploration wells in the Tuchang-Jentse geothermal field in Taiwan. The Delta 47-based temperatures (Delta 47-T; 161(-8)(+8) to 238(-14)(+16) degrees C) and calculated delta O-18(fluid) values (+2.0 to +5.0 parts per thousand) of vein calcites were the highest, indicating mixing of the magmatic/metamorphic fluids with the meteoric water after water-rock interaction in the deep thermal fluid cycle and migration to shallow areas by fault activity. Rhombic calcites with high Delta 47-T (142(-6)(+6) to 185(-4)(+5) degrees C) and low calculated delta O-18(fluid) (-9.1 to -5.3 parts per thousand) correspond to downhole temperatures and measured delta O-18(fluid) of thermal fluids in the exploration wells, respectively, suggesting that the latest thermal fluid was of meteoric water origin. Travertines near faults have low calculated delta O-18(fluid) (-8.9 to -1.0 parts per thousand) and Delta 47-T that are above air temperatures (32(-9)(+9) to 95(-9)(+10) degrees C), implying that the local groundwater was subsequently heated by or mixed with deep fluid. Carbonate clumped isotope thermometry application demonstrates that the carbonate clumped isotope thermometer not only precisely estimates temperature in the carbonate-dominated system using drill cutting samples during the exploration phase, but also reveals the spatial and temporary evolution of geothermal fluids used as a reference for future site selection planning.

Automated summary

The temperature and oxygen isotope composition of fluids in a geothermal system can be determined using carbonate clumped isotope analysis, which is crucial for understanding the thermal structure and hydrothermal fluid characteristics. A study conducted in Taiwan's Tuchang-Jentse geothermal field analyzed the clumped isotope geochemistry of veins and travertines to investigate past tectonic activities and present reservoir conditions. The findings suggest that the high Delta 47-T and low calculated delta O-18(fluid) values of rhombic calcite indicate mixing of magmatic/metamorphic fluids with meteoric water, while the presence of low delta O-18(fluid) and high Delta 47-T in travertines near faults indicates heating or mixing with deep fluid. The application of carbonate clumped isotope thermometry provides accurate temperature estimates during the exploration phase and valuable information for future site selection.