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

Abstract

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 (Δ47) 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 Δ47-based temperatures (Δ47-T; 161−8+8 to 238−14+16 °C) and calculated δ18Ofluid values (+2.0 to +5.0‰) 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 Δ47-T (142−6+6 to 185−4+5 °C) and low calculated δ18Ofluid (−9.1 to −5.3‰) correspond to downhole temperatures and measured δ18Ofluid 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 δ18Ofluid (−8.9 to −1.0‰) and Δ47-T that are above air temperatures (32−9+9 to 95−9+10 °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.