Isotopic Study of Carbonate Minerals from the Sumikawa Geothermal Area and its Application to Water Movement

Abstract

Ninety-one carbonate samples in the Sumikawa geothermal wells (37 core samples from well S-2 and cutting samples from other 9 wells) were analyzed for their carbon and oxygen isotopic compositions to discuss the origin of fluids in equilibrium with them. Chemical compositions as well as CO₂ content of rocks from well SC-1 were also analyzed in details. The δ¹³C values of carbonates studied are -7.8 to -0.6‰ similar to those reported in the world wide geothermal systems. The calculated δ¹³C values of HCO₃^- in fluids or CO₂ gas in equilibrium with the carbonates are -12 to -3 ‰ and -9 to -2 ‰, respectively, and show no good relationship between them and geological features such as rock type, distribution of clay minerals and so on. The δ¹⁸O (SMOW) values of carbonates in the northern wells (low temperature zone at Sumikawa) vary from -4.2 to +12.8‰, whereas those in the southern wells (high temperature zone) are lower than +5‰. The calculated δ¹⁸O (SMOW) values of fluids in equilibrium with the carbonates above 200 mASL are -14 to -10 ‰ similar to those of the local meteoric waters. The CO₂ contents in rocks from well SC-1 abruptly increase up to 10 wt% with increase in depth to 700 mASL, where all of Ca in rocks is fixed as carbonates (calcite), and gradually decrease up to 200 mASL. These features indicate that meteoric water penetrated from surface down to cap rock at 200 mASL without oxygen isotopic shift. In contrast, δ¹⁸O (SMOW) values of fluids in equilibrium with carbonates below 200 mASL show an oxygen isotopic shift up to 0‰. From these results, most of carbonates above 200 mASL are thought to be deposited from meteoric water with high water / rock ratios, whereas carbonates below 200 mASL were formed from meteoric water which suffered oxygen isotopic exchange by water-rock interaction with low water / rock ratios.