Understanding the Burial and Migration Characteristics of Deep Geothermal Water Using Hydrogen, Oxygen, and Inorganic Carbon Isotopes

Geothermal water samples taken from deep aquifers within the city of Kaifeng at depths between 800 and 1650 m were analyzed for conventional water chemical compositions and stable isotopes. These results were then combined with the deuterium excess parameter (d value), and the contribution ratios of different carbon sources were calculated along with distributional characteristics and data on the migration and transformation of geothermal water. These results included the conventional water chemical group, hydrogen, and oxygen isotopes (D-O-18), dissolved inorganic carbon (DIC) and associated isotopes (C-13(DIC)). The results of this study show that geothermal water in the city of Kaifeng is weakly alkaline, water chemistry mostly comprises a HCO3-Na type, and the range of variation of D is between -76.12 parts per thousand and -70.48 parts per thousand, (average: -74.25 parts per thousand), while the range of variation of O-18 is between -11.08 parts per thousand and -9.41 parts per thousand (average: -10.15 parts per thousand). Data show that values of d vary between 1.3 parts per thousand and 13.3 parts per thousand (average: 6.91 parts per thousand), while DIC content is between 91.523 and 156.969 mg/L (average: 127.158 mg/L). The recorded range of C-13(DIC) was between -10.160 parts per thousand and -6.386 parts per thousand (average: -9.019 parts per thousand). The results presented in this study show that as depth increases, so do D and O-18, while d values decrease and DIC content and C-13(DIC) gradually increase. Thus, D, O-18, d values, DIC, and C-13(DIC) can all be used as proxies for the burial characteristics of geothermal water. Because data show that the changes in d values and DIC content are larger along the direction of geothermal water flow, so these proxies can be used to indicate migration. This study also shows demonstrates that the main source of DIC in geothermal water is CO(2)thathas a biological origin in soils, as well as the dissolution of carbonate minerals in surrounding rocks. Thus, as depth increases, the contribution of soil biogenic carbon sources to DIC decreases while the influence of carbonate dissolution on DIC increases.