Estimation of the Deep Geothermal Reservoir Temperature of the Thermal Waters of the Active Continental Margin (Okhotsk Sea Coast, Far East of Asia)

Abstract

Low-enthalpy thermal waters (30–70 °C) with nitrogen as a dominant associated gas are spread within the active continental margin of the Russian Far East (east and north of the Okhotsk Sea Coast) and traditionally are of great importance for recreation and balneology facilities. The thermal waters are chemically classified into three groups: (i) Na–HCO3(SO4) type, with low TDS (0.2 g/L) and lowest temperature (<50 °C) and high pH (9.1–9.3), (ii) Na–SO4 type with TDS (~1 g/L), highest temperature (70 °C) and weak alkaline pH (8.7) and (iii) Ca–Na–Cl type with high TDS (15 g/L), moderate T (59 °C) and neutral pH (7.5). The δ18O and δD values suggest that the thermal waters originate from meteoric water, and they are not isotopically fractionated. Silica and cation geothermometers and thermodynamic equilibrium calculations using the GeoT and PHREEQC programs indicate a reservoir temperature for the Na–HCO3(SO4) type thermal waters of 103–121 °C and for Na–SO4 and Ca–Na–Cl types of 136 and 153 °C, respectively. The evaluation of the mixing degree of the thermal water with cold groundwater shows that the equilibration temperature ranges between 148 and 153 °C. Estimated circulation depths for thermal manifestations range from 2.7 to 4.3 km and may be as great as 6 km.