Geochemistry of mineral waters and associated gases of the Sakhalin Island (Far East of Russia)

Isotopic and chemical data on the mineral water, mud volcanoes fluid and associated gases from the biggest Russian island Sakhalin, together with previous stable isotope data (d18O, dD, 13C), allow elucidation of their origin and general evolution. The water fluid circulation is mainly related to marine environment inducing three distinct types: Na-HCO3-Cl alkali carbonate groundwaters, Na-Cl-HCO3 highly evolved saline and Na-Cl mature groundwaters, indicating different evolution. Chemical evolution of groundwater on Sakhalin Island demonstrated cation exchange and salinization as dominant evolutionary pathways. Isotopic composition of groundwaters varies from meteoric to metamorphic waters. These metamorphic waters consist of water hydration from the clay and seawater are traced in fluids of Yuzhno-Sakhalin mud volcano despite modification by mixing with meteoric waters and water-rock interaction processes. Fault systems that define the areas of highly mineralized water circulation appear to play a major role in the CO2 migration to the surface and CH4 generation. The δ13C(CO2) values have pointed that gas phase in high-pCO2 waters mostly consists of mantle-derived CO2. The carbon isotope signature of methane δ13C(CH4) and δD(CH4) indicates its distinct origin which is specified by tectonics. Methane manifestation in the south of the Sakhalin Island is mainly related to thermogenic reservoirs as they are more often dislocate by tectonics, and crossed by active and permeable faults. The sources of biogenous methane in the north of Sakhalin Island is related to younger and shallower reservoirs, and less affected by tectonic processes. The determinations of 222Rn have allowed observing that maximal radon flux is associated with high pCO2 waters.