Occurrence of geopressured fluids with high δD and low Cl in northwestern Hokkaido, Japan

Stable (D, O) and noble gas isotopic compositions of 23 thermal and groundwaters along the west coast and in the inland region of Northwestern Hokkaido, Japan were analyzed to investigate their origin. The Cl concentrations vary from 45 to 19,300 mg/L and δD values are in the range from −90 to −8‰. The δD-δ¹⁸O plots show a linear relationship which can be explained by a simple mixing of local meteoric water and altered sea water, with an oxygen isotopic shift of 5‰. However, the δD-Cl plot shows that there is also a contribution of an additional water component from different origin. This third component has a δD value of −20‰ and Cl concentration of 6,000 mg/L. This component is regarded as geopressured fluid which is widely distributed in northwestern Hokkaido.