Copyright © 2010 by The Geochemical Society of Japan. owing to interaction with the surrounding rocks. In several areas, fossil sea water was found to have δ18O values of +5 to +8‰ due to the oxygen isotopic shift caused by the interaction, but to retain almost the same Cl concentration as fresh sea water (e.g., Sakai and Matsubaya, 1974; Mizukami et al., 1977; Shibata et al., 2005). At depths below ca. 2 km in sedimentary regions associated with oil and/or natural gas, formation waters tend to be geopressured fluid with pressures between hydrostatic and lithostatic values (e.g., Myers, 1968; Jones, 1970; Oki et al., 1999; Xu et al., 2006). The δD and δ18O data show that most geopressured fluid can be explained as a mixture of fossil sea water and local meteoric water, with an oxygen isotope shift due to rock-water interaction. Recently, fluids that have oilfield-like characteristic, with unusually high δD (–20‰) and low Cl concentrations (ca. 6,000 mg/L) have been observed along faults in the Horonobe area (Ishii et al., 2006). These fluids might have originated as a mixture between sea water and Stable and noble gas isotopic study of thermal and groundwaters in northwestern Hokkaido, Japan and the occurrence of geopressured fluids