Pore-water mobility: Distribution of δ37Cl, 36Cl/Cl, 129I/127I and dissolved 4He concentration in the core drilled in the Mobara gas field, Japan

Abstract We measured 36 Cl/Cl and 129 I/ 127 I isotope ratios, dissolved 4 He concentrations, and δ 37 Cl to estimate the residence time and flow characteristics of pore water in rocks of the Kazusa Group in the Mobara gas field, Japan. We deduced a residence time of 0.28–0.85 Ma for the pore water, based on a secular equilibrium value of 7.05 ± 1.58 × 10 −15 for the 36 Cl/Cl ratio in Pleistocene strata of the gas field. Furthermore, the concentration of 4 He corrected by the solubility equilibrium model in the pore water indicated that residence times varied from 0.09 to 0.62 Ma from the depth of 642 to the depth of 1742 m in the core drilled in the gas field. The pore-water dating results have insignificant differences between the ages of the Kazusa Group formations, compared with the iodine ages from 129 I/ 127 I ratio. Variations of δ 37 Cl in the pore water suggested that mass transport in rocks of the Kazusa Group was dominantly controlled by diffusion.