Pore-water mobility: Distribution of δ 37 Cl, 36 Cl/Cl, 129 I/ 127 I and dissolved 4 He concentration in the core drilled in the Mobara gas field, Japan

We measured Cl-36/Cl and I-129/I-127 isotope ratios, dissolved He-4 concentrations, and delta Cl-37 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 x 10(-15) for the Cl-36/Cl ratio in Pleistocene strata of the gas field. Furthermore, the concentration of He-4 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 I-129/I-127 ratio. Variations of delta Cl-37 in the pore water suggested that mass transport in rocks of the Kazusa Group was dominantly controlled by diffusion. (C) 2012 Elsevier B.V. All rights reserved.