Comparison of stable isotopes, ratios of ~(36)Cl/Cl and ~(129)I/ ~(127)I in brine and deep groundwater from the Pacific coastal region and the eastern margin of the Japan Sea

摘要

Fifty-three samples, including brines associated with oil and natural gas reservoirs and groundwater samples from deep boreholes, were collected from the Pacific and Japan Sea coastal regions in Japan. The ~(129)I/ ~(127)I and ~(36)Cl/Cl ratios, and stable isotopes (δD and δ ~(18)O) are compared to investigate differences related to the geotectonic settings of the two regions. The δD and δ ~(18)O data indicate that brine and groundwater from the Pacific coastal region reflect mixing of meteoric water with connate seawater in the pores of sedimentary rocks. On the other hand, brine and groundwater from the Japan Sea coastal region have been hydrothermally altered. In particular, brines associated with petroleum accumulations at Niigata and Akita showed the same isotopic characteristics as fluids found in the Kuroko deposits of the Green Tuff region in northeastern Japan. There is little difference in the ~(36)Cl/Cl ratios in brine and groundwater from the Pacific and Japan Sea coasts. Most brine and some deep groundwater, except those from the Pleistocene Kazusa Group, have already reached the average secular equilibrium ratio of 9.9±2.7×10 ~(-15) for their mudstone and sandstone reservoirs. There was no correlation between the ~(36)Cl/Cl ratios and differences in geotectonic setting between the Pacific and the Japan Sea coast. The molar I/Br ratio suggests that the I in all of water samples was of biogenic origin. The average ~(129)I/ ~(127)I ratio was 290±130×10 ~(-15) to 294±105×10 ~(-15) in both regions, showing no relationship to the different geotectonic settings. The uncontaminated brine and groundwater samples are likely to have retained the original ~(129)I/ ~(127)I ratios of marine I released from the old organic matter stored in sedimentary rock.