Distribution of Iodine and Its Geochronological Implications for Gas Field Brine in Japan

Global distribution of iodine is very heterogeneous in location and chemical species; 65% of total iodine is produced in Chile as solid nitrate (e.g. caliche) and 30% in Japan as solute mainly in gas field brine. In the latter case, because iodine has a close association with marine organic materials such as algae, iodine had been liberated into the surrounding aqueous phase during the generation of oil and gas and traveled together with oil/gas to the current deposit. The distribution of iodine therefore reflects the environments of accumulation and secondary migration of iodine during its diagenetic processes. Here we present the concentrations of total iodine (127-I) and a long-lived radioisotope (129-I) in gas field brines in Japan to understand the behavior of iodine in response to the development of present geological setting including oil field in terms of its geochronological signals using 129-I. The concentrations of iodine dissolved in gas field brines and hot/cold springs in nearby areas are relatively high compared with the seawater composition, although the chloride concentrations are lower than the seawater. This is due to the delivery of iodine from organic-rich sediments into the current deposits. The 129-I has decayed since the deposition of iodine-rich organic materials from the seawater following the standard decay curve, however, the129-I concentrations are relatively low compared with the age of host sediments of the fluids, which indicates iodine has derived secondary from old sediments responsible for the generation of gas. The 129-I and halogen composition also indicate these fluids mix with pre-anthropogenic seawater during the migration. These results characterize the history of long-term migration of iodine in organic-rich marine system.