Chemical and isotopic compositions of brines from dissolved-in-water type natural gas fields in Chiba, Japan

Abstract

Chemical and isotopic compositions of brines collected from the Minami-Kanto natural gas field in Chiba, central Japan, were measured to investigate their origin and relation to microbial methane formed from the coexisting brines. The isotopic compositions of water in the brines are -3.8 to -1.5‰ and -15 to -1‰ in δ 18 O H2O and δD H2O values, respectively, which are correlated with their chloride concentrations, suggesting that the brines are derived from seawater and that some of them have been mixed with meteoric water. The brines have high concentrations of iodine up to 0.14 g/l which are related to those of dissolved bromine and ammonia. These dissolved chemicals are suggested to have been liberated from marine sediments into pore water through the diagenetic decompositions of organic matter. The hydrogen isotopic compositions of methane (δD CH4 ) in the natural gases are -195 to -146‰. The average isotopic difference between δD CH4 and δD H2O for the samples is about 180‰, suggesting that the methane was formed by microbial activity via a CO 2 -reduction pathway. However, because the data obtained in this study were widely scattered on the δD CH4 -δD H2O plot, it is not possible to confirm that the obtained data follow the linear correlation predicted by Schoell (1980) for methane formation by CO 2 -reduction.

Superiority of K-edge XANES over LIII-edge XANES in the Speciation of Iodine in Natural Soils

“…15,16 The soil profile was flooded with brine water from a tube well containing iodine (iodine concentration: 5.27 mg/kg). The concentrations of iodine in the soil samples at various depths were determined by a method reported in Yamada et al as follows.…”

Radium geochemistry in Na–Cl type groundwater in Niigata Prefecture, Japan

“…Most of the water samples, however, had higher Br/Cl values than that of seawater. These high Br/Cl ratios have been observed in saline groundwater both from oil-and gas-field at Niigata Prefecture (Kamimura et al, 1988) and from Minami-Kanto gas-field Chiba Prefecture, Japan (Imahashi et al, 1996;Maekawa et al, 2006). It is well known that marine organisms enriched in Br and I (Saenko et al, 1978).…”

“…It is well known that marine organisms enriched in Br and I (Saenko et al, 1978). Maekawa et al (2006) considered that high Br/Cl ratio were attributed to diagenetic decomposition of marine organic matter from the linier relationship between additional concentration of Br and I compared with their contents of seawater in saline waters. Thus, it is considered that high Br/Cl ratios in Na-Cl type groundwater samples obtained in this study were due to additional input of Br together with I from diagenetic decomposition of marine organic matter.…”

Stable and noble gas isotopic study of thermal and groundwaters in northwestern Hokkaido, Japan and the occurrence of geopressured fluids

“…Recently, geopressured fluids with unusually high δD (-20‰) and low Cl concentration (ca. 6,000 mg/L) have been observed along faults in several localities in Japan, e.g., Niigata (Oki et al, 1999;Xu et al, 2006), Horonobe (Ishii et al, 2006), Mobara gas field (Maekawa et al, 2006), and Miyazaki (Ohsawa and Ohi, personal comm.). Fluids in mud volcanoes in various part of the world also have such high δD (-20‰) and low Cl concentrations (e.g., Dia et al, 1999).…”