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

Abstract

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 ¹²⁹I/¹²⁷I and ³⁶Cl/Cl ratios, and stable isotopes (δD and δ¹⁸O) are compared to investigate differences related to the geotectonic settings of the two regions. The δD and δ¹⁸O 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 ³⁶Cl/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 ³⁶Cl/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 ¹²⁹I/¹²⁷I 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 ¹²⁹I/¹²⁷I ratios of marine I released from the old organic matter stored in sedimentary rock.

Introduction

Because of the long half-life of ¹²⁹I (1.57 × 10⁷ a), there have been many studies involving its use as a tracer for global mass transport on a geological time scale. The global I cycle (Fabryka-Martin et al., 1985, Fabryka-Martin et al., 1989, Muramatsu and Wedepohl, 1998), the origin of marine I (Fehn et al., 1992, Fehn et al., 2000, Fehn et al., 2007a, Muramatsu et al., 2007), groundwater dating (Fabryka-Martin et al., 1985, Fehn et al., 1992), and methane hydrate dating (Fehn et al., 2000, Fehn et al., 2003, Tomaru et al., 2007, Tomaru et al., 2009a, Tomaru et al., 2009b) have all been studied using the ¹²⁹I/¹²⁷I ratio. The origin of I, and the iodine–chloride cycle in a subduction-related volcanic arc was studied by Snyder and Fehn (2002) and Snyder et al. (2002). The origin of I and ³⁶Cl, the residence time of groundwater associated with hydrocarbons, and the dilution mechanism were discussed by Snyder et al. (2003) and Snyder and Fabryka-Martin (2007). The initial ratio of marine I for dating was discussed by Moran et al., 1995, Moran et al., 1998, while the worldwide ¹²⁹I/¹²⁷I ratio and the global distribution of ¹²⁹I were determined by Snyder and Fehn (2004). The origin and age of I in brines at Mobara in Japan was studied using the I ratio by Muramatsu et al. (2001), while Maekawa et al. (2006), using stable isotope measurements, suggested that the Mobara brine was altered seawater mixed with meteoric water.

Muramatsu et al., 2001, Fehn et al., 2007a, Fehn et al., 2007b, Tomaru et al., 2007 have argued that I and CH₄ were expelled via cold seeps from the Pacific and Philippine Sea plates during their subduction and that the released I migrated with CH₄ and was trapped in the deep strata of the Mobara and Miyazaki gas fields in areas facing the Pacific Ocean.

Brines with high concentrations of I have been found associated with oil and gas production at the eastern margin of the Japan Sea in Niigata, Akita and Hokkaido, areas characterized by thickly piled turbidite layers containing much accumulated organic matter and kerogen, and associated with extensive deep submarine volcanism related to expansion of the Japan Sea (Taira, 1996). On the other hand, the Pacific coast was affected by subduction during 20–70 Ma, when large volumes of oceanic sediments were accreted as the Philippine Sea and Pacific plates subducted beneath the Eurasian and North American plates (Niitsuma, 2007). The Pacific coastal area is characterized by large amounts of old organic matter stored in the accretionary prisms on the continental side of the subduction zones and CH₄ dissolved in cold seepages. Consequently, the geotectonic setting of the Pacific coast in Japan is different from that of the Japan Sea coastal region.

In this study, stable isotopes (δD and δ¹⁸O), and ³⁶Cl/Cl, and ¹²⁹I/¹²⁷I ratios in brines and deep groundwater from the Pacific and Japan Sea coastal regions were investigated to ascertain geochemical differences between the two regions, and to determine if there were differences related to their geotectonic settings.