Origins of hydrocarbons in the Sagara oil field, central Japan

Authors

  • Tomohiro Toki,

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    • Present address: Department of Chemistry, Biology, and Marine Science, Faculty of Science, University of the Ryukyus, Senbaru, Nishihara-cho, Okinawa 903-0213, Japan.

  • Toshitaka Gamo,

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    • Present address: Marine Inorganic Chemistry Group, Department of Chemical Oceanography, Ocean Research Institute, The University of Tokyo, 1-15-1, Minamidai, Nakano-ku, Tokyo 164-8639, Japan.

  • Urumu Tsunogai

    1. Division of Earth and Planetary Sciences, Graduate School of Science, Hokkaido University, N10 W8, Kita-ku, Sapporo 060-0810, Japan (email: toki@sci.u-ryukyu.ac.jp)
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Abstract

Abstract  We collected free-gas and in situ fluid samples up to a depth of 200.6 m from the Sagara oil field, central Japan (34°44′N, 138°15′E), during the Sagara Drilling Program (SDP) and measured the concentrations and stable carbon isotopic compositions of CH4 and C2H6 in the samples. A combination of the CH4/C2H6 ratios with the carbon isotope ratios of methane indicates that the hydrocarbon gases are predominantly of thermogenic origin at all depths. The isotope signature of hydrocarbon gases of δ13inline image < δ13inline image suggests that these gases in the Sagara oil field are not generated by polymerization, but by the decomposition of organic materials.

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