石油,天然ガス鉱床の同位体地球化学

• 中井 信之

  名古屋大学理学部地球科学教室

• 吉田 裕

  名古屋大学理学部地球科学教室

• 安藤 直行

  工業技術院地質調査所

書誌事項

タイトル別名

• Isotopic Studies on Oil and Natural Gas Fields in Japan

抄録

Methane, crude oil and the associated water from oil and natural gas fields in Japan have been analyzed isotopically. In all of natural gas fields studied, gases occur with water in sand and gravel beds of Quaternary to Tertiary, consisting mainly of CH₄, CO₂, N₂ and Ar. δ¹³C values of methane are distributed in a rather narrow range from -64 to -75‰ relative to PDB. Those of CO₂ in gas phase are in ranges from +2.0 to -21.2‰ and carbonate materials dissolved in water from +9.3 to -7.1‰. For all of natural gas well studied, CH₄ is most depleted in ¹³C, whereas CO₂+HCO₃^-+CO₃⁼ in the associated water is most enriched in ¹³C. Carbon isotopic fractionation between CH₄ and CO₂ varies linearly with temperature of associated water. The isotopic temperatures calculated by using Bottinga's fractionation factor agree rather well with temperatures of associated water (13 to 59℃) measured at the ground surface. It can be concluded from these results that CH₄ and coexisting CO₂ produced by unidirectional reactions have reached an isotopic exchange equilibrium through enzyme reactions. This suggests bacterial processes of gas productions. Hydrogen isotopic compositions of associated waters of natural gases vary widely from + 1.9 to -82.2‰ relative to SMOW, depending on sedimentary environments of gas reservoirs- marine and fresh water. δD values of CH₄ range from -148 to -239‰, being smaller than those of associated waters. A linear relationship for δD values was found between CH₄ and the coexisting water throughout all natural gas fields. The linear relationship can be given as follows:<br>δD_CH₄ = δD_H₂O-(160±10) in permil<br> This means that the difference in the hydrogen isotopic composition between CH₄ and water is constantly 160‰ for a large extent of δD values. Since the difference in δD values between CH₄ and water in the isotopic exchange equilibrium should be theoretically less than 70‰, such a large fractionation is supposed to have resulted from a kinetic isotope effect in the process of methane formation in water. From the constancy of the difference in the isotopic composition and the kinetic isotope effect, it is possible to conclude that CH₄ and the associated water have a genetically close correlation and the associated water should be called "connate water". Oil samples studied have been collected from Tertiary formations in Japan. δ¹³C values are rather constant, ranging from -22.2 to -24.7%o for seven oil fields. The carbon isotopic composition does not systematically vary with geologic age.

収録刊行物

• 地球化学

  地球化学 7.8 (1), 87-98, 1974

  一般社団法人日本地球化学会

詳細情報

• CRID: 1390001205742729856
• NII論文ID: 110008679698
• DOI: 10.14934/chikyukagaku.7.8.87
• ISSN: 21885923
• ISSN: 03864073
• 本文言語コード: ja
• データソース種別:
  • JaLC
  • CiNii Articles
• 抄録ライセンスフラグ: 使用不可