石油,天然ガス鉱床の同位体地球化学 [in Japanese] Isotopic Studies on Oil and Natural Gas Fields in Japan [in Japanese]
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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 CH<sub>4</sub>, CO<sub>2</sub>, N<sub>2</sub> and Ar. δ<sup>13</sup>C values of methane are distributed in a rather narrow range from -64 to -75‰ relative to PDB. Those of CO<sub>2</sub> 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, CH<sub>4</sub> is most depleted in <sup>13</sup>C, whereas CO<sub>2</sub>+HCO<sub>3</sub><sup>-</sup>+CO<sub>3</sub><sup>=</sup> in the associated water is most enriched in <sup>13</sup>C. Carbon isotopic fractionation between CH<sub>4</sub> and CO<sub>2</sub> 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 CH<sub>4</sub> and coexisting CO<sub>2</sub> 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 CH<sub>4</sub> range from -148 to -239‰, being smaller than those of associated waters. A linear relationship for δD values was found between CH<sub>4</sub> and the coexisting water throughout all natural gas fields. The linear relationship can be given as follows:<br>δD<sub>CH<sub>4</sub></sub> = δD<sub>H<sub>2</sub>O</sub>-(160±10) in permil<br> This means that the difference in the hydrogen isotopic composition between CH<sub>4</sub> and water is constantly 160‰ for a large extent of δD values. Since the difference in δD values between CH<sub>4</sub> 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 CH<sub>4</sub> 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. δ<sup>13</sup>C 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.
Chikyukagaku 7.8(1), 87-98, 1974
The Geochemical Society of Japan