Carbon isotope geochemistry on natural gases in Japan

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  • Waseda Amane
    JAPEX Research Center, Japan Petroleum Exploration Co., Ltd.

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  • 国内天然ガスの炭素同位体地球化学
  • コクナイ テンネン ガス ノ タンソ ドウイタイ チキュウ カガク

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The genetic origin (abiogenic vs. biogenic, microbial vs. thermogenic), maturity of thermogenic component, compositional change due to migration, and extent of biodegradation of natural hydrocarbon gases in Japan are interpreted using carbon isotope compositions of methane, ethane and propane, and hydrocarbon molecular ratios. Mixing of gases with different origins or different maturities can also be evaluated using gas isotopic and molecular compositions. Abiogenic gas contribution is not detected in the natural gases. Most production gases are classified into thermogenic or mixed microbial/thermogenic except for some water-dissolved type gases of microbial origin. Ethane and propane carbon isotope compositions of thermogenic hydrocarbons are controlled largely by maturity. The gases in the Niigata Basin show generally higher maturity than the gases in the Akita/Yamagata Basin. <br>Carbon isotope analyses of headspace and/or mud gas can be used to delineate subsurface hydrocarbon reservoirs and to understand gas migration and accumulation processes. Carbon isotope compositions in commingled production could be used to allocate contributions from individual production zones if isotopic differences exist between the gases from the contributing reservoirs. Origin of gas seepage in production sites could be investigated by the gas molecular and isotope compositions if enough reference data exist in the area.

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