天然ガスの生成・移動・集積過程における炭素同位体・化学組成の変化

抄録

The origin of natural gas is generally classified into microbial, thermogenic, or mixed according to the carbon isotopic composition of methane （δ¹³C₁） and methane/（ethane＋propane） ratio （C₁/（C₂＋C₃））. To identify compositional changes during migration and accumulation, we examine gases produced from dissolved microbial to free thermogenic origins from the Kanbara district in the Niigata backarc basin, central Japan. Most gases plot near the mixing line between microbial and thermogenic endmembers and several additional features are recognized. 1） Thermogenic gases from deep reservoirs maintain a constant C₁/（C₂＋C₃） ratio of ~9 with increasing δ¹³C₁. 2） No free mixed gas is observed upon incorporation of several percent thermogenic gas in the microbial endmember. 3） Dissolved and free gases in shallow reservoirs exhibit similar or higher δ¹³C₁ and C₁/（C₂＋C₃） values that are distinct from the mixing line. These observations demonstrate that “mixed gas” does not form by simple mixing but that different mechanisms individually control the isotopic and chemical composition. δ¹³C₁ values of the thermogenic gas gradually decrease by an exchange reaction with dissolved biogenic methane in fossil seawater during migration. C₁/（C₂＋C₃） ratios remain constant under deep high-temperature/pressure conditions because natural gas exists as supercritical fluid or dissolved in crude oil. However, C₁/（C₂＋C₃） increases upon fractionation with dissolved gas in subsequently-separated condensate oil and/or re-equilibrium with dissolved gas in pore water under low-temperature/pressure conditions. Different migration paths generate different compositions. For example, migration in shallow carrier beds leads to lower δ¹³C₁ and more methane-rich compositions far from the mixing line. Ongoing accumulation causes a differentiated composition at the migration front to reverse towards the direction of the original thermogenic gas. The composition of dissolved gas in the edge waters is affected by accumulated free gas. In general, only thermogenic gas contributes to “mixed gas” reserves, which do not increase by adding and mixing with microbial gas inferred from decreasing δ¹³C₁, whereas the existence of biogenic methane in carrier beds prevents free gas loss during migration. This indicates that the origin of methane molecules is a mixture of biogenic and thermogenic sources, whereas free gas originated in deep thermogenic. It is not easy to distinguish that the origin of free dry gas with low δ¹³C₁ at very shallow depths is either microbial gas released from edge water or migrated thermogenic gas. Methane hydrates with low δ¹³C₁ beneath the seafloor of the forearc basin may originate from thermogenic gas generated in the accretionary prism.