Origin of methane-rich natural gas in Japan: formation of gas fields due to large-scale submarine volcanism

Abstract

Eighty-seven samples of CH 4-rich natural gas were collected from commercial gas and oil wells in Japan. The 3He/ 4He and 4He/ 20Ne ratios, chemical composition of major, minor and trace components (CH 4, C 2H 6, C 3H 8, CO 2, N 2, O 2, Ar, He, Ne and H 2), and the C isotope composition of CH 4 are reported. Most 3He/ 4He ratios of natural gases in the forearc regions are as low as (1–4) × 10 −7 and are comparable to those from large gas reservoirs in other parts of the world, suggesting that the CH 4 has a biogenic origin. In contrast, the 3He/ 4He ratios of samples from gas reservoirs in the “Green Tuff” region along the Japan Sea vary considerably: high ratios are observed for gases from volcaniclastic reservoirs in the deeper zone, and low ratios are found in shallow reservoirs of sedimentary rocks. The highest 3He/ 4He ratio, (9.37 ± 0.34) × 10 −6, is equivalent to the highest value in the subduction zone. The high 4He/ 20Ne ratios observed in most samples suggest that there is no significant atmospheric contamination. An overall regularity between 3He/ 4He ratios and δ 13C CH 4 values is observed for Japanese natural gases. The δ 13C CH 4 value of gas with a high 3He/ 4He ratio reaches about −35‰, and that of gas with a low ratio is about −75‰. Total C to 3He ratios (ΣC/ 3He) vary over a wide range (6 × 10 8−>9 × 10 12), but those of gases in volcaniclastic reservoirs are in a rather narrow range with a value of ∼2 × 10 9, coinciding with that of a mantle reservoir. The relation between the observed 3He/ 4He ratios and δ 13C CH 4 values of gases, together with their ΣC/ 3He ratios, suggests that CH 4-rich gases in volcaniclastic reservoirs are a mixture of gases from two sources, magmatic and biogenic. The formation of gas reservoirs with high 3He/ 4He ratios may be attributed to large-scale submarine volcanism that occurred in the Middle Miocene. Gas separation from volcaniclastic rocks is expected to have occurred during the process of sea water-rock interaction at a high temperature over a long period in a submarine environment. The Middle Miocene volcanic episode which occurred under extensional stress was quite different from the present arc volcanism, and is thought to have been closely related to the opening of the Japan Sea.