Geochemistry and origin of natural gases dissolved in brines from gas fields in southwest Japan

Abstract

Previous geochemical studies indicated that most natural gases dissolved in brines in Japan are of microbial origin, consisting of methane produced via carbonate reduction. However, some of those from gas fields in southwest Japan contain methane relatively enriched in 13C, whose origin remains to be clarified. To address this issue, chemical and isotopic analyses were performed on natural gases and brines from the gas fields in Miyazaki and Shizuoka prefectures, southwest Japan. Methane isotopic signatures (δ13C ≈ −68‰ to −34‰ VPDB; δ2H ≈ −183‰ to −149‰ VSMOW) suggest that these gases are of microbial (formed via carbonate reduction) or of mixed microbial and thermogenic origin. The relatively high δ2H‐CH4 values and their relationship with the δ2H‐H2O values argue against the possibility of their formation via acetate fermentation. The δ13C‐CO2 values (≈−5‰), together with the slope of the correlation between δ2H‐CH4 and δ13C‐CH4 (Δδ2H‐CH4/Δδ13C‐CH4 ≈ 1), contradict the possibility of their formation via carbonate reduction followed by partial oxidation by methanotrophs. The 3He/4He ratios of the gases from Miyazaki (≈0.11–1.3 Ra) and their low correlation with δ13C‐CH4 values do not support an abiogenic origin. It is inferred therefore that the high δ13C‐CH4 values of natural gases dissolved in brines from gas fields in southwest Japan are indications of the contribution of thermogenic hydrocarbons, although whether abiogenic hydrocarbons contribute significantly to the gases from Shizuoka requires further investigation. This study has clarified that, for the future exploration of natural gases in southwest Japan, we should adopt the strategies for conventional thermogenic gas accumulations, such as checking the content, type and maturity of organic matter in the underlying sedimentary rocks.

Helium and methane sources and fluxes of shallow submarine hydrothermal plumes near the Tokara Islands, Southern Japan

“…The estimated end-member δ 13 C CH4 value of the Tokara Islands is more positive, i.e., enriched in thermogenic methane, than the natural gases in brines from the gas fields in southwest Japan (−38.9 to −67.5‰ PDB 33 ) but more negative than the gases observed in the hydrothermal fluids from mid-ocean ridge regions (−8.6 to −20‰ PDB 5 . However, the estimated end-member δ 13 C CH4 value at the Tokara Islands is similar to the hydrothermal fluids at Minami-Ensei and Yonaguni in the Okinawa Trough (−25 to −26.9‰ PDB 34 ).…”

Biogas production using anaerobic groundwater containing a subterranean microbial community associated with the accretionary prism

“…Groundwater is mainly recharged by rainfall which infiltrates into outcrops or faults, then flows down through the permeable sandstone and is anaerobically reserved in a deep aquifer. Furthermore, it has been reported that a large amount of the natural gas [methane (CH 4 ), > 97%] is present in deep aquifer associated with the accretionary prism (Igari and Sakata, 1989 ; Sakata et al ., 2012 ).…”

The Potential for CH₄ Production by Syntrophic Microbial Communities in Diverse Deep Aquifers Associated with an Accretionary Prism and its Overlying Sedimentary Layers

“…1B and Table S1). These wells were drilled down to 810 to 1,301 m and constructed from tight steel-casing pipes including strainers (Table 1) (Kato et al, 2011;Sakata et al, 2012). Groundwater at these wells is anoxically drawn up to ground level by a water pump.…”

“…This sediment is approximately 10 km thick and contains layers of water-bearing permeable sandstone and water-impermeable shale that are rich in complex organic matter (Maki et al, 1980;Taira et al, 1982). Rainwater and seawater recharge through faults or fracture zones formed by earthquakes that occur at the plate boundaries, and these waters then collect in the deep aquifers of accretionary prisms in which they become anoxic over time (Sakata et al, 2012;Baito et al, 2015). Deep aquifers contain large amounts of groundwater with dissolved and free phase natural gas, mainly methane (CH 4 ) (Kimura et al, 2010;Matsushita et al, 2016;Matsushita et al, 2018).…”

“…The anaerobic deep aquifers associated with accretionary prisms contain large amounts of CH 4 (Kimura et al, 2010;Sakata et al, 2012). CH 4 is a potential greenhouse gas and important energy resource.…”