Gas origin and migration in the Ulleung Basin, East Sea: Results from the Second Ulleung Basin Gas Hydrate Drilling Expedition (UBGH2)

We have investigated the gas chemistry of marine sediments (up to 250 m below seafloor, m bsf) recovered during the Second Ulleung Basin Gas Hydrate Drilling Expedition (UBGH2) to identify and compare the gas origin and its migration at seismically non-chimney, truncated chimney and chimney sites in this basin. The molecular and isotopic data (C1/C2+ ratios > 300, δ13CCH4δ13CCH4 < −65‰, and δDCH4δDCH4 < −181‰) of the gases indicate that methane predominantly originates via microbial carbon dioxide reduction. The carbon isotopic fractionation (ɛc) between methane and carbon dioxide below 30 m bsf has a relatively constant value (ɛc = 66–68) at all sites, consistent with the microbial pathway. In contrary, samples shallower than 30 m bsf display significant differences with the seismic characteristics of each site. ɛc ranges from 65 to 81 in non-chimney sites but shows values as low as 45 in chimney sites, which is agreement with previously reported piston core data (Kim et al., 2012), and thought to reflect two-phase methane transport processes. In chimney sites, deep-sourced methane migrates independently of the carbon dioxide, which remains dissolved in the aqueous phase leading to the extremely low ɛc values observed in the upper 30 m bsf. Upward methane diffusion at non-chimney sites may also add a transport-induced fractionation as evidenced in mixing diagrams, but this process does not significantly affect the ɛc values.δ13CCO2δ13CCO2 shows usually minimum values at the Sulfate–Methane Transition Zone (SMTZ) at all sites. However, in chimney sites, these values have a much more depleted signature, indicating faster rates of methane consumption by Anaerobic Oxidation of Methane (AOM) supported by the higher methane flux. Moreover, here the massive hydrate that forms in shallow sediments incorporates gases with a lower C1/C2 ratio (=higher C2/C1) than the surrounding sediment, as documented in hydrate-bound gas (BG) and void gas (VG) samples. Gas hydrate dissociation during the core recovery is released enriched ethane (C2H6) trapped in gas cage, which leads to have high C2H6 in VG samples.