Gas Migration through the Crystal Lattice of Gas Hydrates during Sedimentation in Geological Time Scale

Massive gas hydrates recovered from various sediment depths at the Umitaka Spur in the Sea of Japan were studied using Raman spectroscopy to determine their burial diagenesis, particularly those initially grown at shallow sediment depths. The samples recovered from sediment depths within 20 m below the seafloor (mbsf) were enriched in H2S. These depths were greater than the present sulfate-methane transition (SMT) zone distributed within approximately 1 mbsf of the coring area. The anaerobic oxidation of methane within the SMT is generally the only potential source of high H2S at these depths, whereas the deepening of the SMT to approximately 20 mbsf, greater than that of the reference areas, is improbable. A potential scenario for the depth discrepancy is the sedimentation of gas hydrates that originally grew around the near-seafloor SMT. The CH4 cage occupancy ratio in each sample becomes <1.08 at sediment depths deeper than 40 mbsf. The more homogeneous CH4 distribution within the gas hydrate at greater depths is attributed to the longer duration of CH4 migration through the lattice cage of the gas hydrate. The lack of H2S in samples at greater depths could also be attributed to the more homogeneous distribution of H2S by migration and escape from gas hydrates at H2S-depleted environments.