Migration paths of methane gas into ocean through gas hydrate-bearing sediments in the Sea of Okhotsk

Abstract

We present high-resolution seismic and hydroacoustic profiles showing chimney structures (gas chimneys) in gas hydrate-bearing sediments and ongoing gas-emission features (gas flares) in the water column in the Sea of Okhotsk. Two kinds of gas chimneys showing different seismic characteristics occur in the sediments

wipeout (WO) and enhanced reflection (ER) chimneys. From the correlation between gas chimneys and gas flares, gas flares are associated exclusively with the ER chimneys, indicating that the ER chimneys are active pathways along which gas is migrating upward from the deeper depth to the seafloor. We interpret that the ER chimney is gas-filling chimneys and the WO chimney is gas hydrate-filling chimney. A scenario on occurrence of two chimneys and their evolution is (1) First the ER chimney is formed due to diffused saturation of gas when warm fluid with gas is migrating upward along the fault into seafloor, (2) After stoppage of warm fluid supply, gas in the ER chimney is converted into gas hydrates. Focused saturation of gas hydrates along the fault makes the narrow WO chimney. (3) New ER chimney is formed at the sites where gas/fluid is migrating upward along other fault. Our results show what happens in the gas-hydrate bearing sediments during and after gases release into the ocean.