Origins and Distribution Patterns of Seabed-derived Shallow Gas in the southwestern Coastal Area, East Sea

Abstract

In the mud-belt off the southeastern coast of Korea, geophysical and geochemical survey from sedimentary layer to water column was performed to confirm the accumulation and release of hydrocarbon gases in the shallow depth of sedimentary layer. The active seepage has been observed by high-resolution (3.5 kHz) seismic profiling that occurs through up to 120 plumes from the topmost sedimentary layer. The plumes are hydro-acoustically detected in the area of 2.7 km2, from 20 to 50 m in water depth range, and found rare In deeper depth. The gas plumes are reached vertically up to 25 m from the seafloor. The gas seepage appears to be explosive enough to form craters, although pockmarks are rare. The carbon isotopes (δ13C) of gases in the sediments (-62.0 to &#8211 39.7 ‰ PDB) are characteristically differentiated from the seawater (-59.8 to &#8211 48.6 ‰ PDB). The gases are considered to be generated from abundant organic carbon (1.5 to 4.0%) in the sedimentary layer and mixed of small amounts of thermogenic gas which are derived from the underlying older sedimentary rocks.ayer. The active seepage has been observed by high-resolution (3.5 kHz) seismic profiling that occurs through up to 120 plumes from the topmost sedimentary layer. The plumes are hydro-acoustically detected in the area of 2.7 km2, from 20 to 50 m in water depth range, and found rare In deeper depth. The gas plumes are reached vertically up to 25 m from the seafloor. The gas seepage appears to be explosive enough to form craters, although pockmarks are rare. The carbon isotopes (δ13C) of gases in the sediments (-62.0 to &#8211 39.7 ‰ PDB) are characteristically differentiated from the seawater (-59.8 to &#8211 48.6 ‰ PDB). The gases are considered to be generated from abundant organic carbon (1.5 to 4.0%) in the sedimentary layer and mixed of small amounts of thermogenic gas which are derived from the underlying older sedimentary rocks.