Seismic evidence of shallow gas in sediments on the southeastern continental shelf of Korea, East Sea (Japan Sea)

High resolution multi-channel seismic reflection data (-1000 km) and multibeam echosounder bathymetry from the southeastern Korean continental shelf of the East Sea (Japan Sea) reveal numerous shallow gas indicators and seepage-related features, such as bright spots, enhanced reflections, seismic chimneys, acoustic blanking, pockmarks, and bathymetric mounds. Bright spots, indicating gas-charged layers, appear as local negativepolarity reflection anomalies (up to 5 km wide) and occur at various stratigraphic levels within a subsurface depth of -320 m. Bright spots covering an area of -60 km2 are clustered at the tip of NE-SW-trending reverse faults in the northeastern and southeastern part of the investigated region, suggesting gas entrapment. Enhanced reflections (ca. 20-km-long) are developed along erosional unconformities and tilted sedimentary layers below them. This suggests that unconformities formed during sea-level low stands in the study area are potential reservoirs and may have acted as potential conduits for lateral migration of gas-rich fluids due to their permeable nature. Some enhanced reflections are formed along interfluves of channels where channel walls cut them, and thus they may potentially act as fluid reservoirs. Seismic chimneys, expressed as vertical disturbances in seismic data, are interpreted as the upward movement of fluids (i.e., either in liquid or gaseous form). Lack of faulting in some seismic chimneys suggests higher permeability in the sedimentary interval, which would allow the migration of deeper-sourced fluids. Pockmarks (up to 500 m in diameter) are typically associated with seismic chimneys in the sub-seabed, suggesting that they were formed by the explosive emission of gas or gas fluids. Some exhibit mound-like features near their crests that are interpreted as carbonate mounds. The locations of mounds above uplifted fault blocks in the central part suggest a structural control on the formation of these seabed features.