ORIGINS AND DISTRIBUTION PATTERNS OF SHALLOW GAS IN THE SOUTHEASTERN COASTAL AREA OF KOREA

Abstract

In the mud-belt off the southeast of Korea, geophysical and geochemical survey from sedimentary layer to water column was performed to confirm the shallow gas accumulation and gas release features. The active seepage of hydrocarbon gases has been observed by high-resolution (3.5 kHz) seismic profiling both in March and July 2007, occurring through up to 120 plumes from the topmost sedimentary layer. The gas seepage appears to be explosive to form craters, although pockmarks are rare. In water column and sediments, the stable carbon isotopes in gases are characterized by large difference between δ13C (-59.8 to -48.6 ‰ PDB) of seawater and δ13C (-62.0 to -39.7‰ PDB) of sediments, respectively. The biogenic gases are considered to be generated from high amount of organic carbon (1.5-4.0%) in the sedimentary layer with mixing of small amount of thermogenic gas which derived through faults.