Geophysical and geochemical observations on actively seeping hydrocarbon gases on the south-eastern Yellow Sea continental shelf

In the southeastern Yellow Sea, active seepage of hydrocarbon gases has been observed by high-resolution (3.5 kHz) seismic profiling both in 1987 and 2001, occurring through a large number of plumes from the topmost pre-Holocene sedimentary layer. It is strong enough to compensate for current speed, extending vertically up to the sea surface. The gas seepage often appears to be explosive to form craters and diapirs, although pockmarks are rare due to the redistribution of mobile palimpsest sands. In core-top seawater and sediments, the gases are characterized by high amounts of C-2, homogenous delta(13)C(1) values and a large difference (19.7parts per thousand on average) between delta(13)C(1) (-55.2 to -53.6parts per thousand PDB) and delta(13)C(2) (-36.8 to -32.5parts per thousand PDB) values. The gases are considered to be generated with a smaller amount of C-1 at the early thermal cracking stage of labile source materials, after which the C-2 gas is enriched in C-13 by diffusion or biological alternation at the generation or accumulation site. The homogenous delta(13)C(1) values may be one of the geochemical characteristics of gases acquired at depth which are less altered in the case of rapid diffusive gas migration to the seafloor.