OS2B-2:A COMPLEX GAS HYDRATE SYSTEM IN THE DONGSHA AREA, SOUTH CHINA SEA: RESULTS FROM DRILLING EXPEDITION GMGS2

    Gas hydrate drilling expedition GMGS2 was carried out in the summer of 2013 and investigated 13 sites in the Dongsha area, South China Sea.  Using a combination of well logging and sampling, it was concluded that nine of the 13 sites investigated contained gas-hydrate-bearing lithologies, including all five of the sites which were cored (Sites GMGS2-05, -07, -08, -09, & -16).  Coring results showed that the Dongsha area is a very active region of methane flux and that gas-hydrate-bearing sediments are common features in the first 200 meters below the sea floor.  Gas-hydrate-bearing lithologies identified from the coring included the following hydrate morphologies: a) disseminated gas hydrate in deeply-buried, fine-grained sediments with gas hydrate saturations of 20-55% of pore volume, b) disseminated gas hydrate in coarse-grained sediments, c) dense, thin veins of gas hydrate in near-surface, fine-grained sediments with gas hydrate saturations equivalent to 10-40% of pore volume, and d) massive gas hydrate layers.  Free methane gas was observed coming directly from boreholes at Sites GMGS2-07 and GMGS2-16.  Site GMGS2-16 also showed strong geochemical evidence for a deep aquifer supplying both free gas and fluids into the system.

    Massive gas hydrate at Site GMGS2-09 was associated with a seafloor carbonate platform, which had been investigated during previous expeditions in the region.  Buried carbonate layers above the massive gas hydrate features discovered at Sites GMGS2-07 & -08 indicate that these formations are likely to have formed initially at the surface similar to Site GMGS2-09, but then subsequently buried to depths of 40-60 meters below seafloor.  These three carbonate/hydrate sites may be examples of a common process in the area.  A layer of disseminated gas hydrate in fine-grained sediments just above the base of gas hydrate stability also appears to be a common feature in the Dongsha region, as it was in the Shenhu region of the South China Sea (Expedition GMGS1).  The near-surface vein hydrate lithology, found at Sites GMGS2-08 & -16 and restricted to 10-30 meters below seafloor, may again be a common feature of the area.  These vein-rich zones have no hydrate directly beneath them, and they do not appear to be part of a standard focused-flow, gas hydrate “chimney” structure.  Dongsha is certainly one of the richest and most complex marine gas hydrate environments studied to date.