Gas geochemistry study in gas hydrate potential area offshore SW Taiwan

It has been found that Bottom Simulating Reflections (BSRs) widely distribute
in offshore southwestern Taiwan which infer the existence of potential gas hydrates
underneath the seafloor sediments. Fluids and gases derived from dissociation of gas
hydrates which usually enriched in methane concentrations would affect the
compositions of sea water and sediments nearby the venting areas. Hence, methane
concentrations of sea waters and sediments become useful proxies for exploration of
potential gas hydrates in one area.

We systematically collected sea waters and cored sediments for dissolved and
pore-space gas analysis through eight cruises: ORI-697, ORI-718, ORII-1207,
ORII-1230, ORI-732, Marion Dufresne (MD) cruise, ORI-758 and ORI-765 from
2003 to 2005 in this study. Some unusual high methane concentrations can be found
in offshore southwestern Taiwan, e.g., sites G23 of ORI-697, N8 of ORI-718, G96 of
ORI-732, MD05-2911(G22), MD05-2912(G24), MD052913(G5A) and
MD05-2914(73205) of MD cruise, GH10 and GH16 of ORI-758 cruise, sites A, C, D,
and H of ORI-765 cruise. The methane concentrations of cored sediments display an
increasing trend with depth. Meanwhile, the water column also showed unusual high
dissolve methane concentrations at sites A and H of cruise ORI-765. It indicates that
gases are venting actively from seafloor in this region. Compiling the available data,
high CH4 concentrations are mainly distributed in active margin SW offshore Taiwan;
nevertheless, no abnormally high concentrations were found in passive margin so far.

In addition, the profiles of methane and sulfate of cored sediments reveal very
shallow depths of sulfate methane interface (SMI) at some sites in this study. The
linear sulfate gradients, low total organic carbon (TOC) and high methane
concentrations imply that sulfate reduction is mainly driven by the process of
anaerobic methane oxidation (AMO) in sediments. Thus, the methane fluxes can be
determined through the gradients of sulfate reduction and steady state solutions to
diffusion equations and show that the methane fluxes are very high, especially at sites
G23 of ORI-697 (4.12×10-2 mmol cm-2yr-1) and N8 of ORI-718 (2.11×10-2 mmol
cm-2yr-1) in offshore southwestern Taiwan. It indicates that there is a
methane-enriched venting source, which may be the product of dissociation of gas
hydrates, in this area.

Some selected gas samples from ORI-718 (N6； N8； G22); ORI-732 (G96)
and MD cruise have also been analyzed for carbon isotopic compositions. The δ13C data of CH4 gases range from -28.3~ -95.0 o/oo and -11.6 ~ -31.6 o/oo for CO2. The
carbon isotopic compositions of methane show that organic gas source is dominated
at shallower depth, however, some thermogenic gases might be introduced from
deeper source in this region.