High-Resolution Seismic Studies of Methane Hydrate and Migrating Gas on the West Svalbard Margin

Tim Minshull

The presence of methane hydrate beneath the seabed west of Svalbard is evidenced by the presence of a widespread bottom-simulating reflector (BSR) on the lower continental slope and by direct sampling with cores. The presence of migrating gas beneath this hydrate is evidenced in seismic data by regions of anomalously low velocity, by loss of high frequencies and by disruption of reflectors. This migrating gas is released directly into the ocean close to the landward limit of hydrate stability, further landward on the shelf, and at isolated locations further seaward. Ocean temperature variations modulate the near-seabed gas transport by driving formation and dissociation of hydrate. During cruises in 2011 and 2012, we acquired high-resolution reflection and wide-angle seismic data in the vicinity of gas escape on the continental slope and over a large pockmark on the nearby Vestnesa Ridge sediment drift. We collected surface seismic data using a 90 cu. in. GI gun source and a 60-m, 60-channel hydrophone streamer, and deep-towed data using Ifremer’s SYSIF vehicle and chirp sources with 220-1050 Hz and 580-2200 Hz sweeps. We recorded both sources on ocean bottom seismometers to determine the velocity structure with high vertical resolution. Velocities were obtained using a semblance-type approach for the chirp source, which is directional and therefore only recorded at a limited range of offsets, and raytracing for the GI-gun source. The seismic reflection data show evidence for the widespread presence of subsurface gas at the slope site. Here, numerous sub-vertical fractures provide conduits for gas transport to the ocean floor. Deeply sourced gas also appears to migrate along stratigraphic horizons and locally pond beneath a thin veneer of glacial and post-glacial sediments. At the Vestnesa pockmark site, strong scattering in Chirp images suggests the presence of localised pockets of subsurface gas within the hydrate stability field, and local increases in seismic velocity above the BSR provide evidence for a concentration of hydrate beneath the pockmark.

AAPG Search and Discovery Article #90177©3P Arctic, Polar Petroleum Potential Conference & Exhibition, Stavanger, Norway, October 15-18, 2013