Fluid-flow Dynamics in Association with Subsea Permafrost and Possible Gas Hydrates in the Kara Sea

Alexey Portnov

We study the West-Yamal Shelf in the Kara Sea, offshore Western Russia. We present new high-resolution seismic data (2-16 kHz) and gas geochemical data from 2012 cruises on the VNIIO’s research vessel. In seismic data, we found extensive acoustic anomalies in the water column, which we interpreted to be gas (bubble) flares rising from the seafloor. These anomalies were widespread throughout the study area, but seemed to be limited to water depths > 20 meters below sea level (mbsl). In one case, we observed a focused, gas-seepage site in ~6m water depth that reached almost to the atmosphere. This seepage site, however, was not detected during subsequent passes over the area. It is unclear why the majority of hydroacoustic anomalies are limited by the 20 m isobath, but it may be because permafrost is still present below the seafloor at these depths and is providing an impermeable layer through which gas and other fluids cannot migrate. We also detected acoustic amplitude anomalies in sediments in the upper 2-5m below seafloor (mbsf). We interpret these acoustic anomalies to also record the presence of gas. They were widespread throughout the study area and occurred in water depths ranging from 0-80 mbsl. The presence of these acoustic anomalies appeared to have no correlation with the distribution of acoustic anomalies detected in the water column. Geochemical analysis of pore water sampled from cores reveals that acoustically transparent zones have methane concentrations that are elevated compared to regions that are not transparent. Deeper seismic data show that acoustic anomalies in sediments near the seafloor are connected by gas chimneys that extend to depths >2000 mbsf. This suggests that some gas is migrating from deeper reservoirs and that it likely has a thermogenic component. In addition to the more widespread and disperse acoustically transparent zones, we discovered two prominent transparent mounds that are 1.5-2 km in diameter and that are elevated ~ 1-3m above the seafloor. These mounds are surrounded by layered sediments which fan downward toward the centre of the mounds. These features bear striking resemblance to the pingo-like features (PLF) that have been studied on the Beaufort Shelf (e.g. Shearer et al., 1971; Paull et al., 2007) and Pechora Sea (Rokos, 2009). These initial results provide targets for drilling and data acquisition in the summer of 2013 and for future research cruises in the Kara Sea. Furthermore, our study allows us to directly compare our observations in the Kara Sea to other regions in the Arctic (e.g. the Beaufort Shelf) where permafrost and/or hydrate degradation may be occurring. This will allow us to begin to have a global understanding of how permafrost and/or hydrate may be responding to warming ocean temperatures in the Arctic. Finally, a better understanding of hydrocarbon seepage at the seafloor is important in assessing the hydrocarbon potential for new exploration regions like the Kara Sea.

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