Abstract

It has recently been recognized that, in addition to low concentrations of widespread natural gas hydrate associated with bottom-simulating seismic reflectors, highly concentrated hydrate can occur in local seafloor fluid venting structures. Such structures extend upward into the regional gas hydrate stability field and sometimes allow gases to escape into the overlying ocean. These hydrate-choked chimneys are especially prospective as an energy resource because they contain high hydrate concentrations. Furthermore, they may be one of the most important conduits into the ocean-atmosphere system for deep methane. We present an analysis of two-dimensional seismic reflection data from offshore Korea that give a complete picture of gas migrating from a deep source zone to feed hydrate-choked vent structures at the seafloor. The gases migrate upward through networks of fractures imaged as steep amplitude striations in both diffuse and concentrated distributions. We present an example of a high-flux gas vent fed through fracture swarms emanating from a 10–15-km-wide catchment zone of source gases residing 3–4 km below the seabed. The geological context and the inferred distribution of hydrate within this feature are consistent with recent models in which seabed gas venting is a consequence of elevated pore fluid salinities that are produced by a high flux of gases migrating independently of the pore water. In contrast, a nearby vent that is not plumbed into the same high-flux system appears to be dominated more by gas-rich liquids. We present these two vent structures as type cases for high- and lowflux fluid escape systems in the seabed. Furthermore, we suggest that since the amount of gas trapped as hydrate within the vents is small compared with the amount in the underlying reservoir, the greatest risk for increased methane input to the atmosphere associated with climate-driven oceanic warming is not the melting of hydrate, but an increase in the number of deep reservoirs able to vent gases through the seabed that occurs as the regional gas hydrate stability zone thins.

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