Migration and venting of deep gases into the ocean through hydrate-choked chimneys offshore Korea

Abstract

It has recently been recognized that, in addition to low concentrations of widespread natural gas hydrate associated with bottomsimulating seismic refl ectors, highly concentrated hydrate can occur in local seafl oor fl uid venting structures. Such structures extend upward into the regional gas hydrate stability fi eld 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 refl ection 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 seafl oor. 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-fl ux 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 fl uid salinities that are produced by a high fl ux of gases migrating independently of the pore water. In contrast, a nearby vent that is not plumbed into the same high-fl ux system appears to be dominated more by gas-rich liquids. We present these two vent structures as type cases for high-and lowfl ux fl uid 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.

Structural-stratigraphic control on the Umitaka Spur gas hydrates of Joetsu Basin in the eastern margin of Japan Sea

“…One characteristic of gas chimneys is that the seismic blanking obscures weak reflectors, while portions of the strongest reflectors can still be observed (Kou et al, 2007). Gas chimneys are widely observed in the Japan/East Sea and in other parts of the world, as reported by Haacke et al (2009), Horozal et al (2009), Riedel et al (2002, Saeki et al (2009), and are often related to the occurrence of gas hydrates.…”

Gas hydrate versus geological features: The South Shetland case study

“…Here, faults and fractures were considered as preferential path-ways for fluids escape in the gas hydrate system. Moreover, several studies focused on depth variation of the hydrate stability zone related to intense fluid flows (Haacke et al, 2009), guided by major faults cutting sediments (Roberts et al, 2006). The presence of fractures and minor faults sometimes can be a key factor due to the fact that they can represent high permeability conduits along with fluid can migrate (Nimblett and Ruppel, 2003;Hornbach et al, 2004).…”

Offshore Antarctic Peninsula Gas Hydrate Reservoir Characterization by Geophysical Data Analysis

“…Around several mud volcanoes, the gas hydrate distribution is strongly influenced by the thermal field [19]. Finally, the depth of hydrate stability zone can be affected by the fluid salinity changing the depth and/or generating a focused flux of gas that can reach the seafloor [20].…”