Decreased stability of methane hydrates in marine sediments owing to phase-boundary roughness

Abstract

Below water depths of about 300 metres, pressure and temperature conditions cause methane to form ice-like crystals of methane hydrate. Marine deposits of methane hydrate are estimated to be large, amassing about 10,000 gigatonnes of carbon, and are thought to be important to global change and seafloor stability, as well as representing a potentially exploitable energy resource. The extent of these deposits can usually be inferred from seismic imaging, in which the base of the methane hydrate stability zone is frequently identifiable as a smooth reflector that runs parallel to the sea floor. Here, using high-resolution seismic sections of seafloor sediments in the Cascadia margin off the coast of Vancouver Island, Canada, we observe lateral variations in the base of the hydrate stability zone, including gas-rich vertical intrusions into the hydrate stability zone. We suggest that these vertical intrusions are associated with upward flow of warmer fluids. Therefore, where seafloor fluid expulsion and methane hydrate deposits coincide, the base of the hydrate stability zone might exhibit significant roughness and increased surface area. Increased area implies that significantly more methane hydrate lies close to being unstable and hence closer to dissociation in the event of a lowering of pressure due to sea-level fall.

Seismic images of gas conduits beneath vents and gas hydrates on Ritchie Ridge, Hikurangi margin, New Zealand

“…Indications for gas migration through the gas hydrate zone have been found both in passive-and active-continental margin settings (Holbrook et al 2002Riedel et al 2002. Heatflow modelling suggests that warm fluids that migrate along faults to the seafloor may locally keep the faults outside the zone of gas hydrate stability, making it possible for gas to migrate well into the regional gas hydrate zone (Wood et al 2002), and perhaps through it into the ocean (Pecher 2002).…”

Methane Venting in Gas Hydrate Potential Area Offshore of SW Taiwan: Evidence of Gas Analysis of Water Column Samples

“…As a result, the high pressure fluids generated by gas hydrate dissociation will permeate upward to the seafloor. Furthermore, according to high resolution seismic data, a chimney structure could provide fluids one of the main migration pathways in gas hydrate bearing layers (Pecher 2002;Wood et al 2002). Such a mechanism can explain the occurrence of submarine mud volcanoes in the gas hydrate potential areas of offshore SW Taiwan, which is tectonically active (e.g., Chiu et al 2006).…”

Feasibility of Large-Scale Ocean CO2 Sequestration

“…The sea floor sediments are highly porous, leading to an assumed sufficient supply of water and a gas-limited situation. However, along with the sea floor venting, seismic surveys attributed wipeout zones, or zones with no organized reflection horizons, similar to gas chimneys, to free gas directed up through the GHSZ , Wood et al 2002. A mass balance on a pressure-cored sample collected at ODP Leg 204 site 1249 led researchers to conclude that free gas must have been present along with dissolved methane and gas hydrate.…”

“…Although the presence of free gas has a solid foundation, the reason for it is still debatable. Two possibilities exist if the thermodynamic predictions are accurate: (a) The system is not in equilibrium because of kinetic and/or transport limitations (Haeckel et al 2004, Torres et al 2004 or (b) the ambient conditions are not being described correctly, including advection of warm fluid (Wood et al 2002) and pore water hypersalinity (Liu & Flemings 2006, 2007Milkov et al 2004).…”