Gas hydrate stability zone, abbreviated GHSZ, also referred to as methane hydrate stability zone (MHSZ) or hydrate stability zone (HSZ), refers to a zone and depth of the

marine environment Marine habitats are habitats that support marine life. Marine life depends in some way on the saltwater that is in the sea (the term ''marine'' comes from the Latin ''mare'', meaning sea or ocean). A habitat is an ecological or environmental ...

at which

methane clathrate Methane clathrate (CH4·5.75H2O) or (8CH4·46H2O), also called methane hydrate, hydromethane, methane ice, fire ice, natural gas hydrate, or gas hydrate, is a solid clathrate compound (more specifically, a clathrate hydrate) in which a large amou ...

s naturally exist in the

Earth's crust Earth's crust is Earth's thin outer shell of rock, referring to less than 1% of Earth's radius and volume. It is the top component of the lithosphere, a division of Earth's layers that includes the crust and the upper part of the mantle. The ...

Description

Gas hydrate stability primarily depends upon

temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied o ...

and

pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and e ...

, however other variables such as gas composition and ionic impurities in water influence stability boundaries. The existence and depth of a hydrate deposit is often indicated by the presence of a bottom-simulating reflector (BSR). A BSR is a

seismic reflection Reflection seismology (or seismic reflection) is a method of exploration geophysics that uses the principles of seismology to estimate the properties of the Earth's subsurface from reflected seismic waves. The method requires a controlled seismi ...

indicating the lower limit of hydrate stability in

sediments Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand a ...

due to the different densities of hydrate saturated sediments, normal sediments and those containing free gas.

Limits

The upper and lower limits of the HSZ, as well as its thickness, depend upon the local conditions in which the hydrate occurs. The conditions for hydrate stability generally restrict natural deposits to polar regions and deep oceanic regions. In polar regions, due to low temperatures, the upper limit of the hydrate stability zone occurs at a depth of approximately 150

meters The metre (British spelling) or meter (American spelling; see spelling differences) (from the French unit , from the Greek noun , "measure"), symbol m, is the primary unit of length in the International System of Units (SI), though its prefi ...

.¹ The maximal depth of the hydrate stability zone is limited by the

geothermal gradient Geothermal gradient is the rate of temperature change with respect to increasing depth in Earth's interior. As a general rule, the crust temperature rises with depth due to the heat flow from the much hotter mantle; away from tectonic plate b ...

. Along continental margins the average thickness of the HSZ is about 500 m. The upper limit in oceanic sediments occurs when bottom water temperatures are at or near 0

°C The degree Celsius is the unit of temperature on the Celsius scale (originally known as the centigrade scale outside Sweden), one of two temperature scales used in the International System of Units (SI), the other being the Kelvin scale. The ...

, and at a water depth of approximately 300 meters.¹ The lower limit of the HSZ is bounded by the geothermal gradient. As depth below seafloor increases, the temperature eventually becomes too high for hydrates to exist. In areas of high geothermal heat flow, the lower limit of the HSZ may become shallower, therefore decreasing the thickness of the HSZ. Conversely, the thickest hydrate layers and widest HSZ are observed in areas of low geothermal heat flow. Generally, the maximum depth of HSZ extension is 2000 meters below the Earth's surface.^1,3 Using the location of a BSR, as well as the pressure-temperature regimen necessary for hydrate stability, the HSZ may be used to determine geothermal gradients.²

Transport

If processes such as

sedimentation Sedimentation is the deposition of sediments. It takes place when particles in suspension settle out of the fluid in which they are entrained and come to rest against a barrier. This is due to their motion through the fluid in response to the ...

subduction Subduction is a geological process in which the oceanic lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, the ...

transport hydrates below the lower limit of the HSZ, the hydrate becomes unstable and disassociates, releasing gas. This free gas may become trapped beneath the overlying hydrate layer, forming gas pockets, or reservoirs. The pressure from the presence of gas reservoirs impacts the stability of the hydrate layer. If this pressure is substantially changed, the stability of the methane layer above will be altered and may result in significant destabilization and disassociation of the hydrate deposit.

Landslides Landslides, also known as landslips, are several forms of mass wasting that may include a wide range of ground movements, such as rockfalls, deep-seated slope failures, mudflows, and debris flows. Landslides occur in a variety of environments, ...

of rock or sediment above the hydrate stability zone may also impact the hydrate stability. A sudden decrease in pressure can release gasses or destabilize portions of the hydrate deposit. Changing atmospheric and oceanic temperatures may impact the presence and depth of the hydrate stability zone, however, is still uncertain to what extent. In oceanic sediments, increasing pressure due to a rise in sea level may offset some of the impact of increasing temperature upon the hydrate stability equilibrium.¹

References

{{DEFAULTSORT:Gas Hydrate Stability Zone Clathrate hydrates Hydrates Hydrocarbons Methane Oceanographical terminology Physical oceanography