At what depth are methane hydrates found? Are they within the ocean's floor or the continental shelves?

Question

I'm confused about the location of methane hydrates deposits - on the one hand i've read they are mostly found within the sediments of continental shelves, which depth is useully around 300m, yet i've also heard the hydrets are most common at the depths of 1000m below surface.

(From the diagrams i've seen it doesnt seem as if the deposits are 700m beneath the continental shelve's sediment- they are drawn as a thin layer- and to add to the mess - some sources claim the hydrates are at the ocean floor,1000m below sea surface!)

links will be really apprecited :)

thanks!

Answer 1

Permafrost- which is methane based within the polar ice caps that actually seep out as the ice slowly melts. it does more damage to the ozone than CO2 hense speeding up the process of global warming.http://www.wesjones.com/climate1.htm Elizabeth Kolbert is one of the best scientists on this matter and will give you in elaborate detail what permafrost is and how dangerous it is becoming.

Answer 2

The National Methane Hydrates R&D Program
All About Hydrates - Necessary Conditions for Methane Hydrate Formation

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Methane actively dissociating from a hydrate mound

Methane hydrate, much like ice, is a material very much tied to its environment—it requires very specific conditions to form and be stable. Remove it from those conditions, and it will quickly dissociate into water and methane gas. A key area of basic hydrate research is the precise description of these conditions so that the potential for occurrence of hydrates in various localities can be adequately predicted and the response of that hydrate to intentional, unintentional, and/or natural changes in conditions can be assessed.

Our current understanding of naturally-occurring methane hydrate indicates that the fundamental controls on hydrate formation and stability are (1) adequate supplies of water and methane, (2) suitable temperatures and pressures, and (3) geochemical conditions. Other controls, such as sediment types and textures, may also exist.

Modes of Formation: Hydrates can form in several ways. In the arctic, there is a growing belief that many hydrate accumulations represent pre-existing free gas accumulations that have been converted to hydrate by subsequent change in environmental conditions (onset of arctic climate post-dated the migration of gas into shallow sandstone “traps”. In the marine environment, hydrate is often considered to have formed from solution, as methane is generated by in-situ microbial processes to the point where the water becomes saturated with methane and hydrate growth begins. There is also a high likelihood that methane hydrate could accumulate in coarser-grained marine sediments by the migration of gas from deeper, warmer zones, up through various pathways such as faults, and into water-bearing shallow sediments where it is then converted to methane hydrate.

Methane is formed in two ways. First, biogenic methane is the common by-product of bacterial ingestion of organic matter (as described in the equation below). (CH20)106(NH3)16(H2PO4) 53CO2+53CH4+16NH3+H2PO4

How methane is produced in shallow subsurface environments through biological alteration of organic matter (with original ratio of Carbon:Nitrogen:Phosphorus of 106:16:1). The equation summarizes successive stages of oxidation by oxygen and reduction by nitrates, sulfates, and carbonates. (from Sloan, 1990)


The same process that produces methane in swamps, landfills, rice paddies, and the digestive tracts of mammals occurs continually within buried sediments in geologic environments all around the globe. Biogenic processes are capable of producing vast amounts of methane, and are considered to be the dominant source of the methane trapped in hydrate layers within shallow sea floor sediments.

Second, thermogenic methane is produced by the combined action of heat, pressure and time on buried organic material. In the geologic past, conditions have periodically recurred in which vast amounts of organic matter were preserved within the sediment of shallow, inland seas. Over time and with deep burial, these organic-rich source beds are literally pressure-cooked with the output being the production of large quantities of oil and natural gas. Along with the oil, the gas (largely methane, but also ethane, propane and other larger molecules) slowly migrates upwards due to its buoyancy relative to water. If sufficient quantities reach the zone of hydrate stability, the gas will combine with local formation water to form hydrate.
Figure 1: Methane Hydrate Phases


Temperatures and Pressures: : Figure 1 shows the combination of temperatures and pressures (the phase boundary) that marks the transition from a system of co-existing free methane gas and water/ice solid methane hydrate. When conditions move to the left across the boundary, hydrate formation will occur. Moving to the right across the boundary results in the dissociation (akin to melting) of the hydrate structure and the release of free water and methane. In general, a combination of low temperature and high pressure is needed to support methane hydrate formation

Geochemical Conditions: In addition to temperature and pressure, the composition of both the water and the gas are critically important when fine-tuning predictions of the stability of gas hydrates in specific settings. Experimental data collected thus far have included both fresh water and seawater. However, natural subsurface environments exhibit significant variations in formation water chemistry, and these changes create local shifts in the pressure/temperature phase boundary (higher salinity restricts hydrate formation causing the phase boundary to shift to the left). Similarly, the presence of small amounts of other natural gases, such as carbon dioxide (CO2), hydrogen sulfide (H2S) and larger hydrocarbons such as ethane (C2,H6), will increase the stability of the hydrate, shifting the curve to the right. As a result, hydrates that appear to be well above the base of hydrate stability (from pressure-temperature relationships) may actually be very close to the phase boundary due to local geochemical conditions. These local variations may be very common, as the act of forming hydrate, which extracts pure water from saline formation waters, can often lead to local, and potentially-significant increases in formation water salinity.
Figure 2: Specific Hydrate Stability for Arctic Permafrost



Figure 3: Typical occurrence of the gas hydrate stability zone on deep-water continental margins. A water depth of 1200 meters is assumed.



Figure 4: At sub-polar latitudes temperatures are too high at every depth for methane hydrate to be stable.


Simplified Examples of Hydrate Stability: Commonly, methane hydrate phase diagrams are presented with pressure being converted to depth to place the diagram in a geologic perspective. In addition, the natural geothermal gradient is shown to indicate the range of temperatures expected to exist as depth (i.e. pressure) increases. The range of depths in which the temperature gradient curve is to the left of the phase boundary indicates the Gas Hydrate Stability Zone (GHSZ). This zone only delineates where hydrates will be stable if they form. Local conditions and a region's geologic history will determine where and if hydrates actually occur within the GHSZ (see our Geology of Methane Hydrates section for more information).

The phase diagram in Figure 2 illustrates typical conditions in a region of arctic permafrost (with depth of permafrost assumed to be 600 meters). The overlap of the phase boundary and temperature gradient indicates that the GHSZ should extend from a depth of approximately 200 meters to slightly more than 1,000 meters. (Note that both the permafrost thickness and pressure/temperature gradients in the chart are examples and can vary with locality, so specially-tailored diagrams must be made before site-specific predictions of hydrate stability can be attempted.)

The phase diagram in Figure 3 shows a typical situation on deep continental shelves. A seafloor depth of 1,200 meters is assumed. Temperature steadily decreases with water depth, reaching a minimum value near 0°C at the ocean bottom. Below the sea floor, temperatures steadily increase. In this setting, the top of the GHSZ occurs at roughly 400 meters—the base of the GHSZ is at 1,500 meters. Note, however, that hydrates will only accumulate in the sediments or as mounds on the seafloor over point sources of methane release.

From the phase diagram in Figure 3 for oceanic settings, it would appear that hydrates should accumulate anywhere in the ocean-bottom sediments where water depth exceeds ~400 meters. However, very deep (abyssal) sediments are generally not thought to house hydrates in large quantities. The reason is that deep oceans lack both the high biologic productivity (necessary to produce the organic matter that is converted to methane) and rapid sedimentation rates (necessary to bury the organic matter) that support hydrate formation on the continental shelves.

The final phase diagram in Figure 4 illustrates why no hydrates are found in interior basins at sub-polar latitudes. At every depth (pressure), subsurface temperatures are too high for methane hydrate to be stable.