Predictable, but Elusive

Gas hydrate deposits exist beneath certain parts of the ocean, and under many regions of permafrost. Methane hydrate forms in these places because it is stable only at high pressure or low temperature.

The deep waters of almost all the world's oceans are cold enough and have enough pressure to stabilize hydrate in the seafloor at depths greater than about 500 meters. Most marine hydrates seem to be confined to the edges of continents where water is sufficiently deep and where nutrient-rich waters unload partially decayed organic material for bacteria to convert to methane. With the right pressure and temperature combination, these methane molecules are eventually entrapped in icy crystalline cages, forming undersea deposits of gas hydrates. Large undersea accumulations have been identified off the shore of Japan; in the Blake Ridge off the US eastern seaboard; on the Cascade continental margin off Vancouver, British Columbia, Canada; and off the shore of New Zealand.

Gas hydrates are also found close to the land surface in permafrost regions because of the low prevailing temperatures. permafrost deposits of gas hydrates have been found in western Siberia and on the Alaskan North Slope.

	Known (white dots) and inferred (black dots) occurrences of gas hydrates. (Courtesy of Keith Kvenvolden, U.S. Geological Survey.
Note: Evidence for only a small proportion of these sites of hydrate accumulation comes from the direct sampling of hydrate material. Most evidence is inferred from other related information, such as data from seismic reflections and remote measurements made by instruments lowered into drilled holes.

As you can see, even though there appears to be an extensive reserve of gas hydrates in many places around the world, all of the deposits are in rather hostile environments. That is, they are located in places where humans do not usually go and where humans cannot function without special equipment and protection.

Playing Hard to Get

In addition to being located in hard-to-get-to places around the world, gas hydrates are playing hard to get in a couple of other ways as well. When we can even find them, they are not very concentrated. Instead, gas hydrates are spread out or dispersed throughout large volumes of solid material. Also, the actual recovery process is difficult because the natural gas is entrapped in solid icy material; energy is needed to release the gas.

To give an idea about the problem of concentration, let's first discuss something that might be a bit more familiar -- gold! A gold mining company looks for places in which the amount of gold in the earth material is one part in 100,000 or better. This means that the company would have to process 100,000 g (100 kg or about 220 lb) of such ore to obtain one gram (about 1/30 oz) of gold. Since the current price of gold is more than $11 per gram, a mining company could spend as much as $9 per gram to extract the gold, and still hope to make a profit.

The concentration picture for gas hydrates is similar, but the economics do not play out the same way. One of the promising hydrate reserves off the coast of the United States has a concentration of one part methane to 161,000 parts of solid material. Although this concentration is close to that sought by a gold mining company for gold, the one gram of natural gas extracted from processing 161,000 g of solid material would be worth less than one-millionth of a dollar, at current prices! Until the value of the gas goes up a lot, or until we discover ways to extract the gas at practically no cost, gas hydrates will have to stay right where they are.

Even assuming that it will eventually be cost effective to tap this extensive energy reserve, there are additional difficulties ahead. Because the gas is locked inside the icy crystalline structure of solid material under pressure and often at low temperature, some kind of energy will be needed to release the gas and bring it to the surface.

These special pressure and temperature conditions have also made it difficult to study gas hydrates. When the pressure and temperature are changed, the hydrate material becomes unstable and changes form. Deep-sea research programs have drilled and cored hydrate-rich sediments and attempted to retrieve samples for shipboard and laboratory study. When the samples were brought onboard and exposed to atmospheric pressure and ambient temperature, however, they depressurized and melted. Few naturally occurring hydrates have survived long enough to be studied.

Pressures and temperatures needed for methane hydrate to be stable (left) are common in areas of permafrost and under much of the world's oceans. For example, conditions typical of the continental slope off the eastern U.S. (red line) allow methane hydrate to exist in the upper hundreds of meters of sediment and at all but the shallowest water depths (right). Solid hydrate does not form, however, unless the concentration of methane exceeds what can be dissolved. Because seawater is never saturated with methane, hydrate is not found within the ocean itself. Only in some places within the sediments does the methane concentration rise sufficiently high to allow hydrate to fill the interstices between mineral grains, occasionally to be exposed as outcrops on the seafloor (as can be found in the Gulf of Mexico).