Gas hydrates are ice-like minerals that form at the low temperatures and high pressures in the deep sea. Hydrates contain gases, such as hydrocarbons, that glue themselves inside symmetrical cages of water molecules to form hydrate crystals. Research submarines are the ideal tool to study hydrates under natural conditions. Submarines make a profound difference in our understanding of gas hydrate because they allow us to see hydrates forming mounds on the seafloor. The mounds are covered with white and orange bacterial mats as well as various filter-feeding bivalves and other strange organisms. Mounds are sometimes surrounded by rings of chemosynthetic organisms such as tube worms and mussels. Actually, the seafloor scene from a research submarine gives the strong visual impression of a silent, serene, and beautiful garden.

Although gas hydrates contain hydrocarbons that are colorless, not all of them are white like snow. Some hydrates from the deep Gulf of Mexico are richly colored in shades of yellow, orange, or even red. The ice-like masses are beautiful, and contrast with the dull gray of deep sea muds. Hydrates from the Blake-Bahamas Plateau in the Atlantic Ocean can be gray or blue. Scientists would like to explain why hydrates show these colors, but so far there is little agreement on reasons. A number of different factors, including oil, bacteria, and minerals, are probably at play in producing the rainbow hydrates .

Almost all gas hydrates are found by drilling in sediments at 10s to 100s of meters depth, but the gulf is different. The Gulf of Mexico is the best natural laboratory in the world for studying gas hydrates because they outcrop on the seafloor as mounds and can be easily sampled in sediments.

Scientists aboard research vessels first found gas hydrates in the deep waters of the gulf in 1983 by taking core samples at sites where oil was naturally seeping out of the bottom. Hydrates have since been recovered in cores from water depths as shallow as 425 meters and at depths greater than 2000 meters.

Methane hydrates

Hydrocarbons captured in gas hydrates come from various sources. Methane hydrates are the most abundant type in the earth's ocean. They occur in deep water and on land in polar areas. A methane molecule only contains one carbon atom and four hydrogen atoms (CH₄). It is a small molecule, and by itself can form one simple type of hydrate, which is known as structure I. Free methane gas appears to exist below hydrate in some areas, sealed in by the overlying impermeable hydrate layer.

In addition, bacteria in marine sediments naturally produce enormous volumes of methane when they feed on plant debris washed into the gulf from rivers and swamps. This "biogenic" methane is often trapped in layers of hydrate that simulate the contours of the seafloor, and can be detected by various seismic techniques commonly used in oil and gas exploration.

Although methane hydrates are most abundant, Texas A&M geochemists find them the least interesting because the methane molecule is so simple.

Surprises in the gulf

Natural oil and gas seeps in the deep Gulf of Mexico also deliver bigger hydrocarbon molecules to the seafloor. An exciting discovery is that larger crystal forms of hydrate exist in the gulf. One of these is structure II hydrate, which can contain methane and other hydrocarbons such as propane, a three-carbon hydrocarbon molecule normally found in petroleum of thermal origin. Structure II hydrate was first produced in laboratory experiments, then in 1983 we made the first discovery of structure II hydrate in the natural environment at a depth of 530 meters.

In 1993 we discovered the first evidence of structure H hydrate in nature at a similar water depth near Jolliet Field. This rare hydrate forms "cages" large enough to hold molecules much bigger than methane. For example, hydrates at Jolliet Field contained abundant iso-pentane, a big, branched-chain hydrocarbon molecule with five carbon atoms. We are excited by the real possibility of discovering other, previously unknown types of hydrate crystals in our natural laboratory, the Gulf of Mexico.

It is no accident that unusual gas hydrates were discovered near Jolliet Field, a site now occupied by a huge oil platform. Oil and gas continually migrate from great depths in the earth's crust toward the gulf floor. Some of the oil and gas is trapped below the sea bottom, but much migrates toward the seafloor where hydrates form. For this reason, natural oil and gas seeps and their associated hydrates are studied as a guide to the presence of subsurface fields.

A future energy source?

At the sea surface, decomposing hydrate feels like cold Alka Seltzer fizzing and popping on your skin. It's fun to light gas hydrates with a match and watch the hydrocarbons burn like a candle, leaving behind only slightly salty water. Watching gas hydrate burn and produce heat shows its value as an energy source. Right now there is worldwide interest in exploiting energy from hydrates.

Published estimates of the total energy reserves trapped in methane hydrate vary considerably, but all the numbers emphasize this single point-the resource potential of methane in gas hydrate exceeds the combined worldwide reserves of conventional oil and gas reservoirs, coal, and oil shale by a wide margin. It is no secret that the world's production of conventional fossil fuels will begin to decline sometime during the next century. At that time, some oil companies may go extinct or we might start referring to them as hydrate companies. A great opportunity to develop new technology will occur, and ocean scientists will have the chance to contribute to the future of our energy supply during that time of change.

The first steps towards hydrate gas production will occur soon, with the goal to recover natural gas by decomposing hydrates from offshore deposits. A research team from India is getting ready to begin an applied research effort to drill wells in deep water, and to learn how to produce gas hydrates. Similar research is proceeding offshore near Japan. The hydrate race has already begun.

The hydrate experiment

Gas hydrates can be grown and carefully studied in the laboratory, but the problem is that laboratory experiments are only simple simulations. The experiments, being simple, are always outfoxed by nature. For example, laboratory hydrates grow only slowly and they are more like slush than ice.

We grew gas hydrates on the deep seafloor for the first time in 1995, using a research submarine in a natural setting (See "Voyage to the depths"). This pivotal experiment showed that hydrate can be rapidly produced from natural gas venting into the sea, emphasizing that economic exploitation of gas hydrates is actually closer than we thought.

Our experimental precipitation of gas hydrate on the gulf floor expands the horizon for energy production. Until now, most thinking centered on gathering hydrate known to be buried in sediments, and allowing it to decompose into fuel and water. Our experiment shows that there might be another way to use gas hydrates to produce energy.

It is possible to drill for conventional gas reservoirs in the sea at great water depths, but it is not always possible to produce that gas because of economic problems associated with long pipelines across unstable continental slopes. Furthermore, pipelines in deep, cold water tend to become plugged with hydrates. Energy companies actually support research to prevent hydrates from forming!

We have an open mind about hydrates as a future energy source. Enormous volumes of natural gas vent naturally to the deep waters of the gulf and at many locations on continental margins offshore. This natural gas supply currently goes unused, and eventually escapes to the atmosphere. At the same time, conventional gas reserves in ultra-deep water will probably never be produced due to the high costs. Why not produce gas hydrates industrially at the seafloor from escaping gas? Manufacturing hydrates from gas currently being lost to the water column could help buffer future energy shocks.

There is a formidable obstacle to using hydrates as fuel. When removed from its high pressure, low temperature environment hydrate decomposes and releases the hydrocarbon gas contained in it. We do not yet have a way to safely transport large amounts of hydrate to production facilities on land.

One new approach to the hydrate transportation problem is to pelletize hydrate or to inflate large bladder-like blimps with hydrate in the deep sea. Submarines could then tow the hydrates to shallower water near the continental shelf, where they could be slowly decomposed to yield fuel and water. Perhaps chemical engineers could design additives to make hydrates more stable at lower pressures and higher temperatures. If they can, hydrate might actually be safer to transport in conventional ships than liquefied natural gas.

Climate change

Producing hydrates from naturally venting gas has implications for global warming with respect to two greenhouse gases, methane and carbon dioxide. Methane is the main constituent of gas hydrates, and is also a fast acting, high-impact greenhouse gas. Methane is at least an order of magnitude more effective as a short-term greenhouse gas than carbon dioxide. Although it is a matter of controversy among scientists, gas hydrates could have impacted the atmosphere several times during the last two million years. Some believe that fluctuating sea levels of the ice ages could have rendered large volumes of gas hydrate unstable, releasing great volumes of methane to the atmosphere.

Producing gas hydrates would redirect methane away from the atmosphere. Using hydrate methane industrially would convert it to carbon dioxide, actually decreasing the short-term effect on atmospheric chemistry and global change. In addition, methane is an environmentally cleaner fuel than oil, coal, or oil shale which all have an immense environmental impact during production and combustion.

Carbon dioxide itself is a gas that forms hydrates. Much recent research focuses on understanding how to deliberately create carbon dioxide hydrates. Perhaps we can find a way to trap carbon dioxide at the seafloor where it would eventually be buried by sediment.

Some giant natural gas reservoirs in Southeast Asia contain more carbon dioxide than hydrocarbons. Normally the carbon dioxide would be separated from the hydrocarbon gas and released to the atmosphere. Separating carbon dioxide at the seafloor and burying it as hydrate would be more environmentally friendly.

Clearly, there is a bright future for gas hydrate research to ensure future supplies of clean-burning energy, to potentially dispose of greenhouse methane, and to understand the role of hydrates as an agent of climate change over geologic time. The question of hydrates as a niche habitat for life also deserves special study (See "Lair of the 'Ice Worm'"). We look forward to addressing these questions in future research projects.

In the serene hydrate environment colorful mats of bacteria blanket sediment over and around a hydrate mound, surrounded by tendril-like tubes of chemosynthetic worms. Free hydrocarbon gas bubbles out of the sediment to begin its 540-meter ascent to the sea surface (Photo by Jonathan Blair).

[26K] Scientists find hydrates around hydrocarbon seeps in the Gulf of Mexico. The seeps often signal the presence of oil and gas reservoirs far below the seafloor.

This mound of gas hydrate grew large enough to break free of the seafloor's surface. Sediment still drapes the top of the mound but on its underside the hyrate burrows of ice worms lie exposed. (Photo by Charles Fisher)

Structure I hydrate crystals form cages that can only hold small hydrocarbon molecules inside. These commonly hold a single molecule of methane (CH₄.

Crystal with more complex structures can contain larger hydrocarbon molecules. We predicted Structure II hydrates would exist based on laboratory experiments, then discovered them in nature at Jolliet Field in the Gulf of Mexico.

Structure H crystal cages can contain iso-pentane, a relatively large, branched-chain hydrocarbon.

Texas A&M oceanographer Ian MacDonald prepares a pressurized container in which to store hydrates for later study in the laboratory. Without the specially designed canister hydrates decompose into gas and water as they are raised from the bottom. (Photo by Charles Fisher)

This colorized image of the ocean surface taken from the space shuttle makes the sea and clouds look like an artist's abstract dabs and brushstrokes. The bright streaks are oil slicks produced by hydrocarbons seeping naturally from seafloor vents.