Geology of Marine Gas Hydrates and Their Global Distribution

Abstract It is generally accepted that the amount of gas in the world's gas hydrate accumulations exceed the volume of known conventional gas resources. Researchers have long speculated that gas hydrates could eventually be a commercial producible energy resource yet technical and economic hurdles have historically made gas hydrate development a distant goal rather than a near-term possibility. This view began to change in recent years with the realization that this unconventional resource could possibly be developed with existing conventional oil and gas production technology. The most significant development was gas hydrate production testing conducted at the Mallik site in Canada's Mackenzie Delta in 2002. The Mallik 2002 Gas Hydrate Production Research Well Program yielded the first modern, fully integrated field study and production test of a natural gas hydrate accumulation. More recently, BP Exploration (Alaska) Inc. with the U.S. Department of Energy and the U.S. Geological Survey have successfully cored, logged, and tested a gas hydrate accumulation on the North Slope of Alaska known as the Mount Elbert Prospect. The Mallik 2002 project along with the Mount Elbert effort has for the first time allowed the rational assessment of the production response of a gas hydrate accumulation. In addition to the gas hydrate production tests in Canada and the US, marine gas hydrate research drilling, coring, and logging expeditions launched by the National gas hydrate programs in Japan, China, South Korea, and India have provided a much deeper appreciation of the geologic controls on the occurrence of gas hydrates. With an increasing number of highly successful gas hydrate field studies, significant progress has been made in addressing some of the key issues on the formation, occurrence, and stability of gas hydrates in nature. This report deals with the assessment of the geologic and engineering factors that control the ultimate resource potential of gas hydrates. This assessment will be conducted mainly though the examination of several of the more successful international gas hydrate research efforts. Introduction Natural gas hydrate is a combination of two common substances, water and natural gas. If these meet under conditions in which pressure is high enough and temperature is low enough, they join to form a gas hydrate. Vast volumes of sediments in the earth's oceans and polar regions are conducive to gas hydrate formation. Numerous research programs have shown that natural gas hydrate is widespread in permafrost regions and beneath the sea in sediments of outer continental margins. Natural gas hydrates, commonly also known as methane hydrates, are clathrates, (from the Greek and Latin words for " cagework??) meaning that " guest?? gas molecule is encaged in a " host?? framework of water molecules. The empty cagework of water is unstable, and requires the presence of encapsulated gas molecules to stabilize the clathrate crystal, at temperatures both above and below the freezing point of water (Figur 1). Chemists have known about gas hydrates for almost 200 years, but mainly as a laboratory curiosity. The petroleum industry began to take an interest in hydrates in the 1930s when gas-hydrate was found plugging natural gas pipelines. The first suggestions that natural gas hydrates might be present in the geosphere can be attributed to Russian scientists (Makogon et al, 1971; Trofimuk et al, 1973; Makogon, 1981), this was confirmed as scientists on deep-sea drilling expeditions discovered that gas hydrates did occur in deepwater sediments along continental margins (Figure 2).

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