Abstract It is generally accepted that the amount of gas in the world's gas hydrate accumulations exceeds 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 has been gas hydrate production testing conducted at the Mallik site in Canada's Mackenzie Delta. The Mallik Gas Hydrate Production Research Well Program has 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 US Department of Energy and the US 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 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 USA, marine gas hydrate research drilling, coring and logging expeditions launched by the national gas hydrate programmes in Japan, India, China and South Korea have also contributed significantly to our understanding of how gas hydrates occur in nature and have provided a much deeper appreciation of the geological 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.

Abstract Vertical seismic profile (VSP) data acquired at the Japanese Petroleum Exploration Co., Ltd. (JAPEX)/Japanese National Oil Corporation (JNOC)/Geological Survey of Canada (GSC) Mallik 2L-38 gas-hydrate research well, Mackenzie delta, Northwest Territories, Canada, were analyzed and combined with surface seismic and downhole well-log data to (1) estimate gas-hydrate concentration around the well and (2) characterize the arctic gas-hydrate accumulations using different scale lengths ranging from 0.3 (sonic log) to 60 m (197 ft) (surface seismic). The interval compressional (P-)wave velocities derived from VSP data are somewhat slower than those from the well-log data. Furthermore, the shear (S-)wave velocities derived from VSP data within the depth interval 600-900 m (1968-2953 ft) are about 20% slower than the sonic-log-derived velocity, implying seismic anisotropy. The spectral ratio of downgoing waves indicates that the P-wave attenuation quality factor of non-gas-hydrate-bearing sediments is about 65, whereas that of gas-hydrate-bearing sediments is about 170. The seismically determined thickness of gas-hydrate-bearing sediments inside a 2.3 × 2.4-km (1.42 × 1.49-mi) area surrounding the Mallik 2L-38 well is about 212 m (695 ft). Porosity obtained from well-log data averages 30%. The average gas-hydrate concentration estimated from the surface seismic data is about 43% of the pore space, and a cubic meter (35 cubic feet) of gas hydrate is 164 m 3 (5792 ft 3 ) of free gas. Therefore, the estimated gas content present in the gas-hydrate-bearing sediments is equivalent to 4.5 × 10 9 m 3 /km 2 (4.1 × 10 11 ft 3 /mi 2 ) of gas at the standard conditions (0°C and 1 atmosphere).

Abstract Identification and quantification of gas-hydrate and free-gas reservoirs in unconsolidated sediments using seismic data are important, because seismic methods are the most promising remote sensing technique for the delineation of such prospects. For many years, the seismic method has been used to detect free-gas reservoirs because of their association with high-amplitude events and a characteristic amplitude relative to offset behavior. However, seismic-data-driven analysis for detecting and quantifying gas-hydrate accumulations is uncommon. Gas hydrate in the pore space increases elastic velocities and reduces the bulk density. Because of these properties, coupled with the fact that gas hydrate tends to be more highly concentrated in high-porosity clean sands and sandstones, the combined effect of varying gas-hydrate saturation and reservoir thickness on seismic amplitude is complex. Depending on saturation, the seismic expression of a gas-hydrate accumulation can be that of a low-amplitude event (amplitude blanking) or a high-amplitude event (bright spot). This complex amplitude relationship with respect to gas-hydrate and gas saturations is described in this article and is used to estimate saturations and thicknesses of reservoirs on the basis of a thin-bed analysis of three-dimensional seismic data. Seismic interpretation of gas-hydrate and free-gas occurrences and saturations of the reservoirs agree well with geologic considerations as described by Inks et al. (2009) . Because the quantitative interpretation is based on a reservoir model appropriate for the North Slope of Alaska, it is emphasized that a caution should be exercised to extend this method to other areas.

Abstract In 1995, the U.S. Geological Survey conducted the first systematic assessment of the in-place natural-gas-hydrate resources of the United States. That study suggested that the permafrost-associated gas hydrates on the Alaska North Slope may contain as much as 590 tcf (16.7 tcm) of in-place gas. However, the Alaska North Slope gas-hydrate assessment failed to identify or characterize the nature of individual gas-hydrate accumulations or prospects. Detailed analysis and interpretation of available two-dimensional (2-D) and three-dimensional (3-D) seismic data, along with seismic modeling and correlation with specially processed downhole well-log data, have lead to the development of a viable method for identifying subpermafrost gas hydrate prospects within the gas-hydrate stability zone (GHSZ) and associated sub-gas-hydrate free-gas prospects in the Milne Point area of northern Alaska. This study has revealed a total of 14 gas-hydrate prospects in the Milne Point area. The seismic data, in conjunction with geophysical modeling results from a related study, was also used to further characterize the thickness and concentration of gas-hydrate occurrences within the delineated prospects. A Monte-Carlo-style statistical analysis of the seismic and well-log-derived reservoir data indicates that the gas-hydrate prospects in the Milne Point area may hold about 668.2 (18.9 bcm) bcf of gas.