Natural gas hydrate occurs worldwide: in oceanic sediments on continental slopes; in deepwater sediments of inland lakes and seas; and in both continental and continental shelf polar sediments. As discussed in the previous section, in oceanic sediments, where water depths exceed about 300m and bottom water temperatures approach 0°C, gas hydrate is found at the seafloor and down to sediment depths of about 1,100m. Te typical depth range for hydrate stability lies 100–500m beneath the seafloor. In polar continental regions, gas hydrate can occur in sediments at depths ranging from 150 to 2,000m. Occurrences of hydrates within the gas hydrate stability zone (GHSZ) are affected by numerous additional factors, including availability of gas, water and geological controls. About 98% of the gas hydrates are believed to be concentrated in oceanic sediments, while the other 2% are in polar landmasses.
8.4.1 How Much Gas Hydrate Exists?
Figure 8.24: Gas hydrate resource pyramid. Gas hydrates exist in a variety of forms that pose different opportunities and challenges for energy resource exploration and production. The left axis displays lithology of the host sediment. The right axis shows associated estimates of natural gas resources. Gas hydrate-bearing sands are the most feasible initial targets for energy recovery. Other occurrences, such as gas hydrate-filled fractures in clay-dominated reservoirs, may become potential energy production targets in the long-term future.
Gas volumes are often cited in units of trillion cubic feet (Tcf), and there are approximately 35.3 cubic feet in a cubic metre. It is estimated that resources of methane in natural hydrate reservoirs range anywhere from 105 2.8 x 106
to Tcf, or around 2.8 x 1015 to 8 x 1015 m3 , indicating that
more carbon is contained in methane hydrate than in all other organic carbon reservoirs on earth combined. Tese estimates, however, include hydrate in low-grade shale
deposits as well as in high-grade sand deposits. Only a fraction of the methane sequestered in global gas hydrate deposits is
likely to be both concentrated and accessible enough to ever be considered a potential target for energy resource exploitation. Te relative amounts of gas hydrate in the global system
can be illustrated by the hydrate resource pyramid (Figure 8.24), which captures the distribution of sequestered methane among the major types of global gas hydrate deposits. Only the hydrates at the top of the pyramid – a small subset of the hydrate deposits – are likely to be considered viable as a source of commercial quantities of natural gas.
Figure 8.25: Seismic attribute co-blend map (RMS amplitude/coherence) showing sand channels in excess of 150m thick. The bright yellow and orange colours highlight zones with high seismic amplitudes characteristic of sand channels. The displayed interval shows several generations of sand deposits within the gas hydrate stability zone. If charged with gas they could form prospective targets for gas hydrate exploration.
Mud and coarse silt occurrences: At the top of the pyramid lie high permeability sediments in permafrost areas. Te amount of gas hydrate in these settings globally is relatively small, but permafrost- associated gas hydrates might be the easiest to commercialise, particularly in areas with well-developed infrastructure from conventional hydrocarbon production, such as the Alaskan North Slope. Gas hydrate resources housed in marine
sand reservoirs are also obvious major targets for any longer-term development of gas hydrates as a resource. Highly permeable marine sands with moderate to high gas hydrate saturations are considered the best targets for resource development. Recent logging-while-drilling in the Gulf of Mexico has identified geologic units with inferred hydrate saturations as high as 80%. Reservoir quality is expected to increase
with increasing grain size. However, the primary control of importance may
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Reichel and Gallagher, 2014
Courtesy: Ray Boswell
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