Estimates of worldwide hydrate resources are large, but they are also uncertain because of inherent difficulties in determining the amount of gas hydrate present in ocean sediments. Estimates of gas-hydrate concentrations across a deepwater site can vary widely. For example, estimates of the volume of gas existing in gas hydrates and as free gas on Blake Ridge offshore South Carolina (USA) range from about 70 trillion m3 over an area of 26 000 km2 (Dickens et al., 1997) to about 80 trillion m3 for a larger area of 100 000 km2 (Holbrook et al., 1996). Discrepancies between some estimates of hydrate concentrations can partly be attributed to poor understanding of how gas hydrates are distributed in their host sediments. In particular, estimates based on seismic measurements, if not supported by reliable rock physics models and by in-situ observations, can be inaccurate.

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