Unlike most sediment and rock lithologies, natural gas hydrates cannot be readily sampled at depth and studied at the surface. They are highly unstable at or near standard temperatures and pressures and, when cored by conventional equipment, will rapidly dissociate into a mixture of gas, water and sediment before they can be retrieved and examined. Direct sampling is possible only with specialized apparatus, such as pressure-core equipment; even in this case, however, dissociation begins immediately upon exposure of recovered samples, significantly altering original in situ conditions and thereby rendering tenuous the results of any detailed study, including characterization of hydrate distribution over a given interval of recovered sample. These realities have made it necessary for geosci-entists to discover or devise various proxy methods for identifying natural gas hydrates in the subsurface. At this point in the history of hydrate study, there exists a sizeable number of such proxies. They primarily include: seismic reflection events ("bright spots"); velocity changes (between hydrate-bearing and non-hydrate-bearing intervals); sonic transit times; log resistivities; gas volumes; salinity changes (chloride concentration); methane/ethane ratios; and thermal anomalies. To a varying extent, the utility of these identifiers has been confirmed by limited sampling of actual hydrates using specialized coring apparatus. However, because such unequivocal examples of sampling are few and specific to certain settings, they cannot be justifiably extended to all hydrate occurrences.
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