Introduction 11 Motivation

The occurrence, distribution, properties, and hydrocarbon reservoir potential of natural gas hydrates in marine sediments continue to be enigmatic questions in marine geoscience. The fact that natural gas hydrate is metastable and affected by changes in pressure and temperature makes its observation and study difficult under laboratory conditions.

As is now well known, gas hydrates are crystalline solids formed of a cage of water molecules surrounding a natural gas molecule under specific conditions of relatively high pressure and low temperature. The supply of gas also must be sufficient to initiate and stabilize the hydrate structure [Sloan, 1990; Kvenvolden, 1993]. These restrictive thermodynamic conditions are satisfied on many continental slopes and rises around the globe. Most information on the regional distribution and geological environment of hydrate occurrence comes from remote geophysical measurements, especially seismic studies. The most common mapping tool is the characteristic BSR—the large bottom-simulating seismic reflector that is commonly associated with the base of high-velocity hydrate and the top of low-velocity free gas. Some promising initial surveys of sub-seafloor electrical resistivity and seafloor complicance also have been carried out. However, all such remote methods require calibration or "ground truth" with in situ data from drill holes. In addition, since hydrate is unstable

under atmospheric conditions it is extremely difficult to recover and retain samples for study in the laboratory. Dissociation of even massive hydrate samples into water and methane gas occurs after only a few minutes of exposure to room temperatures and pressures. Because of this instability, downhole measurements provide the primary ground truth data for the mapping of gas hydrate using surface geophysical surveys. Downhole measurements also provide calibration data for remotely-measured physical properties and the concentrations of hydrate and free gas that variations in these properties represent. Especially critical relations are those between velocity increase and hydrate concentration, velocity decrease and gas concentration, and similar relations for electrical resistivity. Therefore, in situ detection methods that provide ground truth information in drill holes hold an important key to studying gas hydrate occurrence and concentration.

In situ methods have improved our understanding of gas hydrate occurrence and distribution significantly in recent years. Given the continued scientific interest in gas hydrate investigations, the planning and use of new in situ detection methods and technologies will undoubtedly continue to advance in the future. Simply developing a new strategy to measure in situ properties in closely spaced drill holes, for example, could produce a three-dimensional isopach map of the distribution of hydrate below the seafloor. Such maps would provide ground truth information over a significantly wider area than in a single drill hole and more fully constrain the interpretation of a seismic survey over the same area.

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