The Indirect Approach Geologic Features That Favor Gas Hydrate Occurrence

As in oil and gas exploration, a principal role of seismic methods in gas hydrate studies is not necessarily to directly detect the hydrate, but rather to characterize the structure and stratigraphy of the hydrate/gas deposit by obtaining an accurate image of the subsurface structure. Indeed, to date no unambiguous direct hydrate reflections have been observed, so that hydrate occurrence must be inferred indirectly. (Gas, in contrast, has a distinctive seismic signature, and the free gas zone underlying hydrate deposits can often be imaged directly). Before drilling an oil exploration hole, numerous parameters (e.g., stratigraphic information, porosity, permeability, structure, etc.) are investigated to test the likelihood of hitting an oil reservoir. Research to apply a similar strategy for gas hydrate prediction is underway (Collett, 1996). However, our knowledge of gas hydrate formation in nature probably has to improve significantly before we have a reliable set of parameters to investigate the likelihood of gas hydrate occurrences.

Many of these parameters cannot be determined without information from drilling and sampling. Seismic surveys, however, provide detailed 2D and 3D structural images, as well as crucial extensions of stratigraphy away from boreholes. In particular, seismic imaging provides critical maps of:

• layers that are known to contain gas hydrates.

• the "source rock" of the gas, and clues as to whether the gas originates in a deep thermogenic reservoir or in more distributed biogenic source zones.

• conduits that supply fluid and gas into the hydrate stability zone. Layers of high permeability (particularly faults) may allow gas to migrate upward.

• conduits that allow escape of gas through the hydrate stability zone. We still don't know how methane migrates through the GHSZ without getting "trapped" as hydrate. Faults may play a key role in this process.

• possible traps for gas. In particular, it is not clear by what mechanism gas is trapped at the BSR, or whether stratigraphic traps within the hydrate stability zone might focus methane flux, leading to elevated gas hydrate concentration.

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