Many scientists predicted that gas hydrate forms in fractures or lenses in fine-grained sediments, but only in the last decade were gas hydrates found in complex fracture systems on continental margins. Gas hydrate-filled fractures were captured on both in situ borehole images and in x-ray imaged pressure cores. These new discoveries of gas hydrate as fill in fractures have been a boon to the gas hydrate community, yet, very little is known about the features and dimensions of a gas hydrate-filled fracture reservoir. Geophysical prospecting techniques, such as exploration seismic and controlled source electromagnetic surveys have not been able to detect a gas hydrate-filled fracture reservoir.
In this dissertation, I aim to define the marine gas hydrate-filled fracture reservoir. Three offshore drilling expeditions, known as the gas hydrate Joint Industry Project Expeditions 1 and 2 in the Gulf of Mexico and the Indian National Gas Hydrate Program Expedition 1 on the Indian continental margins, are the sources of the geophysical well log and core data used in this dissertation.
In the following five chapters, I show that gas hydrate often forms in shallow, unconsolidated, fine-grained sediments in near-vertical fractures. Gas hydrate-filled fractures are planar features, but likely only extend a few meters in breath. Gas hydrate-filled fracture systems are likely controlled by in situ methanogenesis and or methane solubility.
The near-vertical nature of the gas hydrate-filled fractures causes anisotropic conditions in geophysical logging measurements made in vertical boreholes. Measured resistivity is most affected by the anisotropy, producing high resistivities in near-vertical gas hydrate-filled fracture systems. Thus, using measured resistivity to calculate gas hydrate saturation produces unreliable results.
Gas hydrate-filled fractures in the same hole usually have similar strike orientations. The fracture orientations are used to determine the shallow stress directions in hole. The stress directions orient with bathymetric contour lines showing shallow stress is chiefly affected by changes in seafloor topography.
|Advisor:||Goldberg, David S.|
|School Location:||United States -- New York|
|Source:||DAI-B 71/03, Dissertation Abstracts International|
|Subjects:||Geophysics, Petroleum Geology|
|Keywords:||Continental margins, Fractured reservoirs, Gas hydrates, Well logs|
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