Can Fractures in Soft Sediments Host Significant Quantities of Gas Hydrates

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The Gulf of Mexico Hydrate Research Consortium has collected several types of data in and around Mississippi Canyon Lease Block 798 (MC798), an area of the northern Gulf of Mexico where fine-grained sediment occurs at the sea floor and where hydrates have been sampled. Swath bathymetry, heat-flow measurements, core samples, and subbottom profiles were collected. Hydrate was grown in the laboratory in sediments subsampled from the cores to demonstrate that the surficial sediments in MC798 are conducive to hydrate formation. Herein, data are presented and results discussed. It is postulated that significant quantities of hydrate could form in fine-grained sediments by filling fracture porosity produced by polygonal faulting. Analyses of cores combined with laboratory experiments indicate that conditions in MC798 are conducive to the formation of polygonal faults. Heat-flow measurements indicate that the hydrate stability zone is about 400 m (1312 ft) thick. Its upper 100 ms or so appears on two-dimensional (2-D) subbottom profiles to be fine grained. Small, near-vertical fractures indicated by features called brooms are common there. Thus, it is possible that a polygonal fault system exists in the upper 100 ms (75 m [246 ft] at 1500 m/s [4921 ft/s]). It is acknowledged that 2-D profiles cannot demonstrate this conclusively. Conclusive proof would require a three-dimensional (3-D) data set with sufficient resolution to demonstrate interconnectivity among the small faults. If polygonal faulting exists, gas and water could circulate through the fractures and be exposed to smectite-rich clays, a situation favorable to hydrate formation. X-ray images of pressure cores have documented hydrate accumulation within small, nearly vertical fractures in fine-grained sediments. Thus, it is possible that polygonal fault systems could host significant accumulations of hydrate in the Gulf of Mexico.