Investigations of the structural and hydrologic context of gas hydrate deposits on Hydrate Ridge, Oregon.

摘要

Scientific interest in seafloor gas hydrate deposits has grown over the past two decades with the recognition of the significant volume of methane stored in these compounds. Speculation on their influence on the global carbon cycle, marine slope stability, and as a future energy resource has dominated the literature, yet much remains to be learned about the fundamental controls on their distribution in the subsurface. This work uses data from Ocean Drilling Program Leg 204 to examine the influence of lithology, geologic structure, and stress state on the distribution of gas hydrate at Hydrate Ridge, Oregon. The primary results of these studies highlight the importance of permeability in controlling fluid flow and ultimately hydrate distribution.; A study of cold thermal anomalies identified in continuous down-core records of thermal structure acquired by an infrared camera was combined with core based observations of lithology to examine the linkages between lithologic permeability changes and hydrate distribution. The spatial association between hydrate and silt horizons is stronger at the crest of the ridge than on the flanks, a trend that is consistent with focused up-dip migration of fluids along permeable lithologic conduits.; The influence of fracturing and stress state on hydrate distribution and morphology were examined in a study of resistivity-at-the-bit images. Chaotic hydrate-filled fractures were found on the ridge crest, where no preference in fracture orientation was identified. In contrast, the primary orientation of the hydrate-filled fractures on the ridge flanks parallels the strike of the ridge indicating a rotation of the principal stress directions across the ridge and a change in the phase of the fluid source of the hydrate deposits.; Finally, geophysical log data and laboratory consolidation tests from the eastern slope basin point to variations in fluid pressure that correspond to changes in sedimentation rate. A lack of permeable conduits in the uppermost 130 mbsf creates a highly overpressured section unlikely to channel fluids into the ridge.; The ultimate picture of hydrate distribution painted by these studies is one of heterogeneity related to permeability and stress state. Such an environment is not an ideal target for resource development.