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Solitary Waves in Low Permeability Sediments: A Possible Mechanism for Rapid Methane Transport in the Eugene Island Field, Gulf of Mexico Basin


Hydrocarbons in the Eugene Island Field, offshore Louisiana appear to have migrated through 2-4 km of low permeability muds and shales at surprisingly high rates as high as 100's of meters per year (m/yr) to reach their present day shallow Plio-Pleistocene reservoirs. Recent seismic and thermal data suggest that hydrocarbons may have traveled in part as discrete and episodic fluid pressure pulses along the Red growth fault. Initial calculations in the present study were aimed at evaluating the ability of solitary waves to transport oil. These calculations showed solitary wave formation to be limited to a narrow range of source sediment permeabilities between 10-24 and 10-25 m and for the waves to travel at a maximum rate of order 10-3 m/yr for distances of 1-2 kilometers. The flux of oil transported by the solitary waves was found to be too low to account for the amount of oil present in the Eugene Island reservoirs. Solitary waves however could be much more effective at transporting methane because of the lower density and viscosity of methane compared to oil. This hypothesis was tested by modeling the Red fault as a one-dimensional vertical profile saturated with methane, with a maximum of about 5.12 MPa of overpressure emplaced instantaneously at a depth of 4.5 km. Though work is ongoing to improve the accuracy and stability of the numerical solution, the results obtained thus far show that methane-saturated solitary waves can migrate vertical distances of more than a kilometer at rates of at least 100's of m/yr. In contrast to oil-saturated solitary waves, methane-saturated solitary waves in the models do not leave behind a wake of elevated fluid pressure that inhibits repetitive solitary wave formation. However, in order for solitary waves to form, pressure generation rates significantly greater than the 30 Pa/year predicted from compaction disequilibrium and hydrocarbon generation would be needed to overcome the high diffusivity of methane at a fault permeability between 10-18 and 10-19 m. Thus, provided a geologic mechanism exists for rapid fluid pressure generation, solitary waves would be capable of very rapid methane transport in low permeability sediments.