OIL TRANSPORT BY SOLITARY WAVES IN ELASTIC POROUS MEDIA: A CASE STUDY OF THE EUGENE ISLAND FIELD, GULF OF MEXICO BASIN

Hydrocarbons in the Eugene Island field, offshore Louisiana that are concentrated in Plio-Pleistocene sand reservoirs at depths between 0.5 and 3 km appear to have risen from older Tertiary source sediments at depths of at least 4.5 km at surprisingly high rates (up to 100’s of m/yr) given the low permeability of the intervening predominantly mud and shale sequence. Much of the hydrocarbon flow appears to have been channeled episodically along the Red growth fault, whose permeability is a sensitive function of fluid pressure. Strongly overpressured elastic porous media that possess such a permeability relationship are predicted from theory to generate solitary waves, which have the potential to transport pore fluids at rates much greater than expected from conventional Darcian mechanics. The purpose of the present study was to evaluate the rate of formation and oil transport for solitary waves generated under the conditions present at Eugene Island.

A two-dimensional numerical model of the formation of the Eugene Island field incorporating the deposition and compaction of its sediments, the generation of oil and methane from kerogen, the transport of heat, and the flow and pressure evolution of oil, water, and gas indicated pore pressure in the source sediments to increase at an average rate of 30 Pa/yr. A further one-dimensional model of an oil-saturated vertical flow path indicated that solitary waves could form in the hydrocarbon source region but only within a narrow window of very low permeability between 10⁻²⁴ and 10⁻²⁵ m² and at high degrees of overpressuring between 91 and 93% of lithostatic. Solitary waves were able to reach velocities on the order of 10⁻³ m/yr, several orders of magnitude greater than the background Darcian flow, but were only able to ascend 1-2 km before dissipating to ambient fluid pressure and porosity. Thus, solitary waves are unlikely to have played a significant role in charging any but the deepest reservoirs at Eugene Island based on a model of steady pressure increase and diffusive pressure change. This model for oil transport by solitary waves could be more important in fields where hydrocarbon source sediments and reservoirs are separated by less than one kilometer.