Abstract

Several lines of evidence support kilometer-scale upward migration of fluids in the Gulf of Mexico Basin: discharge of hypersaline brines at the sea floor; long-term, natural hydrocarbon seeps and microseeps; gas chimneys; lead-zinc mineralization in salt dome cap rocks; and allochthonous brines in Cenozoic sediments. We explore the hypothesis that upward fluid transport in geopressured sediments is caused by buoyancy-driven propagation of isolated methane-filled fractures. In other words, instead of fluid migrating along a fixed network of interconnected pores or fractures, fluid enclosed within an isolated fracture is transported upward by hydrofracturing the mechanically weak geopressured sediments. Thus, the fluid-filled fracture propagates upward through the sediments. Hydrofracture is driven by the pressure difference (buoyancy) between the enclosed methane and the surrounding sediments. Our results show that methane-filled fractures with half-lengths of a few meters should propagate upward through geopressured sediments with velocities of hundreds of meters per year or higher. As methane-filled fractures increase in volume and decrease in density with decreasing confining pressure, they develop the potential to entrain and transport more than 1000 kg/m3 of oil or brine. Methane-filled fractures should propagate to the surface unless they are trapped beneath a layer that has high fracture toughness, such as salt, or are absorbed when they intersect a permeable (>10 md) sand layer.

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