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ABSTRACT

The emplacement of allochthonous salt bodies in the Northern Gulf of Mexico and their subsequent deformation to form secondary salt features involves the upward movement of salt along discrete feeder conduits. The detachment of allochthonous salt from a deeper source results in collapse of these conduits. Structural disruption associated with this collapse creates a permeability pathway to allow enhanced fluid migration from depth into the shallower section. Some of the high pressure fluids migrating upward along these permeability conduits will impinge on a permeability barrier created by the horizontal to sub-horizontal base of allochthonous salt sheets. Additional high pressure fluids associated with shale compaction and dewatering will also move upward to the base of salt permeability barrier. The constant influx of high pressure fluids into the zone immediately below salt prevents the shale in this zone from undergoing normal compaction, resulting in the formation of a lithologically distinct “gumbo zone.” This gumbo zone has been encountered in many of the subsalt wells drilled in the Gulf of Mexico. Abnormally high pore pressures are often associated with this gumbo zone beneath the salt sheets covering the southern shelf area, offshore Louisiana. Formation pressure gradients within this zone can be as much as 0.05 psi/ft (0.9 ppg) above the regional pressure gradient.

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