Fluid Migration and Formation Pressures Associated with Allochthonous Salt Sheets in the Northern Gulf of Mexico

William M. House, John A. Pritchett

Allochthonous salt sheets emplaced into Pliocene and Pleistocene age section in the Northern Gulf of Mexico are often associated with formation pressures in excess of the expected regional pressures. Abnormally high pressures are encountered in the section immediately below the base of salt where formation pressure gradients can be as much as 0.04 psi/ft (0.8 ppg) above the regional pressure gradient. These abnormally high pressures are associated with a lithologically distinct gumbo zone, and the pressures within this zone can approach formation fracture pressures. Pressure gradients below the gumbo zone regress to the regional gradient. Formation of the gumbo zone is facilitated by a constant influx of high pressure fluids through an interval adjacent to the base of sal . These high pressure fluids prevent the section from undergoing normal compaction and dewatering. Over a geological time frame this zone acts as a flushing zone. The emplacement of allochthonous salt bodies and their subsequent deformation to form secondary salt features, such as salt sheets and rootless salt domes, involves the upward movement of salt along discrete conduits. The collapse of salt feeder conduits during this deformation process creates permeability pathways which allow for enhanced fluid migration into shallower section. Field evidence indicates that hydrocarbons in some currently producing fields have migrated along collapsed salt feeder conduits and through holes in the salt sheets to reach their current position.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995