ABSTRACT

The basal salt shear model entails an initial salt sheet emplacement stage with salt extruding or intruding near the sea floor. Subsequent sediment loading upon the sheet drives salt withdrawal and suprasalt sediment deformation. Salt withdrawal occurs via pure shear within the salt sheet. As sediments thicken over the salt sheet, overpressures develop in a subsalt transition zone. These anomalously high pore fluid pressures facilitate simple shearing beneath the sheets by reducing the effective normal stress, thereby allowing lateral movement of the sheets and their overlying sediments with minimal force.

Evidence supporting the basal salt shear model includes: (1) lithologically distinct transition zones beneath salt sheets, (2) overpressures in these transition zones, (3) stratigraphic sections above salt repeated below salt, (4) compressional features in front of salt sheets, (5) thick sediment escarpments near salt sheet toes, and (6) low velocity zones near salt sheet bases.

Characteristics of the basal salt shear model significantly impact subsalt hydrocarbon exploration. The overpressured shear zone may be a path for hydrocarbon migration, a seal, or a reservoir, depending upon its local characteristics. One may be able to estimate the hydrocarbon column height of a nearby reservoir in contact with the shear zone based on the detection of hydrocarbons in the shear zone. Also, hydrocarbon reservoirs trapped against shear zones may have larger hydrocarbon columns and different updip limits than expected. Furthermore, this model implies that subsalt reservoir sands may be younger and originate from a more basinward depositional environment than otherwise expected. Finally, geophysicists should consider anomalous velocities near the base of salt sheets when depth migrating seismic data.