An Emplacement Model for Allocthonous Salt Sheets with Implications Toward Subsalt Exploration

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An Emplacement Model for Allocthonous Salt Sheets with Implications Toward Subsalt Exploration

R.D. Baud, J.L. Haglund, J. L. Hunt, R.L. Rocque, and A.P. Daigle
Minerals Management Service, 1201 Elmwood Park Boulevard, New Orleans, Louisiana 70123
e:mail richie_baud@smtp.mms.gov

Abstract

A model for Allocthonous salt sheet emplacement is presented to explain observed overthrusting of thick sediment columns above these sheets. This model, termed 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.

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Figure 1.	During stage 1 of the basal salt shear model, the salt sheet is emplaced as a salt extrusion directly onto the sea floor or as an intrusion near the sea floor. During stage 2, sediment loading upon the sheet drives salt withdrawal and suprasalt sediment deformation. Salt withdrawal occurs via pure shear within the salt sheet. During stage 3, sediments thicken over the salt sheet and overpressures develop in a subsalt "transition zone." Anomalously high pore fluid pressures drive simple shearing beneath the salt sheet. Suprasalt sediments are overthrust with the salt.
Figure 2.	The reverse fault seen here protrudes from the base of a salt sheet in the southern Ship Shoal and Ewing Bank areas, Gulf of Mexico. This fault is interpreted as a subsalt transition zone extension. (Seismic line reproduced courtesy of Diamond Geophysical Service Corporation.)
Figure 3.	Repeated section interpreted in the OCS-G 5809 (Ewing Bank 988) #1 and #1ST wells. This results from the Ewing Bank Thrust Fault, which cuts both wells.
Figure 4.	Thick suprasalt overburdens are required to induce overpressures in the transition zone. This overthrust section, near the Sigsbee Escarpment in the Walker Ridge area, Gulf of Mexico, is approximately 3500 feet thick at its leading edge.
Figure 5.	Low velocity zones (LVZ) near the base salt in the OCS-G 12635 (Garden Banks 165) #2 well. These zones may represent subsalt sediments intermixed with salt along a shear zone. Log curves shown are soectroscopy gamma ray (GR), deep induction enhanced borehole corrected resistivity (IDEB), and sonic (DTLN).