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

The text of this paper is available without cost via direct download
from the web site,
"http://www.gomr.mms.gov/homepg/whatsnew/papers/gp9602.html", and is in
Adobe Acrobat's pdf format. The paper's corresponding figures may be
viewed directly from a browser. Within the paper, the authors introduce the
basal salt shear model and provide evidence to support this mode of
salt sheet emplacement. The model can also place constraints on
hydrocarbon exploration. The following is the article's abstract
taken from the web page.

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
occursvia 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
apath 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.