An Emplacement Model for Allochthonous Salt Sheets with Implications Toward Subsalt Exploration
A model for allochthonous salt sheet emplacement is presented to explain overthrusting of thick sediment columns above these sheets. This model proposes that initial emplacement, either as extrusive sheets or horizontal intrusions near the sea floor, sometimes yields to shearing along thrust fault zones immediately beneath the salt sheets. Anomalously high pore fluid pressures drive shearing beneath the sheets by reducing the effective nominal stress, thereby fadlitating lateral movement of the sheets and their overlying sediments.
Several observations support this model: (1) stratigraphic sections above salt repeated below salt, (2) thick sediment escarpments near the toes of some salt sheets, (3) compressional features in front of some salt sheets, (4) low velocity zones near the base of some salt sheets, (5) lithologically distinct zones beneath many salt sheets, and (6) overpressured sediments below many salt sheets.
The mode of salt sheet emplacement has significant implications toward subsalt hydrocarbon exploitation. Our model is consistent with the following implications: (1) subsalt strata may be younger and originate from a more landward depositional environment than otherwise expected, (2) the overpressured shear zone may be a path for hydrocarbon migration, a seal, or a reservoir, (3) one may be able to estimate the hydrocarbon column height of a nearby reservoir based on the detection of hydrocarbons in the shear zone, and (4) hydrocarbon reservoirs trapped against shear zones may have significanty different updip limits and depletion patterns than expected.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California