Stress Evolution in Sediments Around a Rising Salt Diapir

Abstract:

We model the evolution of a salt diapir during sedimentation and study how deposition and salt movement affect stresses within the sedimentary wall rocks. We model the salt as a solid visco-plastic material and the sediments as a poro-elastoplastic materials, using a generalized Modified Cam Clay model. The salt flows because ongoing sedimentation increases the average density within the overburden sediments, pressurizing the salt. Stresses rotate within the sediments, such that the maximum principal stress is perpendicular to the contact with the salt. The minimum principal stress is in the circumferential direction, and drops near the salt. The mean stress increases near the upper parts of the diapir, leading to a porosity that is lower than predicted for uniaxial burial at the same depth. We built this axisymmetric model within the large-strain finite-element program Elfen. Because we simulate sedimentation simultaneously with the movement of the salt, our study offers two major achievements distinct from previous work on salt-diapir and sediment interaction: the salt is not kinematically prescribed and the stresses within the basin develop as a function of both the depositional process and the loading from the salt. Our results highlight the fact that forward modeling can provide a detailed understanding of the stress history of sediments close to salt diapirs; this is critical for predicting stress, porosity, and pore pressure in the wall rocks and more generally understanding earth processes related to salt systems.