Mahdi Heidari

Initiation and growth of salt diapirs in tectonically stable settings: Upbuilding and megaflaps

We use finite element modeling to show that upbuilding can be a significant component of salt diapir growth in tectonically stable systems when basin sediments are elastoplastic mudrocks. The ability of such sediments to deform plastically and the dependence of their strength on confining pressure enable structural thinning, which allows salt to pierce through a relatively thick roof. Once pierced, the originally continuous roof uplifts to form a megaflap. We show that the evolution to an upturned megaflap adjacent to a salt stock causes shortening of the bedding layers in the radial and vertical directions and extension in the hoop (circumferential) direction. These deformations lead to significant shear strains within the sediments; as a result, in some areas within the upturned megaflap, mudrocks have reached their maximum level of shear resistance and are failing. Thinning and shear failure of sediments are also significant near salt walls, despite the absence of out-of-plane deformation. We illustrate that cross-sectional area and bedding line lengths are not necessarily preserved. Based on our results, we re-evaluate traditional assumptions of kinematic restoration and show that established workflows may not properly restore salt systems that interact with shallow plastic sediments. Finally, we show that when wall rocks are deformable, salt diapir shapes are not necessarily a simple function of sedimentation and salt flux rates (qfx/A) and that the diapir hourglass shape might result from lateral deformation of the megaflap.

Stress and Pore Pressure in Mudrocks Bounding Salt Systems

We simulate the evolution of stress and pore pressure in sediments bounding salt systems. Our evolutionary geomechanical models couple deformation with sedimentation and porous fluid flow. We find that high differential stresses develop near rising diapirs and below salt. Salt emplacement induces significant excess pressures that are comparable to the weight of the salt sheet. In addition, we show that the shear-induced component of the excess pressures is significant. We also find that low effective stresses result in low strength, which enables salt growth. We model salt as a solid viscoplastic and sediments as poro-elastoplastic materials, and calibrate the consolidation properties based on experimental testing on smectite-rich mudrocks typical of those in the Gulf of Mexico. Our approach can be applied to design stable well bores and provide insight into macroscale geological processes. Overall, we show that transient evolutionary models can provide estimates of stress and pore pressure in many geologic systems where large strains, pore fluids, and sedimentation interact. We close with a discussion of the need to better understand material behavior at geologic stress and timescales.