Deformation of Siliciclastic Stringers in a Layered Evaporite Sequence (LES): Insights From Geomechanical Forward Modeling

Layered evaporite sequences have been documented from various rifted margins, including the South Atlantic and the Red Sea. The intervening sedimentary layers in such sequences can undergo large deformation and present drilling hazards associated with high pore fluid pressures and/or rubble zones. Although physical and numerical models provide insights to the deformation of such layers, the former are limited in terms of scalability of material failure parameters to natural examples, while the latter predominantly focus on massive salt and adjacent frictional-plastic sediments. In this paper we present a 2D evolutionary large strain finite element model of a salt diapir in an idealized layered evaporite sequence (LES). Gravitational loading and sedimentation provide the driving force for halokinesis. Salt is assigned a temperature-dependent non-Newtonian rheology, whereas the sediments are assigned a non-associative cap-plasticity model that supports both compaction and shear localization. The model results suggest that mechanical stratification plays an prominent role in the evolution of a LES. Stresses and strains in the sediment layers evolve in a complex manner and are predominantly controlled by their structural position. The presence of multiple salt layers in a LES decouples the deformation at different depths such that poly-harmonic folds can develop near the salt diapir. Structural dip and position, in addition to curvature, impact the deformation within the sedimentary layers. Geomechanical forward models also provide directional guidance on the likely variations in in-situ stresses and in well planning in LES settings.