Numerical modelling of gravitational sinking of anhydrite stringers in salt (at rest)

Abstract

A large number of salt bodies contain layers of anhydrite material which is generally referred to as “stringers”. The movement and deformation of embedded anhydrite bodies are processes which are not yet fully understood. It is observed that stringers tend to sink towards the bottom of salt bodies at velocities highly dependent on the mechanical properties of both salt and anhydrites, with given density contrast between salt and denser anhydrites. The rheological differences between salt and the embedded anhydrites are a major issue, contributing to the complexity of the problem. On a geological timescale, the salt behaves as a Newtonian or a power-law fluid. The anhydrite stringers present elastic or brittle properties under certain conditions. Finite Element Modelling (FEM) has been employed in this study by using the FEM package ABAQUS (SIMULIA, Dassault Systems) in order to numerically simulate the sinking of an anhydrite stringer embedded in the salt. Furthermore, numerical modelling of isolated anhydrite stringers in salt at rest is compared with observations of stringers in seismic data. FEM simulation of the anhydrite stringer sinking and the gravitational sinking of anhydrite blocks embedded in the salt will be studied and demonstrated with two different methods of rheology, respectively. The study results indicate that sinking velocity is closely related to several factors, including the viscosity, the thickness of the stringer, as well as the density contrast between stringer and salt for a given viscosity. The results also prove that anhydrite stringer fragments do not sink significantly over the geological timescale if the halite is deformed by non-Newtonian viscosity. But, when Newtonian viscosity is dominant, the fragments are likely to sink hundreds of metres through the Zechstein salt during a few Ma. In conclusion, the modelling of the sinking of anhydrite or anhydrite inclusions provides an important scope for understanding the movement and deformation of embedded stringers.