The role of stress on chemical compaction of illite shale powder

Abstract

AbstractExperimental compaction curves for mud are key factors in sedimentary basin models but they are poor at predicting porosity evolution beyond 2.5 km depth, where chemical compaction dominates shale diagenesis. This study presents the results of experimental simulation of diagenesis and low-grade metamorphism of an illite shale powder. By applying amphibolite facies conditions using high-pressure–high-temperature deformation apparatus, the transformation of illite to phengite that takes place under anchizone and epizone metamorphic conditions could be accelerated and completed within laboratory timescales. In accordance with observation of naturally compacted shales, our experimentally compacted metapelites range in porosity from 1% to 16% and contain authigenic phengite, biotite, and quartz. By compacting illite shale powder under both lithostatic and non-lithostatic conditions, the role of stress or tectonic forces during diagenesis could be evaluated. Based on microstructural and chemical similarities we infer that both types of samples compact in the same way, indicating that chemical compaction is unaffected by differential stress. Basin evolution models can therefore extend the 1D generalization of the stress field to depths where chemical compaction takes place. However, as chemical compaction enhances mineral alignment, anisotropy of physical properties such as permeability and sonic velocities can change substantially.