Making Mudstones: insights into material behavior through resedimentation experiments

We explore how composition controls permeability, stiffness, and fabric in 'resedimented' mudstones. We mix slurries of natural mudstone (e.g. Boston Blue Clay and Gulf of Mexico Mudstone) with other components (e.g. silt) and we subject them to uniaxial compression to effective stresses up to 40 MPa. We show that increasing the silt fraction in mudstones results in a transition from a single porosity to a dual porosity structure. This dual porosity structure results from the development of large pore throat pathways associated with silt-to-silt contacts in the mixture. We describe how fabric evolves with increased effective stress due to increasing alignment of platy materials that rotate during uniaxial compression. We illustrate how pore throats evolve from approximately circular to elongate and how this results in the development of permeability anisotropy from isotropic at low effective stresses to anisotropic (Kh/Kv >2) at 40 MPa. Finally, we show how stiffness increases during compression. As compression proceeds, increasing grain-to-grain contact increases the stiffness. With the resedimentation approach, the effects of sample variability and sample disturbance are removed and the fundamental material behavior of mudstones is illuminated. The results can be used to simulate the evolution of mudstone properties during burial and the associated flow that must occur. More broadly, the results provide insight into the material properties of mudstones given particular burial histories that allow us to predict their flow behavior, and their sealing capacity.