The Impact of Grain Size on the Hydromechanical Behavior of Mudstones

The consolidation behavior of mudstone is controlled by the clay fraction and affects a wide range of deformation and fluid transport processes in accretionary wedge systems. I analyze the compression and permeability behavior of six sediment mixtures based on uniaxial resedimentation and constant rate of strain consolidation data. The sediment mixtures are composed of varying proportions of hemipelagic mudstone and silt-size silica resulting in clay fractions ranging from 56% to 32% by mass. The hemipelagic mudstone is from Site C0011 drilled seaward of the Nankai Trough, offshore Japan, during Integrated Ocean Drilling Program Expedition 322. I show that porosity and compression index consistently decrease and permeability consistently increases with decreasing clay fraction over vertical effective stresses ranging from 0 to 21 MPa. Backscattered electron microscope images reveal that matrix porosity declines and large, jagged pore throats are being preserved in compaction shadows between silt grains as the clay fraction decreases. I compare the behavior of reconstituted samples with that of intact core and field measurements and interpret that increasing creep with depth and different mudstone fabrics explain differences in their compression curves. Finally, I provide empirical compression and permeability models that describe the evolution of porosity (void ratio) and permeability with vertical effective stress and as a function of grain size. Characterizing the in situ hydromechanical properties of subduction inputs is critical in order to relate input sediments to those at frontal thrust regions and predict pore fluid pressures and compaction-driven fluid sources within the outer part of the accretionary prism.

Automated summary

The consolidation behavior of mudstone in accretionary wedge systems is influenced by clay fraction, with porosity decreasing and compression index decreasing, while permeability increases as clay fraction decreases. Microscopic images reveal that as clay fraction decreases, matrix porosity decreases, and jagged pore throats are preserved between silt grains during compaction. Comparisons with intact core and field measurements show differences in compression curves due to increasing creep with depth and variations in mudstone fabrics. Empirical models describe the evolution of porosity and permeability with vertical effective stress and grain size. Understanding in situ hydromechanical properties of subduction inputs is crucial for predicting pore fluid pressures and compaction-driven fluid sources in the outer part of the accretionary prism.