Hydromechanical Effects of Microorganisms on Fine-Grained Sediments During Early Burial

Microorganisms are known to change fluid flow and permeability processes in subsurface environments, although this has only been demonstrated for coarse-grained sediments and fractures. For fine-grained sediments (mudstones), however, little is known about the effects of microorganisms on hydromechanical properties. Here, we investigate the influence of microorganisms on the porosity, permeability, and compressibility of fine-grained sediments. We perform resedimentation experiments with and without microorganisms added to two reconstituted, fine-grained sediment samples. These sediments were collected from the Ursa and Brazos-Trinity Basins in the Gulf of Mexico during Integrated Ocean Drilling Program Expedition 308. Microorganisms caused a systematic, yet small increase in compression index for both sediments. Changes to permeability caused by microorganisms, while relatively minor, were greater for the Ursa sediment than the Brazos-Trinity sediment. Additionally, the effect of microorganisms on permeability is greater at higher porosities and lower vertical effective stresses. Differences in permeability behavior between the two sediments are likely due to differences in sediment properties and nutrients for microbial growth. We therefore suggest that the effectiveness of microorganisms at altering fluid flow in fine-grained sediments is dependent on burial depth (porosity as a function of vertical effective stress) and the grain size, pore and pore throat size, and specific surface area of a sediment. Characterizing the effects of microorganisms on the hydromechanical properties of fine-grained sediments can aid in bioclogging practices around contaminated sites in terrestrial environments and further our understanding of the controls on pore pressure near the sediment-water interface in marine environments.