Hydromechanical Effects of Micro-Organisms on Fine-Grained Sediments During Early Burial

Micro-organisms are known to change fluid flow and permeability processes in subsurface environments, but this has only been demonstrated for coarse-grained sediments and fractures. For fine-grained sediments (mudstones), little is known about the effects of micro-organisms on hydromechanical properties. Here, we investigated the influence of micro-organisms on the porosity, permeability, and compressibility of fine-grained sediments. We performed resedimentation experiments with and without micro-organisms 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. Micro-organisms caused a systematic, yet small increase in compression index for both sediments. Changes to permeability caused by micro-organisms, while relatively minor, were greater for the Ursa sediment than the Brazos-Trinity sediment. Additionally, the effect of micro-organisms 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 micro-organisms 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 micro-organisms on the hydromechanical properties of fine-grained sediments can further our understanding of the controls on pore pressure near the sediment-water interface in marine environments and aid in bioclogging practices around contaminated sites in terrestrial environments. Plain Language Summary Micro-organisms dwell in the pore space between sediments (porosity) all across Earth's surface. This could potentially affect the rate at which porosity is lost as clay-rich sediments are buried (compressibility) and the ease at which fluids flow through clay-size sediments (permeability). In this paper, we investigate if and how micro-organisms change the compressibility and permeability of finegrained sediments (sediments dominated by clay-size grains). To do this, we experimentally compress these fine-grained sediments in the vertical direction with micro-organisms added and without micro-organisms added (control experiment) while measuring sediment porosity, compressibility, and permeability. We find that micro-organisms cause a small increase in the rate of porosity loss during compression and a small decrease in permeability. However, the amount of permeability decrease caused by micro-organisms is dependent on multiple properties of the clay-size sediment. These findings can be used to help understand how water pressures in ocean sediments can become elevated, which could lead to damage of seafloor infrastructure, or benefit geotechnical engineering practices that use micro-organisms to prevent fluid flow around contaminated sediments.

Automated summary

Micro-organisms have an influence on the porosity, permeability, and compressibility of fine-grained sediments, which is dependent on sediment properties, nutrients, and vertical stress. Understanding the effects of micro-organisms on the hydromechanical properties of sediments can contribute to our knowledge of pore pressure control in marine environments and aid in bioclogging practices in terrestrial environments.