Mudstones compose nearly 70% of the volume of sedimentary basins, yet they are among the least studied of sedimentary rocks. Their low permeability and high compressibility contribute to overpressure around the world. Despite their fundamental importance in geologic processes and as seals for anthropogenic-related storage, a systematic, process-based understanding of the interactions between porosity, compressibility, permeability, and pore-size distribution in mudstones remains elusive. I use sediment mixtures composed of varying proportions of natural mudstone such as Boston Blue Clay or Nankai mudstone and silt-sized silica to study the effect of composition on permeability and compressibility during burial. First, to recreate natural conditions yet remove variability and soil disturbance, I resediment all mixtures in the laboratory to a total stress of 100 kPa. Second, in order to describe the systematic variation in permeability and compressibility with clay fraction, I uniaxially consolidate the resedimented samples to an effective stress equivalent to about 2 km of burial under hydrostatic conditions. Scanning electron microscope images provide insights on microstructure. My experiments illuminate the controls on mudstone permeability and compressibility. At a given porosity, vertical permeability increases by an order of magnitude for clay contents ranging from 59% to 34% by mass whereas compressibility reduces by half at a given vertical effective stress. I show that the pore structure can be described by a dual-porosity system, where one rock fraction is dominated by silt where large pores are present and the majority of flow occurs and the other fraction is dominated by clay where limited flow occurs. I use this concept to develop a coupled compressibility-permeability model in order to predict porosity, permeability, compressibility, and coefficient of consolidation. These results have fundamental implications for a range of problems in mudstones. They can be applied to carbon sequestration, hydrocarbon trapping, basin modeling, overpressure distribution and geometry as well as morphology of thrust belts, and an understanding of gas-shale behavior. / text

Uniaxial consolidation experiments on Mass Transport Deposit (MTD) and non-MTD core samples from Ursa Basin, Gulf of Mexico, show MTDs have a lower porosity at a given effective stress compared to adjacent non-MTD sediments; a behavior observed in additional experiments on lab remolded Ursa core and resedimented Boston Blue Clay (BBC). I hypothesize debris flow action remolded the sediment: removing its stress history through shearing action, resulting in dense sediments at shallow depth. I supplement testing this hypothesis through lab remolding of BBC (in addition to Ursa clay) due to the greater availability and knowledge of this material. Ursa MTDs record multiple submarine slope failure events within the upper 200 meters below sea floor (mbsf); the most prominent is labeled MTD-2. MTDs have lower porosity and higher bulk density than surrounding, non-MTD, sediment. Porosity ([phi]) is 52% at 125mbsf – immediately below MTD-2; whereas [phi] is 46% at 115mbsf – within MTD-2. Comparison of non-MTD samples to MTD-2 samples, and intact to remolded samples, shows a decrease in sediment compressibility (Cc) within the MTD-2 and remolded sediments. Permeability within Ursa mudstones also declines with porosity according to: log (k) = A[phi] - B. Permeability is slightly higher within MTD-2; however grain size analysis indicates lower clay content in MTD-2 versus the non-MTDs. Pre-consolidation stress interpretations from the experiments show a linear trend in both MTD and non-MTD sediments, indicating both geologic units depict the same pore pressure profile. Remolding via debris flow explains the origin of MTDs at Ursa and governs the evolution of this geologic unit to its dense, highly consolidated, state today. At some point, slope failure triggered movement of the sediment down slope in form of a debris flow. The shearing action of the debris flow weakened the sediment, reducing its ability to support the overburden. As consolidation resumed, the remolded sediment followed a new, less steep, Cc curve. Within the geologic record, a distinctive dense, shallow unit is preserved; evidence for historical slope failure. / text