[1] In the Ursa Basin, Gulf of Mexico, in situ mudstone permeability near the seafloor declines from 1.1 Â 10 À16 to 5.8 Â 10 À19 m 2 over a depth of 578 m. We can reproduce this in situ permeability-porosity behavior through consolidation experiments in the laboratory. We use uniaxial constant-rate-of-strain consolidation experiments to measure permeability-porosity relationships and derive in situ permeabilities of 31 mudstone samples collected at Integrated Ocean Drilling Program (IODP) Sites U1324 and U1322. Although these mudstones have similar grain-size distributions, permeability at a given porosity varies significantly between the samples due to small variations in composition or fabric. We calculate an upscaled permeability relationship based on the observed permeability variation in the samples and determine a resultant large-scale permeability anisotropy of around 30. Based on this upscaled relationship and observations of in situ pressure, we calculate upward fluid flow rates of 0.5 mm/yr. We find that given the observed compressibility, permeability, and the geologic forcing at Ursa, overpressures are predicted as observed in the subsurface. The primary mechanism for overpressure generation at Ursa is sediment loading due to rapid burial. Low vertical permeabilities, accompanied by high sedimentation rates, can cause severe overpressure near the seafloor, which controls fluid flow and can reduce slope stability as observed in the Mississippi Canyon region. Such flow systems, especially at intermediate depths on passive margins, are important due to their control over macroscale behavior such as topographic gradient of continental slopes and submarine landslides, but have been largely understudied in the past.

Slow slip on preexisting faults during hydraulic fracturing is a process that significantly influences shale gas production in extremely low permeability “shale” (unconventional) reservoirs. We experimentally examined the impacts of mineralogy, surface roughness, and effective stress on permeability evolution of slowly slipping faults in Eagle Ford shale samples. Our results show that fault permeability decreases with slip at higher effective stress but increases with slip at lower effective stress. The permeabilities of saw cut faults fully recover after cycling effective stress from 2.5 to 17.5 to 2.5 MPa and increase with slip at constant effective stress due to asperity damage and dilation associated with slip. However, the permeabilities of natural faults only partially recover after cycling effective stress returns to 2.5 MPa and decrease with slip due to produced gouge blocking fluid flow pathways. Our results suggest that slowly slipping faults have the potential to enhance reservoir stimulation in extremely low permeability reservoirs.

We conducted Atterberg limits measurements, particle size analyses, and uniaxial constant rate of strain consolidation experiments on six sediment samples, which were prepared in the laboratory using resedimentation, to characterize the material and analyze compression and flow behavior. We performed all experiments in the GeoMechanics Laboratory at the University of Texas at Austin (Texas, USA). The six samples are sediment mixtures composed of varying proportions of Nankai silty claystone, which was homogenized from a large number of discrete samples that were collected during Integrated Ocean Drilling Program (IODP) Expedition 322 from Site C0011, and silt-size silica from US Silica. The particle size distributions vary from 56% to 32% clay-size particles with no sand present. The compression index (C c) systematically decreases with decreasing clay-size fraction. For clay-rich mixtures, C c also significantly decreases with vertical effective stress (σ′ v), whereas silt-rich mixtures show constant C c. Vertical intrinsic permeability decreases with increasing σ′ v and varies loglinearly with porosity. Slopes of this log-linear relationship vary between 11.8 and 9.8 for mixtures from 56% clay to 32% clay. At a given porosity, vertical permeability increases by two orders of magnitude for clay contents ranging from 56% to 32%.

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