## 1. Introduction

[2] The evolution of mudrock permeability during burial has been studied for decades. Porosity can decrease from 0.9 to as little as 0.05 over a few kilometers depth with a corresponding decrease in permeability of up to 8 orders of magnitude [*Neuzil*, 1994]. The log of permeability generally decreases linearly with porosity, and at a given porosity mudrock permeability can vary by up to 3 orders of magnitude [*Neuzil*, 1994]. The permeability anisotropy of uniform (nonlayered) mudrocks, defined as the ratio of the horizontal to vertical permeability, typically increases with compression [*Basak*, 1972; *Daigle and Dugan*, 2011; *Dewhurst et al*., 1998; *Yang and Aplin*, 2007]. Changing mudrock porosity, and hence permeability, in a sedimentary basin directly affects fluid migration, consolidation rates, and overpressure generation [see *Broichhausen et al*., 2005; *Bethke*, 1989]. Numerical models are often used to simulate the complex behavior of a sedimentary basin. Some such models assume the permeability of a given stratum to be constant in all directions [e.g., *Ungerer et al*., 1990], while others include anisotropy at a constant value independent of mudrock porosity [e.g., *Bekele et al*., 2001]. While there is a mature approach to describing how bulk permeability varies during compression, the evolution of permeability anisotropy is not as well understood.

[3] There has been limited work to describe the evolution of permeability anisotropy of uniform, nonlayered mudrocks with compression. *Clennell et al*. [1999] showed that anisotropy increases from 1.1 to 3 for remolded pure clays, synthetic silty clays, and natural clays. *Leroueil et al*. [1990] studied marine clays and found the permeability anisotropy to range from 1.1 to 2.5, with some mudrocks exhibiting decreasing anisotropy with compression. *Basak* [1972] determined that the permeability anisotropy of kaolinites is dependent on soil structure and varies between 1 and 1.6. Finally, *Yang and Aplin* [1998, 2007] report permeability anisotropy measurements on intact mudrock cores varying between 1.15 and 11.8, but note that high anisotropy (>4) implies specimen heterogeneities such as layering. All of these studies suggest that in uniform mudrocks, the permeability anisotropy varies modestly between 1 and at most 3 or 4 in the first few kilometers below seafloor.

[4] It is generally inferred that the development of permeability anisotropy is due to the rotation of platy particles during uniaxial compression. Numerous models have been developed to describe the permeability anisotropy as a function of particle alignment. *Daigle and Dugan* [2011] derive one such model using geometry to compute the flow path tortuosity based on mean particle orientation, particle aspect ratio, and porosity. Their model predicts that the permeability anisotropy, given as the square of the tortuosity anisotropy, will not exceed ∼10 for 80% strain in a typical Illite–Kaolinite mudrock under uniaxial compression [*Daigle and Dugan*, 2011]. Both *Daigle and Dugan* [2011] and *Arch and Maltman* [1990] show that higher-permeability anisotropy (>10) in homogenous mudrocks is possible only when uniaxial compression is combined with simple shear. Similarly, *Yang and Aplin* [1998] derived a semiempirical model to capture the evolution of the pore geometry with compression. Their model predicts the permeability (horizontal or vertical) to within a factor of ±3; however, as result is unable to accurately predict the permeability anisotropy. Ultimately, theoretical advances in our understanding of permeability anisotropy are constrained by the limited availability of field and laboratory measurements.

[5] A key control on understanding permeability evolution is developing a quantitative understanding of how particle alignment develops during compression. *Martin and Ladd* [1975] reveal that 80% of the particle orientation at 10 MPa effective stress in slurry resedimented Kaolinites occurs below 0.1 MPa effective stress. A common approach is to model particle rotation with the *March* [1932] model which proposes that particle rotation begins at very small strains and continues asymptotically to very large strains. Yet, there is disagreement as to whether particle alignment occurs with increasing compression. *Clennell et al*. [1999] did not find any relationship between particle alignment and permeability anisotropy development for four different mechanically compressed remolded mudrocks up to 4 MPa. They identify clustering of clay particles around larger silt grains as well as nonuniformities in the micro fabric and suggest these as limiting factors for permeability anisotropy development.

[6] We experimentally investigate the evolution of permeability anisotropy with increasing stress and decreasing porosity for mechanically compressed mudrocks deformed in a uniaxial strain field. We first present a method to measure the permeability of Resedimented Boston Blue Clay (RBBC) using cubic specimens which allows for measurement of both the vertical and horizontal permeability of the same specimen. The permeability anisotropy is measured separately for each specimen. With this approach, we measure multiple specimens at different porosities to define the porosity versus log permeability trend for RBBC. Our new approach removes specimen variability from the permeability anisotropy measurement and contrasts other methods which require two specimens to compute the permeability anisotropy [e.g., *Clennell et al*., 1999; *Leroueil et al*., 1990]. We discuss the analytic methods used to determine the permeability anisotropy of a single specimen from directional constant head measurements. Image analysis of Backscattered Scanning Electron Microscope (BSEM) images taken at varying stress levels reveal that platy particles are initially randomly oriented at low stress, and become more oriented to the horizontal with increasing applied stress. Finally, we apply the *Daigle and Dugan* [2011] model to predict the permeability anisotropy of our specimens and find good agreement with our experimental measurements. We show experimentally that platy particle alignment can explain permeability anisotropy development in mudrocks, and that the *March* [1932] model does not capture the platy particle orientation we measure.