Comparing Compression and Shear Behavior of Intact and Resedimented Mudrock to Illuminate the Rheology of Mudrocks

We performed compression and strength measurements on intact and resedimented cores from offshore West Africa from a depth of 975 meters below the seafloor. The in-situ vertical effective stress was 7.9 MPa, and the yield stress of the intact core was ∼21 MPa. We uniaxially consolidated the intact specimen to 15 MPa and unloaded it to undo coring, sampling, and re-saturation effects. We uniaxially consolidated the resedimented specimen to ∼22 MPa effective axial stress and unloaded it to reach the in-situ void ratio. We then performed undrained shear. The intact and resedimented specimens were within the elastic zone (inside yield surface) and had similar void ratios before shearing. We observed a higher peak shear strength (at ∼lower axial strain) and significant post-peak strength softening in the intact material. Both the materials start similarly in the mean effective stress versus shear stress plot. However, after shear stress of ∼4 MPa, the resedimented material departs significantly to the right from the straight ∼vertical stress path of the intact material, indicating dilative behavior. We interpret that the bonding in the intact material delays dilative behavior until just before the peak. Overall, this work identifies a key difference between intact and resedimented behavior due to bonding caused by creep and/or diagenesis in the intact material over geological time. 1. INTRODUCTION The fine-grained siliciclastic sediments start as weak, unconsolidated materials when initially deposited. However, over time and with burial at depth, these sediments undergo various mechanical and chemical processes that lead to develop them stronger (Maltman, 1994). A proper characterization of the geomechanical behavior of these mudrocks is essential for ensuring wellbore stability, predicting reservoir behavior, evaluating seal integrity, etc. Intact mudrocks derived from deep coring on the world's continental margins often demonstrate yield stress much greater than the maximum past geologic stress when loaded at lab rates (Ewy et al., 2020; Finnegan, 2020; Gutierrez et al., 2008). This overconsolidated state may record 1) creep and/or 2) bonding occurring over geologic time. On the other hand, it is common to use resedimented mudrocks to study in-situ stress state and strength (e.g., Casey et al. (2016)), assuming they would successfully reproduce in-situ properties. We systematically test intact and resedimented specimens, bringing them to a similar void ratio, and explore their undrained shear behavior to illuminate the rheology of mudrocks and fully understand in-situ rock properties.