Shales: from the atomic scale to the rock-mass scale

Fine-grained sediments, mudrocks and shales have unique fabric and pore topology that reflect their mineral composition and formation history. Atomic-scale clay-clay electrical interactions coexist with the micron-scale mechanical interactions between silicate and carbonate grains, clay tactoids and organic matter; layering adds cm-scale vertical heterogeneity. The resulting strata define the performance of km-scale natural and engineering systems including oil and gas reservoirs and the long-term geological storage of CO2 and nuclear waste. This study presents the building blocks and physical evidence that support a new multi-scale numerical simulation approach for fine-grained sediments, mudrocks and shales. Atomic-scale studies show the effect of isomorphic substitution and adsorbed water molecules on clay tactoid stiffness. Pore-scale analyses based on SEM images reveal spherical pores in organic matter and elongated/aligned pores bound by clay tactoids. Particle-scale simulations capture fabric evolution including tactoid alignment facilitated by organic matter deformation to accommodate to the evolving mineral fabric. The resulting tactoid and organic matter alignment gives rise to shale fissility.