Microstructural and mechanical property characterization of Argillaceous, Kerogen-rich, and Bituminous shale rocks

Shale rocks have become an indispensable natural gas and oil source. Hence, the knowledge of the mechanical properties of shales is critical for field applications. In this work, we selected three types of shales (argillaceous, kerogen-rich, and bituminous) and conducted detailed chemical and microstructural characterization along with mechanical property measurements by nanoindentation. The three shale samples have highly distinct mineral compositions. The argillaceous and kerogen-rich shales have soft matrix phases - muscovite and kerogen, respectively. The bituminous shale, on the contrary, has no distinct matrix phase and is rich in carbonates. Young's modulus and hardness were observed to be predominantly affected by the mineral composition. The kerogen-rich shale has the lowest Young's modulus and hardness, followed by the argillaceous shale, while the bituminous shale is the stiffest and hardest. Young's modulus is anisotropic for all shales, but hardness does not follow this trend. The three shale samples also show varied fracture behavior. Apparent cracking and spallation were noted in the argillaceous and bituminous shale, but not in the kerogen-rich shale. Cracks, when activated, tend to propagate along the bedding plane-parallel direction, regardless of the loading direction. We anticipate the new information and knowledge generated from this work has a significant contribution to applications such as drilling and hydraulic fracturing.