For unconventional shale plays, what physical property defines the 'brittleness' of a rock, making it suitable for effective hydraulic fracture stimulation?

The brittleness of shale is defined by its elastic properties, specifically the Young's modulus and Poisson's ratio. Young's modulus is a measure of rock stiffness, representing the ability of the rock to resist deformation when subjected to stress. A higher Young's modulus indicates a stiffer, more brittle rock that fractures easily. Poisson's ratio is a measure of how much a rock expands late....