Shale Anisotropy and Natural Hydraulic Fracture Propagation: An Example from the Jurassic (Toarcian) Posidonienschiefer, Germany

Cores recovered from the Jurassic (Toarcian) Posidonienschiefer (Posidonia Shale) in the Lower Saxony Basin, Germany, contain calcite-filled fractures (veins) at a low angle to bedding. The veins preferentially form where the shale is both organically rich and thermally mature, supporting previous interpretations that the veins formed as hydraulic fractures in response to volumetric expansion of organic material during catagenesis. Despite the presence of hydrocarbons during fracturing, the calcite fill is fibrous, and so, the veins appear to have contained a mineral-saturated aqueous solution as they formed. The veins also contain myriad host-rock inclusions having submillimetric spacing. These inclusions are strands of host rock that were entrained as the veins grew by separating the host rock along bedding planes rather than cutting across planes. The veins therefore produce significantly more surface area – by a factor of roughly 5, for the size of veins observed – compared to an inclusion-free fracture of the same size. Analysis of vein geometry indicates that, with propagation, a fracture surface area increases with fracture length raised to a power between 1 and 2, assuming linear aperture-length scaling. As such, this type of fracture efficiently dissipates elastic strain energy as it lengthens, stabilizing propagation and precluding dynamic crack growth. The apparent separation of the host rock along bedding planes suggests that the mechanical weakness of bedding planes is the cause of this inherently stable style of propagation.