Shale anisotropy and natural hydraulic fracture propagation: An example from the Jurassic (Toarcian) Posidonienschiefer, Germany
Hooker, JN; Ruhl, M; Dickson, AJ; et al.Hansen, LN; Idiz, E; Hesselbo, SP; Cartwright, J
Date: 5 February 2020
Article
Journal
Journal of Geophysical Research: Solid Earth
Publisher
American Geophysical Union (AGU)
Publisher DOI
Abstract
Cores recovered from the Jurassic (Toarcian) Posidonienschiefer (Posidonia Shale) in the
Lower Saxony Basin, Germany, contain calcite filled fractures (veins) at low angle to
bedding. The veins preferentially form where the shale is both organic rich and thermally
mature, supporting previous interpretations that the veins formed as ...
Cores recovered from the Jurassic (Toarcian) Posidonienschiefer (Posidonia Shale) in the
Lower Saxony Basin, Germany, contain calcite filled fractures (veins) at low angle to
bedding. The veins preferentially form where the shale is both organic 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 sub-millimetric 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 five, for the size of veins observed—compared to
an inclusion-free fracture of the same size. Analysis of vein geometry indicates that, with
propagation, 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
Camborne School of Mines
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