ABSTRACT: The Antrim Shale is a fractured Devonian gas reservoir in northern Michigan with multiple natural fracture sets. At least three regional fracture sets are observed in the Antrim Shale which developed independent of local structure. Vitrinite reflectance data constrain the maximum burial of the Antrim Shale in the productive trend to about 6000 ft (1800 m) near the end of the Paleozoic era. Stress history analysis supports a model in which NW-striking fractures formed by natural hydraulic fracturing (NHF) as hydrocarbons were generated near the end of the Alleghenian orogeny. NE-striking fractures formed as a result of cooling and unloading during nearly 4000 ft (1220 m) of basin uplift since the Permian, and are aligned with the contemporary NE-directed maximum horizontal stress in the mid-continent region. Continued uplift and local reversal of the minimum and intermediate principal stress resulted in a second set of NW-striking cross fractures, orthogonal to NE-striking fractures.
Elastic property data indicate that the organic-rich, black shale has the lowest Young's modulus, Poisson's ratio, and fracture toughness in a sequence containing gray" shales and limestones. It is also the lithology which is preferentially fractured, in contrast to other reservoirs where the stiffest lithology is most fractured. Fracture preference and containment in layered sedimentary rocks is not only a function of lithology and rock properties, but depends on the mechanism of fracture. The mechanism of NHF favors lithologies with low Poisson's ratio, low Biot poroelastic parameter, low fracture toughness and high fluid pressures. Fracturing under conditions where strain (bending, layer-parallel stretching) or uplift (thermoelastic contraction) mechanisms are dominant favors fracturing in stiff, brittle lithologies with high Young's moduli.
