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"Regional" fractures may form slowly, at depth, in extension, and parallel to regional horizontal tectonic compression. Such fracture sets can be common in reservoirs despite the absence of flexure of the strata, and are commonly important contributors to reservoir permeability. Such fractures make good targets for deviated wells. High formation pore pressure is crucial to creating low effective confining stresses and brittle rock properties in the reservoirs (conditions conducive to fracturing) despite deep burial, but pore pressure does not exceed the least principal stress and does not cause tensile fracturing: the mechanism of natural hydraulic fracturing is incompatible with the characteristics of most regional fracture sets. Rather, many regional fractures form duri g far-field compression, initiate at locally induced tensile stresses caused by flaws in the rock, and propagate in the plane of the maximum and intermediate compressive stresses. The differential stress required for initiation and propagation of regional fractures is well below that necessary for shear failure. In the presence of high pore pressure, fractures can be held open at depth by a tectonic, basinwide dilatancy of the strata, and the open void space commonly becomes mineralized.
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