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Fracturing is a mechanism for strain in rock. In many folds developed in sedimentary rock, fracturing is the primary mechanism of strain. The amount of strain produced by fracturing is a function of the amount of offset on individual fractures and the fracture spacing. As a consequence, the assessment of strain in the rock, either by direct measurement or with models, can be used to predict relative fracture density or spacing through a structure. The radius of curvature approach, commonly used to assess fracture distribution on folds, is based on this strain approach, but it is appropriate only for special situations. The shortcomings of the radius of curvature approach are demonstrated, qualitatively, with outcrop examples of thrust-associated anticlines.
Numerical modeling is an alternative and more general technique for predicting the strain, and hence fracture density, developed in geological structures. Comparison of measured shear fracture (microfault) density with the strain derived from numerical models of forced-folded sandstone illustrates the viability of this approach. In addition to strain, numerical models predict the orientation of the principal stress and the magnitude of the mean stress. Whereas it is accepted that principal stress orientations dictate fracture orientation and sense of offset, the effects of mean stress on fracture development are not well established. It appears, however, that mean stress influences the amount of offset
and opening on fractures, factors very important to production in fractured reservoirs.
The appropriateness of a numerical model depends completely on the specified input which consists of: (1) the boundary condition that produced the structure, and (2) the material behavior of the rock. Consequently, it is possible that a model with improper input may produce the desired fold geometry yet provide inaccurate information pertinent to fracture prediction. Thus, debate over the nature of boundary conditions, such as exists in thrust terrane, has implications even in the realm of fracture prediction.
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