structural geology, propagation mechanics and - Stanford School of ...

esults therefore suggest that the generally anastomosing pattern of bands
captured in the x1-x2 plane would appear as a more consistently parallel, less
interconnected pattern in the x1-x3 plane (Figure 4.24). This interpretation has
yet to be confirmed by observing outcrop exposures with the requisite
orientation, which are rare at best.
4. Strongly parallel band propagation in the x1-x3 plane would tend also to retard
CB interactions in the x1-x2 plane. Given the degree of anastomosis
nonetheless present in Aztec outcrops, this suggest that the paleo state of
stress in the x1-x2 plane was likely more nearly isotropic than our 2-D
simulations suggest.
5. These interrelationships of magnitude and direction between the components
of principal paleo stress and the resulting patterns of band propagation have
significant implications for predicting 3-D permeability in aquifers and
reservoirs containing CBs. Specifically, because permeability along the trend
of a band pattern decreases with increasing connectivity, and the minimum
principal permeability (k3 ≤ k2 ≤ k1) is directed orthogonal to the dominant
band orientation (Sternlof et al., 2004), k1 due to a subsurface band array will
parallel the presumed direction of paleo σ33 r , k2 will parallel σ22 r and k3 will
parallel σ11 r (Figure 4.24). The relative magnitudes of the principal
permeabilities can likewise be inferred from the presumed relative magnitudes
of the paleo principal stresses.
6. By the same token, the orientations and relative magnitudes of all three
principal paleo stresses can be inferred from directional variations observed in
the degree of anastomosis revealed by a well-exposed (or imaged) CB array.
8. Acknowledgements
We are deeply indebted to John Childs for his ever cheerful and able assistance in the
field, and thank Ovunc Mutlu for help with the numerical code. This work was funded by
the U.S. Department of Energy, Office of Basic Energy Science, Geosciences Research
Program under grant DE-FG03-94ER14462, awarded to David Pollard and Atilla Aydin
at Stanford University. Additional support was provided by the Stanford Rock Fracture
Project.
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