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

the discontinuous nature of the pattern and the availability of preferred flow paths
crossing a minimum number of DBs. Nonetheless, the anastomosing parallel pattern
studied still could induce a significant effective permeability anisotropy between the
maximum (band-parallel) and minimum (band-normal) directions. For kb/km=10 -2 about a
0.5 order-of-magnitude anisotropy results, for kb/km=10 -3 the anisotropy increases to
about 1.3 orders of magnitude.
7. Summary
We have presented a numerical method for calculating the effective permeability
induced by any given pattern of deformation bands (DBs), and then used it to assess the
2-D effective permeability for volumes of porous sandstone containing each of three
characteristic DB patterns present in the Aztec sandstone of the Valley of Fire State Park,
Nevada—parallel, cross-hatch and the dominant anastomosing.
For substantially parallel patterns, we found that band-parallel effective permeability
remains relatively unaffected for realistic DB volume fractions up to 20%. Likewise,
band-parallel effective permeability is only slightly affected by changes in the
permeability contrast between the DBs and the host rock. By contrast, band-normal
effective permeability drops even for low volume fractions of DBs and is extremely
sensitive to varying internal DB permeability. Parallel band patterns such as those
observed in the Aztec can induce permeability anisotropy of up to 2 orders of magnitude,
with the maximum principal permeability aligned in the band-parallel direction. For
idealized parallel patterns, our modeling results exactly match those computed
analytically.
Cross-hatch band patterns can be analyzed numerically, or in a somewhat simplified
way using a combination of numerical and analytical methods. They yield highly
anisotropic effective permeability reductions for acute intersection angles close to 0°, and
isotropic effective permeability for angles close to 90°. For relatively regular, evenly
spaced patterns, the direction of maximum permeability bisects the acute angle of
intersection. For DB volume fractions of ~10% and acute intersection angles of ~80 o ,
such as found in the Aztec, the cross-hatch pattern can produce up to a 1.5-order-of-
magnitude reduction in bulk permeability and more than an order-of-magnitude
permeability anisotropy.
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