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

Porosity and permeability estimates for the CB range from about 9-12% and 1-20
mD, with the results from the three bed types substantially overlapping (Figure 5.7). This
suggests that permeability within the band is approximately homogeneous and isotropic.
The combined average estimates for CB porosity and permeability are 10.6% and 5.1
mD. It turned out that exactly half of the 3-D pore-structure realizations for the CB—12
for the coarse-grain beds and nine each for the fine-grain and mixed-grain beds—yielded
no connected flow paths and so returned zero permeability. Given that the minimum non-
zero permeability result was 1.2 mD, we deemed all zero values to be unrepresentative
and discarded them (see Conclusions). Our average estimate for CB permeability of 5.1
mD is therefore a maximum, and we suggest that the true value is probably around 3 mD,
given that the average including zeros is 2.5 mD.
4.2. Comparison to measured values
Our computational approach yields permeability estimates for the Aztec sandstone of
about 1,000 mD along bedding, 800 mD across bedding and 5 mD within CBs, which
generally cut across bedding at high angle. Permeability reduction due to compaction
within the bands is therefore estimated to be at least two orders of magnitude—the
middle of the range reported for deformation bands in general (see Introduction).
Antonellini and Aydin (1994) report minipermeameter measurements for both matrix and
compactive deformation bands from the Navajo sandstone of southeastern Utah,
recognized as a depositional and chronological equivalent of the Aztec (Marzolf 1983;
Blakey 1989). Their results do not distinguish between bedding parallel and bedding
normal permeability. Flodin et al. (2005) provide a handful of additional lab
measurements for the Aztec matrix, also without reference to bedding orientation.
Together, these measurements—ranging from 123 mD to 5,991 mD in the matrix (mean
of 1,768 mD, and from 1.35 mD to 38.33 mD in the bands (mean of 14.14 mD)—
correspond well with our computational estimates, while being somewhat higher (Figure
5.7). We ascribe this discrepancy to spatial permeability variation within the sandstone,
as much as to any systematic inaccuracy in the computational method, noting again that
the estimates derive from a single sample location.
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