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

In this paper, we use numerical methods and computer codes developed by Durlofsky
(1991; 1992) to quantify the influence of realistic DB patterns on bulk sandstone
permeability in two dimensions (2-D). We restrict ourselves to 2-D analyses so as not to
obscure the essence of the approach—or the potentially important impact of DBs on bulk
sandstone permeability—with the additional conceptual and computational complexity
inherent to 3-D treatments. Also, insofar as DBs are themselves grossly tabular/planar
features, with most arrays consisting of just one or two dominant orientations, the DB
pattern visible on any 2-D observation plane (e.g. outcrop face) that is oriented at high
angle to the bands will be substantially similar for all adjacent parallel planes. That is,
pattern and permeability continuity in the third dimension can reasonably be assumed.
Nonetheless, the numerical methods and analyses presented here could naturally be
extended to 3-D.
We apply the 2-D method to three distinct and characteristic DB patterns—parallel,
cross-hatch and anastomosing—as recognized and mapped within the æolian Jurassic
Aztec sandstone of the Valley of Fire State Park, Nevada (Figure 6.1). The Aztec
sandstone constitutes an excellent exhumed analog for active æolian sandstone reservoirs
potentially affected by DBs, such as the Anschutz Ranch East Field in the Nugget
sandstone of Wyoming, Idaho and Utah (Lindquist, 1988). The Nugget is considered a
chronostratigraphic and depositional equivalent of the Aztec (Marzolf, 1986), and
Anschutz Ranch comprises a large anticlinal trap within the Nugget. Production evidence
for Anschutz Ranch indicates that lowered porosities and permeabilities related to
abundant DB fabrics (observed in core and outcrop) have degraded overall reservoir
quality, introduced flow barriers and produced anisotropic behavior (Lewis, 1993).
Production problems attributed to deformation bands have also been reported for the
Nubian Sandstone in Egypt and for sandstone reservoirs located offshore from Nigeria
(Olsson et al., 2003).
3. Deformation bands
Deformation bands are thin, tabular, bounded features that accommodate pore-loss
compaction, often in association with shear displacement, in sandstones via granular
rearrangement, cataclasis and chemical diagenesis (Aydin, 1978; Aydin and Johnson,
1983; Du Bernard, 2002; Engelder, 1974; Jamison and Stearns, 1982) (Figure 6.2).
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