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

exert appreciable fluid flow and transport effects at scales of practical relevance to
resource management in aquifers and reservoirs has remained an open question.
To begin addressing this issue, we present a series of flow simulation scenarios that
honor the exact details of individual compaction band arrays mapped over an outcrop area
of some 150,000 m 2 (37 acres) in the Aztec sandstone of southeastern Nevada (Figure
7.1). The simulations were conducted using a discrete-feature model in which the bands
are represented as one-dimensional elements assigned realistic petrophysical properties.
This effort represents the application of state-of-the-art flow modeling to a data set of
unprecedented detail and scale, which was derived from a real geological system that
serves as an exhumed analog for active sandstone aquifers and reservoirs. Analog
outcrop-based permeability and flow-modeling studies, whether focused on gross
sedimentological heterogeneities (e.g. White and Barton, 1999; Stephen and Dalrymple,
2002) or on fine-scale structural features (e.g. Taylor et al., 1999; Sternlof et al., 2004),
can provide valuable insights into the natural variability of inter-well flow and the
balance of parameters governing optimal (or at least improved) well placement.
The Aztec sandstone as exposed in the Valley of Fire State Park lies about 60 km
northeast of Las Vegas (Figure 7.1, map inset). The Aztec is a fine to medium-grained,
sub-arkosic æolian sandstone some 1,400 m thick, which was deposited as large
amplitude cross beds in a back arc basin setting from Late Triassic to Middle Jurassic
time (Bohannon, 1983; Marzolf, 1983). It exhibits a mean grain size of about 0.25 mm,
porosity ranging from 15% to 25% and permeability ranging up to several Darcys
(Antonellini and Aydin, 1994; Flodin et al., 2005). Extensive arrays of sub-parallel,
anastomosing compaction bands (CBs) pervade the upper 600 m of the Aztec exposed
over an area of more than 10 km 2 . Apparently associated with tectonism of the
Cretaceous Sevier orogeny, these bands at high angle to bedding comprise the oldest
structural fabric present, which subsequently was overprinted by shear bands, joints,
sheared joints and strike-slip faults (Hill, 1989; Taylor and Pollard, 2000; Eichhubl et al.,
2004; Flodin and Aydin, 2004; Myers and Aydin, 2004; Sternlof et al., 2005).
As observed in the Aztec and similar sandstones, CBs are discrete, tabular, bounded
features of porosity-loss compaction accommodated by granular rearrangement, grain
crushing and chemical diagenesis that exhibit little or no net shear and tend to weather
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