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