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

Though grossly systematic, anastomosing DB patterns are strongly irregular in detail
and so their permeability impact can only be assessed using the numerical method. For
this study, a single representative outcrop array from the Aztec was studied. The resulting
permeability effects were substantially similar to those of a comparable, idealized parallel
pattern, though somewhat muted. For the anastomosing pattern, DB connectivity along
trend acts to reduce effective band-parallel permeability, while lack of continuity across
the pattern enhances band-normal permeability. Nonetheless, for the representative
pattern studied, permeability anisotropies of up to 1.3 orders of magnitude are possible,
with bulk permeability reductions of more than an order of magnitude.
8. Discussion and conclusions
In numerically computing the effective permeability of deformation band (DB) arrays
in sandstone, we have demonstrated, their potential to induce order-of-magnitude bulk
permeability reductions and anisotropies at scales relevant to reservoir production. But do
such permeability reductions and anisotropies influence fluid flow in actual field
practice? While it is beyond the scope of our current research effort to address this issue
directly—by correlating DB occurrence with pump test or production data, for example—
such an effort clearly is indicated. We encourage petroleum geologists to be vigilant for
evidence of DBs in core, well bore logs and outcrop, and to investigate the connection
between DB occurrence and otherwise unexplained departures from expected production
performance.
This being said, evidence does exist for the substantial impact on permeability and
fluid flow of DBs in sandstone. For example, in the Aztec patterns of diagenetic
alteration fronts indicative of paleo fluid flow clearly demonstrate that DBs and DB
arrays acted as baffles to flow at a variety of scales (Eichhubl et al., 2003; Taylor and
Pollard, 2000). In addition, direct production evidence for the influence of geological
heterogeneities—including DBs—has been noted in the Aztec-equivalent Nugget
sandstone of the Anschutz Ranch East Field in Wyoming, Idaho and Utah (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).
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