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

Abstract
Low-porosity, low-permeability compaction bands (CBs) form a pervasive planar
fabric throughout the upper 600 meters of the 1,400-meter-thick æolian Aztec sandstone
of southeastern Nevada, an exhumed analog for aquifers and reservoirs of similar
lithology. The existence of CBs, only recently recognized, raises issues of practical
significance. How do they impact local permeability? Can they influence fluid flow and
transport at production scales? When, how and why do they form? Can their presence,
geometry and effects be forecast in the subsurface from sparse data?
To address these questions, we applied a range of techniques, including: outcrop and
thin-section observations and measurements; structural and tectonic analysis; scanning
electron microscopy; detailed field mapping and aerial photograph interpretation;
mechanical analysis and modeling; computational permeability estimation and fluid-flow
simulation. Although derived entirely from the Aztec sandstone, our results offer a more
general phenomenological understanding of CBs and their effects.
Up to centimeters thick and hundreds of meters long, CBs comprise thin discs of
uniaxial porosity-loss compaction that form anastomosing arrays generally symmetric to
the maximum compression. Idealized as anticracks, CB propagation and pattern
development can be modeled using a linear-elastic boundary element method.
Mechanical interaction between CBs is inversely proportional to differential stress, such
that connectivity within a fabric, and its directional impact on fluid flow, can be
estimated from the stress state in which it formed, and vice versa. Computational
estimation corroborates measured permeability reductions of 10 -3 inside CBs. At outcrop
scales, this yields bulk permeability reductions up to 10 -2 and anisotropy up to 10 1 . Flow
simulations performed on a detailed, 40-acre CB map reveal significant practical impacts:
pumping and injection pressures increase three-fold; reservoir production efficiency
depends on proper alignment of the well array to the CB fabric, and contaminant
transport channels along the CBs regardless of the regional pressure gradient.
We interpret CBs in the Aztec as resulting from the tectonic compression of a high-
porosity, low-cohesion sandstone at modest confining pressures (