Cataclastic Fault Zones in Nature and Experiments: Impact of Textural Evolution on Petrophysical Properties
Sankar Muhuri1, Eric Flodin2, and Martha Gerdes1
1 ChevronTexaco, Bellaire, TX
2 Indiana University Purdue
University Fort Wayne, Fort Wayne,
Cataclastic faults and deformation bands are common features in deformed
porous sandstones with negligible clay content. The presence of these features
in a reservoir has a proven impact on recovery factors and producibility.
Extensive outcrop data and experimental analyses have shown that these
deformation features form under a range of mean stresses and displacements in
the shallow crust (<6 Km). Petrophysical analyses of faults in Valley of Fire
State Park, Nevada suggest evolution of fault zone grain and pore-throat size
distribution and permeability as a function of fault displacement. Laboratory
shear experiments with synthetic gouge zones that relate fault slip to evolution
of fault zone mean grain size, skewness
, and fractal dimensions are in close
agreement with the outcrop analyses. In general, the fractal dimension of gouge
zones evolves to a steady state of around 2.6 both in experiment and nature. The
similarity of the fractal dimension estimated from both outcrop and the
laboratory suggests that this value is fundamentally controlled by the grain
breakage process in fault zones whereby dissimilar sizes can coexist in
proximity. Integration of outcrop and experimental data enables us to
investigate the efficacy of a transform between fault slip and fault rock
permeability for cataclastic faults in clean sandstones. Such a transform is
necessary, due to the fact that current methodologies to compute fault zone flow
properties for reservoir simulation rely primarily on the abundance of host rock
clay content and fail to predict the flow behavior of low clay content rocks.
AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005