Cataclastic Fault Zones in Nature and Experiments: Impact of Textural Evolution on Petrophysical Properties

Sankar Muhuri¹, Eric Flodin², and Martha Gerdes^1
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