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Exploring the Formation, Microtexture, and Petrophysical Properties of Deformation Bands in Porous Sandstone

S. G. Ahlgren
University of Arizona, Dept. of Geosciences, Tucson AZ

To optimize exploration and production efforts, one must understand the distribution, geometry, and petrophysical properties of subseismic structural heterogeneities that impede subsurface fluid movement. Deformation bands, commonly found in porous sandstone, are one such type of sealing structure. They are cataclastic shear fractures marked by negligible across-strike permeability induced by near elimination of host porosity through grain-scale comminution. As a result, these seemingly minor, millimeter to meterscale structures may have a profound negative impact on subsurface fluid flow. The sealing capacity of a deformation band is dependent on the degree of cataclasis as well as the amount of interstitial cement within the band, although the exact timing and mechanism of cement deposition remains unclear.

The primary goal of this research is to document the relationship between physical and chemical conditions favorable for deformation band formation and the impact these conditions have on the hydraulic properties of the bands that form. Additionally, this research will be the first to explore how deformation bands develop under elevated pore fluid pressure and low differential stress, as suggested by field examples. After creating deformation bands in sandstone core samples, microcrack orientations and densities will be determined using optical and scanning electron microscopy, as will the preferred locations of cement deposition. These structural data will be correlated with whole-core and microscale permeability measurements and integrated into a predictive model of deformation band formation and their associated impact on fluid flow.

AAPG Search and Discovery Article #90902©2001 AAPG Foundation Grants-in-Aid