Spatial variation of microstructure and petrophysical properties along deformation bands in reservoir sandstones

A series of deformation bands from various reservoir sand-stones deformed at different burial depths have been studied with respect to microstructural and petrophysical variations. In many of the examples explored, the internal microstructure, porosity, and permeability vary along the bands at the centimeter or even millimeter scale, changing and in most cases reducing the ability of the bands to act as barriers to fluid flow. Porosity varies by up to 18% and permeability by up to two orders of magnitude. Such petrophysical variations are found along different types of deformation bands, but the range depends upon the deformation mechanisms, in particular on the degree of cataclasis and dissolution in cataclastic and dissolution bands, and on the phyllosilicate content in disaggregation bands. For cataclastic bands, the grain-size distribution changes along the bands with regard to the degree of cataclasis. Furthermore, the increased specific surface area of the pore-grain interface as a result of cataclasis causes higher permeability reduction in cataclastic bands than in other types of deformation bands. Phyllositicate content can influence the thickness of phyllosilicate bands. However, no apparent correlation between thickness and intensity of cataclasis in the studied cataclastic deformation bands is observed.