The influence of the cataclastic matrix on the petrophysical properties of deformation bands in arkosic sandstones

This study investigates how the intensity of the cataclasis deformation mechanism influences petrophysical properties, such as pore diameter, grain size, porosity, and permeability, in deformation bands of arkosic sandstones and conglomerates affected by normal fault zones in the Rio do Peixe Basin, Brazil. We analysed in detail the cataclastic matrix within the deformation bands, defined as the amount of fine particles (<1000 μm) generated by grain crushing that surrounds the coarse particles. We collected data in deformation bands exhibiting different textures such as protocataclastic, cataclastic, and ultracataclastic textures, and we performed structural, microstructural, and petrophysical analyses to establish the geometry, temporal relations, kinematics evolution, and deformation mechanism responsible for the formation and evolution of the deformation bands. Deformation bands evolved progressively with increasing grain comminution, which results in different cataclastic textures within a normal fault zone. These textures influence petrophysical properties, mainly porosity. For example, an increase in the cataclastic matrix generates a decrease in macropores, affecting the pore access. The arkosic and arkosic-lithic sandstones present a more selective intragranular fracturing of feldspar grains than of quartz grains. This fact indicates some degree of mineralogical control on the grain size distribution and porosity. As a result, in the ultracataclastic textures, quartz clasts are relatively well preserved, and feldspar grains are almost completely destroyed. Our study indicates that it is possible to estimate the relative porosity reduction of deformation bands by analysing the amount of cataclastic matrix. These results could greatly influence our understanding of the reservoir properties in faulted arkosic sandstones.