Variation of thickness, internal structure and petrophysical properties in a deformation band fault zone in siliciclastic rocks

Abstract

We studied the geometry and petrophysical properties of fault core and damage zones of a deformation band fault zone in the arkosic sandstones of the Cretaceous Antenor Navarro Formation in the Rio do Peixe Basin, Brazil. The present work includes results from fieldwork and laboratory measurements. We performed structural, geometric, and permeability measurements in the field. These measurements were performed on a longitudinal scanline along the fault core and 18 orthogonal scanlines crossing both fault core and damage zones in the footwall and hanging wall. Drone images were taken from the field locality to study the geometric relation between fault core and individual deformation bands in the damage zone. Porosity, capillary pressure and grain size analyses were performed on the samples. The studied fault is normal and composed of three linked segments, a central E-W segment which is hard-linked to two NE-SW segments tipping toward east and west. Our results show that fault core is the thickest on the E-W segment. The damage zone is narrower around the central part (E-W segment) of the fault and widens westward in both hanging wall and footwall, beyond this point the hanging wall damage zone is wider than the footwall damage zone. The thickest part of fault core has the lowest porosity and permeability and the highest capillary threshold pressure, therefore, increasing the fault sealing capacity. Permeability measurements parallel and perpendicular to fault dip show an anisotropy, in which the permeability perpendicular to fault dip is one order of magnitude lower than dip-parallel permeability measurements. This work highlights the effect of hard linkage between the E-W and NE-SW segments of the studied fault on the petrophysical properties of fault rocks and emphasizes the influence of such linkage on hampering the fluid flow along the breached segment of the fault.