The effect of fault-induced compaction on petrophysical properties of deformation bands in poorly lithified sandstones

Although it is well documented that during faulting there is an overall fault zone porosity loss in porous sandstone, little is known about the effect of shear-induced compaction on subsequent deformation bands petrophysics. We studied the impact of fault-induced compaction on deformation band's petrophysical properties developed in poorly lithified sandstones of the Rio do Peixe Basin, Brazil. We compared undeformed sandstone and deformation bands associated with syn-rift normal faults and post-rift strike-slip faults by integrating structural, petrophysical, mineralogical and uniaxial compressive strength (UCS) measurements. The sealing behavior was interpreted from capillary pressure obtained from mercury injection porosimetry and in situ air permeability. Host rock consists of poorly lithified sandstone with 25–30% porosity, permeability of 1.3 Darcy, capillary pressure of 5 psi, and UCS of 20 MPa. The normal faults developed from soft-sediment conditions with moderate to intense cataclasis, and resulted in an overall fault zone compaction (deformation bands and zones between deformation bands) with localized porosity and permeability loss in deformation bands (10% and 12 mD) and rock volumes between deformation bands (20% and 0.7 Darcy). Compared to syn-rift deformation bands, the strike-slip deformation bands show similar porosity (12–13%), but lower permeability (1–10 mD) and greater capillary pressures (up to 114 psi). XRD analysis on clay minerals indicates that both syn-rift and post-rift faults formed under similar shallow burial depths (<1–2 km). Therefore, we propose a model that quantifies fault-induced compaction and may predict fault sealing in structurally complex, deformation bands-dominated sandstone reservoirs affected by multiple tectonic stages.