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Abstract In this contribution we show the variation of the uniaxial compressive strength (UCS) and petrophysical properties of different architectural elements of fault zones affecting poorly lithified coarse sandstone and conglomerates of the Rio do Peixe basin, NE Brazil. We worked on three distinct outcrops; (1) non-deformed (protolith); (2) single fault zone, presenting moderate strain; and (3) complex fault zone, presenting a high strain intensity, where multiple and well developed fault cores occurs. To characterize the structural domains, we performed scanlines and uniaxial geomechanical survey across deformation bands and clustering zones. We combined geomechanical data with porosity data acquired thru image analysis and gas expansion analysis. We acquired data on deformation band cores and zones between deformation bands (i.e. , deformed zones that do not present deformation bands). Our results show that the complex fault zone exhibits a strong increase on the rock resistance, thus presenting UCS values three times higher than those obtained on the single fault zone outcrop and four times higher when compared with the non-deformed outcrop. It suggests that there is a direct relation between strain and rock resistance. The porosity is also strongly affected by the deformation, exhibiting an inverse relation with the strain intensity. Complex fault zone presents up to 75% of porosity reduction, while single fault zone, only about 10% of reduction. The petrophysical properties were calculated in the complex fault zone, where the resistance of the DBs was as high as twice that of the zone without bands, which had effects on the Young's and the incompressibility's moduli. The results suggest that even though DBs generate up to a 40% increase in the UCS and a 75% decrease in the porosity, the structural domains where the rock is located strongly controls the geomechanical and petrophysical properties, thus, directly affecting the anisotropy of a reservoir. [ABSTRACT FROM AUTHOR]

Hydrothermal silicification and deformation bands influence the physical properties of porous siliciclastic rocks. However, the impacts generated by the coexistence of these two processes on fault zone flow properties, such as porosity and permeability reductions, are still debatable. We integrated structural, geomechanical, and petrophysical data to investigate the influence of hydrothermal silicification on the physical properties of a fault zone. The fault affects Precambrian crystalline rocks and porous sandstones in the Rio do Peixe Basin, northeastern Brazil. The results indicate that quartz cementation is confined to a halo along the fault. Silicification decreases away from the fault zone toward the basin, which we subdivided into three main zones: (1) nonsilicified sandstone – Zone I, (2) moderately silicified sandstone – Zone II, and (3) intensely silicified sandstone – Zone III. The elongated geometry, the thickness of the silicified body, the proximity to the fault, and the high silica cement concentration indicate an external silica source. Nevertheless, we also propose internal sources related to feldspar dissolution. The primary porosity is rarely preserved in Zone III, while the secondary porosity is filled with silica cement. The primary sandstone porosity is well-preserved in Zone II, while many dissolved feldspar grains are not filled with cement. The petrographic and petrophysical analysis (cataclastic matrix amount) indicates that cement precipitation also occurred inside the deformation bands. Silicification increased the uniaxial compressive strength (UCS) by one order of magnitude in the fault damage zone relative to the protolith. On the other hand, the porosity and permeability in the silicified fault zone exhibit a reduction of two and four orders of magnitude, respectively, relative to the undeformed and nonsilicified protolith. This study indicates that silicification along a fault zone has crucial implications for the reservoir properties of siliciclastic rocks, strongly reducing the fluid flow properties and increasing the UCS values. [Display omitted] • We recorded hydrothermal silicification in a deformation band fault zone at the Rio do Peixe Basin, Brazil • We assessed how hydrothermal silicification affects the physical properties of arkosic sandstone in a fault zone • Hydrothermal silicification splits the fault zone into non-silicified, moderately silicified, and intensely silicified zones • Percolation of hydrothermal fluid and precipitation of silica cement occur within deformation bands and adjacent host rocks • Silicification increases UCS by 65% and decreases porosity-permeability by up to 2-4 orders of magnitude in fault zones [ABSTRACT FROM AUTHOR]