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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]

One of the most significant problems in the characterization and recovery of Aptian carbonate reservoirs, especially the Brazilian pre-salt, has been the lack of precise estimates of permeability. The heterogeneity of the permeability of the carbonate reservoirs occurs owed great different of the pore form occurred mainly by diagenetic processes. In this study, propose a joint and integrated methodology to estimate the permeability in the reservoir. To achieve this goal, porosity-permeability core data, image logs, applying the Rock Types concepts, analysis of the results of well log Nuclear Magnetic Resonance (NMR), modeling the well logs resistivity (laterolog and induction), as well as estimating the specific surface of the rock using images of section 2D from the pugs, to quantitatively estimate the permeability of the reservoir based on the petrophysical properties of the rocks. In the study it was possible to identify different pore systems distributed in eight Hydraulic Flow Units (HFU) determined from the pore groove radii. The Nuclear Magnetic Resonance (NMR) log it served to separate area of the spectrum corresponding to the small pores from the area corresponding to the large pores. The resistivity logs were analyzed with the purpose of estimating the direction (vertical, horizontal and dipping) and thickness of the fractures, which were also modeled with the purpose of identifying the invasion of the drilling fluid. The specific surface area was obtained by image processing algorithms. The results showed an acceptable precision of this methodology to estimate the permeability in carbonate reservoirs that have in their composition fragments of stromatolites and associated bioclastics, found partially or totally dolomitized. • The Hydraulic Flow Units with the highest velocity zones coincide with the fractured. • Dissolution improves reservoir quality in both spherulites and stromatolites. • The presence of fractures does not affect the quality of the reservoir studied. • The digital image analysis allows estimation of the petrophysical parameters. • The radial resistivity image identified the drilling mud in the fracture zones. [ABSTRACT FROM AUTHOR]