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This study investigates hydrothermal silicification along rift border faults in the Cretaceous Rio do Peixe Basin, Brazil, through field geology, petrography, mineralogy, and fluid inclusion microthermometry. We distinguish three main structural-diagenetic domains: (1) a non-silicified fault damage zone, (2) a silicified damage zone composed of deformation bands and centimeter-scale quartz veins in the hanging wall and footwall, respectively, and (3) a silicified fault core with meter-scale wide quartz veins and mosaic breccia textures. We recognize three main events of silicification in the fault zone. The first silicification event occurs after the widespread development of deformation bands in the hanging wall damage zone that may reach 270 m in width. During subsequent deformation events, multiple fracturing episodes were accompanied with precipitation of the second quartz generation in hydraulic mosaic breccias and the third quartz cement in vein stockworks. Shallow burial conditions of faulting (<2 km), combined with fluid inclusion homogenization temperatures of the second quartz generation, indicate that silicification occurred in a hydrothermal system characterized by fluid temperatures in the range of 71–391 °C (mean value of 141 °C). We conclude that hydrothermal silicification was imprinted during the late synrift stage, when the rift border faults were already established. This contribution has implications for understanding how structural processes may control the geometrical, petrographic and petrophysical attributes of hydrothermal products. • Syn-Kinematic silicification occurs along rift border faults. • Three generations of hydrothermal silicification were recognized. • The second quartz generation confirms hydrothermal fluids having an average temperature of 141 °C. • Silicification occurred during late rifting and subsequent fault reactivation. [ABSTRACT FROM AUTHOR]

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]

In this work we describe the deformation mechanisms and processes that occurred during the evolution of cataclastic deformation bands developed in the feldspar-rich conglomerates of the Rio do Peixe Basin, NE Brazil. We studied bands with different deformation intensities, ranging from single cm-thick tabular bands to more evolved clustering zones. The chemical identification of cataclastic material within deformation bands was performed using compositional mapping in SEM images, EDX and XRD analyses. Deformation processes were identified by microstructural analysis and by the quantification of comminution intensity, performed using digital image processing. The deformation bands are internally non homogeneous and developed during five evolutionary stages: (1) moderate grain size reduction, grain rotation and grain border comminution; (2) intense grain size reduction with preferential feldspar fragmentation; (3) formation of subparallel C-type slip zones; (4) formation of S-type structures, generating S-C-like fabric; and (5) formation of C′-type slip zones, generating well-developed foliation that resembles S-C-C′-type structures in a ductile environment. Such deformation fabric is mostly imparted by the preferential alignment of intensely comminuted feldspar fragments along thin slip zones developed within deformation bands. These processes were purely mechanical (i.e., grain crushing and reorientation). No clays or fluids were involved in such processes. [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]

The control of normal fault growth by preexisting basement structures is still a worldwide matter of debate. The objectives of the present study are (i) to investigate where rift faults nucleate in continental-scale ductile shear zones and how these zones influence fault architecture; (ii) how subvertical basement fabric controls listric fault geometry and evolution; (iii) how long reactivation of faults lasts; and (iv) which are the implications of fault reactivation for the oil industry. We contribute to these issues by defining the 3-D relationship between preexisting ductile strike-slip shear zones, the geometry and evolution of subsequent rift-related normal faults using airborne magnetic, 2-D and 3-D seismic reflection, and structural data from the Rio do Peixe Basin, a Cretaceous intracontinental basin in northeastern Brazil. Our results indicate that Precambrian mylonitic foliation and faults share a common strike at the surface, but not a common dip, mostly because the Precambrian shear zones are planar and the reactivated faults are listric (curved). The combined magnetic anomaly and seismic reflection help understand the 3-D geometrical relation between inherited structures and Cretaceous brittle reactivation, which is not obvious from the field data. As inferred from subsurface information, the mylonitic foliation (sub-vertical dip) and shear zone boundary (ca. 60° dip, parallel to brittle faulting) are not parallel. The basin-bounding faults reactivated along ductile shear zones that separate major terranes with mechanical contrast. We conclude that the map view spatial coincidence of Precambrian ductile shear zones and main brittle faults can be deceiving. Mylonitic foliation and faults are parallel along strike but not along dip, indicating that the geometry of rift faults is not controlled by the mylonitic foliation, but by the mechanical contrast between tectonometamorphic units separated by ductile shear zones. • Basin-bounding fault reactivation occurs along shear zones that bound major terranes. • Shear zones control fault nucleation but not their subsequent growth. • Magnetic data are critical for characterizing subsurface basement-basin relations. • Faults curve down dip, thus departing from both foliation and shear zone boundary. • Master faults cross-cut preexisting basement structures along dip but not along strike. [ABSTRACT FROM AUTHOR]

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. • Breaching of faults influences the fault core thickness, damage zone width, and the faults' petrophysical properties. • The thickest part of fault core has the lowest porosity and permeability, whereas the highest capillary entry pressure. • Permeability is reduced up to three orders of magnitude in the fault core compared to the host rock. • Permeability perpendicular to fault dip is one order of magnitude less than permeability parallel to fault dip. • Damage zone is wider in the hanging wall than the footwall after the breaching occurs on the fault. [ABSTRACT FROM AUTHOR]