Influence of hydrothermal silicification on the physical properties of a basin-boundary fault affecting arkosic porous sandstones, Rio do Peixe Basin, Brazil

Abstract

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.