Influência da silicificação hidrotermal nas propriedades físicas de arenitos arcoseanos afetados por zona de falha, bacia Rio do Peixe, NE do Brasil.

Abstract

Porous sandstones are one of the main types of reservoir rocks in a conventional petroleum system, making it necessary to study their petrophysical and geomechanical properties and hypotheses that can modify them, such as the silicification process and the presence of features such as deformation bands. However, the joint impact of both processes is, as yet, unknown. The Brejo das Freiras Sub-basin, belonging to the Rio do Peixe Basin, located in northeastern Brazil, is a possible scenario to investigate the impact of both processes on a siliciclastic reservoir, as it presents porous sandstones (Antenor Navarro Formation) with deformation bands and hydrothermal silicification, both related to a fault zone. Therefore, this work combines geological and structural field data, petrographic and petrological analysis, chemical analysis (SEM/EDS and BSE) with geomechanical and petrophysical data (2-D porosity and 2-D permeability, obtained from thin sections, and in situ permeability, obtained from a portable minipermeameter). From these data, petrophysical and geomechanical characterizations and the generation of porosity and permeability models were performed. Our results indicate that the occurrence of deformation bands and hydrothermal silicification in porous sandstones can promote considerable changes, such as: (1) porosity reduction of up to 64% in the region between deformation bands of intensely silicified sandstones; (2) porosity reduction in up to 1 order of magnitude inside the deformation bands, in relation to the region between deformation bands of silicified sandstones; (3) 2-D permeability reduction of up to 3 orders of magnitude in the region between bands of intensely silicified sandstones; (4) reduction of 2-D permeability in deformation bands of silicified sandstones; (5) 3-D permeability reduction of up to 2 orders of magnitude in the region between deformation bands of intensely silicified sandstones; (6) clay minerals authigenesis; (7) deformation bands and region between bands of intensely silicified sandstones with very small 3-D permeability differences; (8) region between deformation bands with greater geomechanical strength than deformation bands, in intensely silicified regions, making the deformation bands show no relief in this context. These modifications have an effect on the three models, which have visible similarities to each other, in addition to producing differential erosion, as they make the intensely silicified sandstone more geomechanically resistant than nonsilicified sandstones and than sandstones with intermediate silicification. The in-situ permeability indicates that the regions between strain bands had reductions of up to 2 orders of magnitude in the mean values of insitu permeability. Deformation bands and region between bands may present equivalent insitu permeability values, suggesting the precipitation of silica cement inside the deformation bands.