Numerical modeling of brittle tectonic deformation in poorly consolidated sandstones

Abstract

This work presents the results of a study about the effects of brittle deformation on the distribution of porosity and permeability in poorly consolidated sandstones. The study was carried out in an outcrop of the Resende Formation, located in the Volta Redonda Basin (State of Rio de Janeiro, Brazil), which has been described by other authors as a geomechanical analog of poorly consolidated Maastrichtian sandstone reservoirs in the Campos Basin. This outcrop is composed of fluvial deposits of poorly consolidated sandstones interlayered with green siltstones, characteristics of the Resende Formation. According to previous works on the outcrop, the deformation that operated in these deposits results from the superposition of three of the tectonic events that occurred in the Volta Redonda Basin – a right-lateral strike-slip deformational event (TD), a left-lateral strike-slip deformational event (TS), and finally a Holocene extensional event (E2). These events produced a graben structure, several minor secondary faults and deformation band zones. To better understand how the evolving process of those structures affected the distribution of the original porosity and permeability through the outcrop, a series of finite element numerical models were done in this study, based on the continuum mechanics. In these models, only the final extensional tectonic event in the Volta Redonda Basin, E2, was considered. Results showed a higher volumetric loss caused by deformational compaction in the hanging wall of the main outcrop structures. These models are consistent with the permeability distribution measured in the outcrop and with the observed rock-staining patterns derived from the iron-precipitation process associated with zones of restricted fluid flow, and reveal notable differences concerning tectonic strain distribution between the hanging wall and footwall blocks. Although the uncertainty in the geomechanical parameters is a challenge, the technique can improve geological modeling of reservoirs for exploratory purposes, especially in poorly consolidated and fractured sandstones related to the extensional structural style of Brazilian offshore basins.