Abstract
Rock permeability is sensitive to the deformation caused by the stresses acting in oil reservoirs. These stresses can be either tectonic, generated mainly by the migration of the lithospheric plates, or lithostatic, caused by the weight of rock layers. Under reservoir conditions, stresses produce anisotropies in the pore spaces. Thus, the recognition of the relation among pore distribution, permeability and stress is important for decisions on planning, development and exploitation of oil reservoirs. The main objective of the present work was to study the relation between tectonic stresses and permoporosity of a reservoir analog in the Açu Formation, Potiguar Basin, Brazil. Oriented plugs were collected, according to the directions of the operating stresses in the basin; these directions were derived from earthquake data (focal mechanism), wells (breakout and anaelastic strain recovery - ASR), fault analysis, joints and dissolution structures, in Quaternary rocks less than 1.0 Myr. From these methods, it was concluded that the preferential direction of the maximum horizontal stress (SHmax) in Potiguar Basin ranges from EW to NW. An outcrop analysis led to the identification of seven lithofacies from which the samples were collected to be used in petrophysical and petrographic characterization. In qualitative terms, it was observed that the porosity found in thin sections is predominantly intergranular, followed by the intragranular (mainly in the interior of the feldspars), inter-crystalline (between the calcite crystals), and oversized types. The petrophysical data indicate that there is no significant difference between porosity and permeability values in both stress directions (SHmax and Shmin). Nevertheless, when porosity values obtained from the description of thin sections are analyzed, it is observed that the highest values are found along the direction of maximum stress. A possible explanation for this discrepancy with petrophysical data is the impossibility of pointcounting, in contrast with the porosimeter, in quantifying microporosity present in diagenetic clays. The foregoing data afford no evidence of the influence of tectonic stress over porosity and permeability in the Açu Formation outcrops. Permoporosityanisotropy, if present, may be caused by other factors such as lithofaciology, that is, sedimentary depositional conditions.
