Characterization of Shear Zones in Porous Sandstone of Sergipe-Alagoas Basin, Brazil

By

Fernando Santos Correa¹, Hung Kiang Chang¹

(1) State University of Sao Paulo, Rio Claro, Brazil

 The Sergipe-Alagoas Basin, located in northeastern Brazil, hosts on the border excellent outcrops with very good exposition of deformed rocks, representative of fault zones. The studied outcrop comprises pre-rift fluvial sandstones of Serraria Formation (Neo Jurassic). The absence of detailed information about fault in subsurface made necessary to study outcrop rock analogous with similar conditions of petroleum reservoir, characterizing in greater detail the geometry structure, as well as its petrographic and petrophysical properties, aiming to improve the structural models of fault compartmented fields. Geometrically, the deformation bands present eye-like geometry and high density of deformation bands above of slip plane. Rock deformation occurs through grain comminuition without clay, and with low cementation. Band thicknesses vary from millimetric to three-meter thick and may reach up to 3 m in more advanced stages of deformations. Petrophysical experiments show large heterogeneity in permeability values, varying from 3 to 4 orders of magnitude, between deformed and non-deformed rock. Porosity range reaches one order of magnitude. Microscopic analyses in thin section allowed the identification of several deformation bands characteristics, such as fractures and grain orientation, as well as evidence of the generation processes seal structures. Some experiments of capillary pressure set for the sealing capability of deformation bands are as high as 241m of oil column, which make them efficient traps. Initially, the confining pressure causes increase of porosity due to dilation with grain rotation. The deformation increases when the confining pressure reaches fracture level of grains, causing porous collapse, decreasing the petrophysical properties.