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.