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Deformation Bands in Chalk From the Oseberg Field, Northern North Sea, Norway

Abstract

Deformation bands are narrow tabular zones of grain reorganization in porous and granular rocks formed under deformation. In this study we present a detailed description of deformation bands from the upper Maastrichtian to Danian chalk of the Oseberg Field. Description of the deformation bands is based on optical microscopy, SEM and computed tomography (CT) using core, thin sections and plug data. The deformation bands observed in the chalk of the Oseberg Field generally occur at high angle to bedding (60° to 90°), they typically form a conjugate pattern and they tend to be preferably developed in relatively clean chalk with high porosity. SEM imaging shows that the deformation bands are associated with a porosity reduction from ca. 35 % in the host rock to 10 % or less within the deformation bands. The chalk has grain size from 2 to 10 μm and the deformation bands have an apparent thickness of 50 to 500 μm. The thickness is significantly less than reported previously for deformation bands in sandstone and carbonate grainstone. It is suggested that the thickness of the deformation bands is partly controlled by the grain size of the deformed material. In many previous studies of chalk deformation, authors referred to similar features as hairline fractures. We believe that in many cases these are in fact deformation bands which have not been recognized as such, due to their sub-millimetre scale. Based on geometry and internal characteristics, it is suggested that the studied deformation bands can be classified as compactional shear bands. It is suggested that the majority of the deformation bands formed during vertical mechanical compaction of the chalk in a stress regime where the maximum compressive stress axis is vertical, and in zones adjacent to faults where they formed in a locally variable stress regime dependent on fault orientation and offset. Preliminary analysis of plug measurements indicates that the presence of deformation bands has a minor effect on the porosity-permeability relationship in chalk. Further studies are necessary to conclude on the dynamic effects of deformation bands in chalk.