Deformation bands in chalk, examples from the Shetland Group of the Oseberg Field, North Sea, Norway

Deformation bands are described in detail for the first time in carbonate rock from the subsurface and in chalk from the North Sea. The samples are from 2200 to 2300 m below sea level, in upper Maastrichtian to Danian chalk in the Oseberg Field. The deformation bands were investigated using thin-section analysis, SEM and computed tomography (CT). There is a reduction in porosity from 30 to 40% in the matrix to ca. 10% or less inside the deformation bands. They have apparent thicknesses ranging from less than 0.05-0.5 mm and have previously often been referred to as hairline fractures. Their narrowness is probably the reason why these features have not previously been recognised as deformation bands. The deformation bands in chalk are very thin compared to deformation bands in sandstone and carbonate grainstones which have mm to cm widths. This is suggested to be due to the fine grain size of the chalk matrix (2-10 μm), and it appears to be a positive correlation between grain-size and width of deformation bands. The deformation bands are suggested to have been formed as compactional shear bands during mechanical compaction, and also related to faulting.