Deformation bands in chalk - control on distribution and mechanism of formation
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Deformation bands are narrow tabular zones that accommodates strain through the reorganization of grains by shearing and/or volumetric deformation in porous and granular rocks. Deformation bands in porous sandstones are widely documented, fewer studies are done on deformation bands in porous carbonate rocks. There are even fewer published examples of deformation bands in porous chalk. Inspired by this fact, the current study presents the results of detailed thin-section and SEM analyses of deformation bands present within carbonate rocks from the Upper Maastrichtian to Danian chalk from the Oseberg Field area, northern North Sea. The studied deformation bands are hosted in chalks from the Hardråde/Jorsalfare Formation in the Oseberg fault block in association with stylolites, located 2200 to 2300 m below the sea level. The deformation bands found in the chalk are very thin with a μm-scale thickness, i.e. much thinner than mm- to cm-thick deformation bands typically found in porous sandstones and carbonate grainstones. Two types of deformation bands have been described and classified in the chalk by structural analysis and microscopic investigations: (1) bed-parallel compaction bands (CBs) and (2) bedding-oblique solution-compactive shear bands (SCSBs). For the structural analysis of the deformation bands in the studied cores, methods include recording the frequency of deformation structures (the amount of deformation structures bisecting the scanline per 0.2 m) along scanlines and the orientations of the different deformation structures that bisect the scanline. The microstructural characterization of the deformation bands involved thin-section analysis by means of the optical microscope and SEM. The porosity measurements of the deformation bands and the host rock were performed by the ImageJ software. Both deformation bands and extension fractures seem to be present in the chalk as what has been referred to a hairline fractures" or healed fractures". Due to the difficulty of distinguishing the two different deformation structures at the macroscopic scale, it has been concluded that microscopic analysis is necessary to distinguish between the two. In this work, the deformation bands are suggested to have been formed due to burial and differential compaction, where the maximum stress axis (σ1) was oriented perpendicular to the bedding. Differential compaction could have occurred due to lateral variation in the thickness to the fault basin filled sediments comprising of the syn-rift sediments of Jurassic and the post-rift sediments of Cretaceous overlain by Cenozoic sediments. Evident by the top Cretaceous sediments shaping out as an open anticline, the differential compaction would have applied even more strain to the bedding than burial compaction alone. The deformation band features where not easy to observe in SEM-images. Their narrowness, with an apparent thickness of about 30-220 μm, could be one of the reasons why these features easily have been overlooked in many chalk reservoirs. The grain size reduction has occurred solely by disaggregation of grains and pressure solution mechanism. It is suggested by this study that the stylolites is the baffle and controls the conduits of fluid flow, with some influence by the deformation bands within the studied chalk reservoir.