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Abstract

In this study, Upper Cretaceous Shetland Group mudstone cuttings from a range of depths in the Northern North Sea, have been studied using X-ray diffraction, mercury porosimetry and electron microscopy. Millimetre to micrometre mudstone textures have been quantified using image analysis of backscattered electron microscope images. Relatively shallow samples (1615 m) have isotropic mudstone fabric (no alignment of clay minerals), are dominated by smectite and have porosity values of approximately 35%. In contrast, more deeply buried samples (3300 m) have developed an anisotropic fabric (distinct alignment of clay minerals), are dominated by illite and have porosity values of approximately 22%. The change in mineralogy is due to smectite replacement by illite, which occurs simultaneously with porosity-loss and fabric development during progressive burial. Image analysis of differentially buried mudstones has proved to be a rapid, flexible and quantitative method for characterizing mudstone textures. The coincidence of mineralogical evolution with textural development and compaction implies that the transformation of smectite to illite occurs by dissolution and precipitation and that chemically facilitated compaction may contribute to porosity loss.

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