--> Abstract: Porosity Loss, Fabric Development and the Smectite-to-Illite Transition in Upper Cretaceous Mudstones from the North Sea: An Image Analysis Approach, by Richard H. Worden, Delphine Charpentier, Quentin Fisher, and Andrew C. Aplin; #90039 (2005)

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Porosity Loss, Fabric Development and the Smectite-to-Illite Transition in Upper Cretaceous Mudstones from the North Sea: An Image Analysis Approach

Richard H. Worden1, Delphine Charpentier2, Quentin Fisher3, and Andrew C. Aplin4
1 University of Liverpool, Liverpool, United Kingdom
2 CREGU, Vandoeuvre les Nancy Cedex, France
3 Leeds University, Leeds,
4 Newcastle University, Newcastle, United Kingdom

The way in which highly porous smectite-rich mud lithifies to become compacted, illite-dominated mudstone is largely unknown. Upper Cretaceous Shetland Group mudstone cuttings from a range of depths in the Northern North Sea, UK, have thus been studied using X-ray diffraction, X-ray fluorescence, mercury porosimetry, wireline log analysis and scanning and transmission electron microscopy. Millimetre to micrometre mudstone textures have been quantified using image analysis of backscattered electron microscope images. Relatively shallow samples (1615m) have anisotropic mudstone fabric (no alignment of clay minerals), are dominated by smectite and have porosity values of approximately 35%. In contrast more deeply buried samples (3300m) have developed an isotropic fabric (distinct alignment of clay minerals), are dominated by illite and have porosity values of approximately 15%. Bulk chemistry remains largely constant with depth except for progressive loss of sodium. The change in mineralogy is due to smectite replacement by illite and occurs simultaneously with porosity-loss and the development of defined fabric, clay mineral alignment, during progressive burial. Image analysis of differentially-buried mudstones has proved to be a rapid, flexible and quantitative method for characterising mudstone textures. The coincidence of mineralogical evolution from smectite-rich to illite-rich rock with textural development and compaction imply that the transformation of smectite occurs by dissolution and precipitation and that chemically-facilitated compaction may contribute to porosity-loss.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005