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Petroleum Geoscience

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Mineralogical control on mudstone compaction: a study of Late Cretaceous to Early Tertiary mudstones of the Vøring and Møre basins, Norwegian Sea

Department of Geosciences, Post Box 1047, Blindern, University of Oslo, NO-0316 Oslo, Norway(c.b.peltonen{at}geo.uio.no)

The Late Cretaceous to Early Tertiary sediments of the Vøring and Møre basins are predominantly composed of fine-grained mudstones. Variations in the mineralogy and chemistry of these mudstones provide information regarding facies, provenance and burial history, and may also be used to predict rock properties. Over 300 cuttings’ samples from five wells were analysed by XRD. The results show significant changes in mineralogy as a function of burial depth, as well as important lateral variations throughout the basins. Eocene mudstones with up to 55% smectite probably represent a northern equivalent of the Balder Formation (North Sea). The underlying Late Cretaceous sequence probably had less primary smectite derived from volcanic ash, as indicated by the lower iron content. The distribution of smectite is also limited by its thermal stability, thus providing important constraints on the temperature history. These mudstone sequences may appear to be relatively homogeneous based on gamma-ray and shale volume calculations from wireline logs, but mineralogical and geochemical analyses from cuttings show that they vary significantly in composition. The smectite content is greatest in the south (c. 55%) and decreases significantly northward (c. 20%), indicating a marked regional control on velocity/porosity–depth curves. Mudstones containing high smectite content are characterized by lower velocities, lower densities and higher porosities when compared with published burial curves. Stratigraphic and regional variations in velocity and density are important for seismic interpretation and are significant for basin modelling.

KEYWORDS: diagenesis, smectite, clay mineralogy, shales, XRD