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Velocity-depth trends in Mesozoic and Cenozoic sediments from the Norwegian Shelf: Reply

¹ Statoil Research Center, 7005 Trondheim, Norway; vidar.storvoll@statoil.com
² Department of Geociences, P.O. Box 1047, University of Oslo, 0316 Oslo, Norway; knut.bjorlykke@geo.uio.no
³ Department of Geociences, P.O. Box 1047, University of Oslo, 0316 Oslo, Norway; nazmul.haque@geo.uio.no

The first 20% of the full text of this article appears below.

Based on logs from 60 wells located on the Norwegian Shelf, we concluded that compaction and velocity trends vary greatly with depth in different types of mudstones and shales (Storvoll et al., 2005). Japsen (2006), however, argues that compaction of shales can be represented by a linear velocity-depth trend as a function of the overburden stress and is therefore mainly sensitive to overpressure. In our opinion, the treatment of mudstones and shales as one rock type during depth conversion of seismic data, pore-pressure prediction, or basin modeling represents a descriptive approach, which fails to recognize the great variability in their composition and the complex processes involved during sediment compaction. We welcome this opportunity to explain and defend our interpretations.

Sediment compaction is controlled by the primary sediment composition and the mechanical and chemical changes during burial. Each primary lithology, with different mineralogy, grain size, sorting, carbonate content, etc., will respond differently to the varying compactional processes, and they will therefore not show the same increase in velocity with depth. It has been established that different clay minerals have different stress-strain curves, and that smectite behaves very differently from kaolinite and illite (Chillingar and Knight, 1960; Aplin et al., 1995). The compressibility of mudstones is also strongly influenced by the salinity of the pore water (Rosenqvist, 1962; Muller, 1967), and the thermal stability varies greatly for different clay minerals (Bjørlykke, 1998). These variations in mechanical and chemical compaction explain the scatter in porosity-depth data (e.g., Giles, 1998) and the varying sonic and seismic response for different types of shales.

Some of our data, particularly from the Mesozoic section, show velocity-depth trends that are similar to previously published averaged trends by Hansen (1996), Hermanrud et al. (1998), Japsen . . . [Full Text of this Article]