Ultrasonic velocities of North Sea chalk samples: Influence of porosity fluid content and texture

  • Røgen B
  • Fabricius I
  • Japsen P
 et al. 
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The acoustic properties of the pure chalk of the Upper Cretaceous Tor Formation have been studied on core material from the Dan and South Arne fields, Danish North Sea. Detailed investigations were carried out on 56 vertical plug samples with porosities ranging from 14% to 45%, i.e. measurement of P- and S- wave velocities of dry and water-wet samples and microtextural image analysis on backscatter micrographs. Porosity is the primary control on sonic velocities and elastic moduli of chalk with identical pore fluids. For chalk samples dominated by fine carbonate particles (mud) we obtain a well-defined relation between elastic moduli and porosity, which can be modeled using a modified upper Hashin-Shtrikman bound. The trend is defined by a bulk and shear modulus at zero porosity of 54 and 22 GPa, and by a high-porosity end-member at 47%. We find that compaction, grain size distribution, and possibly mineralogy, rather than pore-filling cementation, control the variation of porosity for the samples.

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