Effects of diagenesis and clays on compressional velocities in rocks

Authors:
  • CTCarol Tosaya
    Carol Tosaya
  • ANAmos Nur
    Amos Nur
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Abstract
Experimental results at ultrasonic frequencies in brine‐ or water‐saturated samples cut perpendicular to bedding demonstrate that compressional velocities vary linearly with porosity and volume‐fraction clay in detrital silicate rocks characterized by pores with low aspect ratios. P‐wave velocity is more sensitive to porosity than clay content, obeying the empirically derived expression Vp = −2.4C − 8.6ϕ + 5.8 at 800 bars confining pressure and 400 bars pore pressure, where the units of Vp are kilometers per second, ϕ is the volume fraction pores, and C is the volume fraction clay. Deviations from this equation are less than ± 2% with the exception of the high pore aspect ratio St. Peter sandstone, which has a compressional velocity that is 17% higher than the predicted value. The equation is insensitive to the chemical compostion of the dominant clay mineral and apparently insensitive to the location of the clay grains with respect to the mineral framework, e.g., in free pore space or in contact zones between mineral grains of lower compressibilities.

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