Porosities, velocities, and densities of rocks

From SEG Wiki
Jump to navigation Jump to search

Problem 5.3a

Assume that sandstone is composed only of grains of quartz, limestone only of grains of calcite, and shale of equal quantities of kaolinite and muscovite. For sandstone, limestone, and shale saturated with salt water , what porosities are implied by the densities shown in Figure 5.3a? (Mineral densities are: ; ; ; , all in g/cm.)


Gardner et al. (1974) plotted the log of velocity against the log of density for sedimentary rocks and obtained the empirical relation known as Gardner’s rule:

Table 5.3a. Rock densities and porosities.
Rock Density range Density av. Mineral density Max Av. Min
Ss 2.00–2.60 g/cm 2.35 g/cm 2.68 g/cm 41% 20% 5%
Ls 2.20–2.75 2.55 2.71 30 10 0
Sh 1.90–2.70 2.40 2.72 48 19 0

being in g/cm and or 0.23 when is in m/s or ft/s, respectively. The rule is valid for the major sedimentary rock types, but not for evaporites or carbonaceous rocks (coal, lignite).

When a porous rock is saturated with a fluid, its density is given by the equation


being the porosity, and the densities of the fluid and rock matrix, respectively.


The density ranges in Table 5.3a were obtained from Figure 5.3a. The mineral densities are for , the values for shale being averages for kaolinite and muscovite.

We solve equation (5.3b) for , obtaining

Figure 5.3a.  Histogram of rock densities (from Grant and West, 1965).

The histogram in Figure 5.3a does not encompass the complete range of samples and the range limits have been picked somewhat arbitrarily. Porosity in rocks ranges from about 50% to 0%. The upper limits of the density range sometimes exceed the mineral densities, hence heavier minerals must be present in the rocks; in these cases we assume that . We take as the fluid density.

Problem 5.3b

What velocities would be expected for the density values in Table 5.3a according to Gardner’s rule? Plot these on Figure 5.3b.


We solve equation (5.3a) for the velocity , obtaining

The velocities in Table 5.3b are plotted as triangles on Figure 5.3b.

Table 5.3b. Velocities obtained from densities in Table 5.3a.
Ss 1.8 (41%) 3.4 (20%) 6.8 (0%)
Ls 2.6 (30%) 4.6 (10%) 6.8 (0%)
Sh 1.4 (48%) 3.4 (19%) 6.8 (0%)
Note: The values in parentheses are the porosities.
Figure 5.3b.  P-wave velocities from different sources. (1) Grant and West, 1965; (2) Kearey and Brooks, 1984; (3) Lindseth, 1979; (4) Mares, 1984; (5) Sharma, 1997; (6) Sheriff and Geldart, 1995; (7) Waters, 1987.

Problem 5.3c

From Figure 5.3c, what densities would you expect at 7500 ft and how do these compare with Figures 5.3d and 5.3e from offshore Louisiana?

Figure 5.3c.  Porosity versus depth (from Atkins and McBride, 1992).
Figure 5.3d.  Average sand density between 7000 and 8000 ft (courtesy Geophy. Develop. Corp.).
Figure 5.3e.  Average shale density between 7000 and 8000 ft (courtesy Geophy. Develop. Corp.).


Using and from Figure 5.3c, equation (5.3c) gives , which is in accord with Figure 5.3d. Using , equation (5.3c) gives , which is slightly lower than most values in Figure 5.3e.

Continue reading

Previous section Next section
Relation between lithology and seismic velocities Velocities in limestone and sandstone
Previous chapter Next chapter
Geometry of seismic waves Characteristics of seismic events

Table of Contents (book)

Also in this chapter

External links

find literature about
Porosities, velocities, and densities of rocks
SEG button search.png Datapages button.png GeoScienceWorld button.png OnePetro button.png Schlumberger button.png Google button.png AGI button.png