Page 109 - The Geological Interpretation of Well Logs

- SONIC OR ACOUSTIC LOGS -

»
2 INTERVAL TRANSIT TIME INTERVAL TRANSIT TIME wift al
2 microseconds per loot 200 160 100, 80 79 80 so 40

T
= T9’Ry2’Ro
3 hh40 90 VELOCITY kiloleei/second

L § 1 2 1 L 10 J. 1 1s pa dl po 2s $9
ll
20

Igneous
oll
a
gas =) | = water metamorphic =
oy \y = = =
yelocities|lowered anhydrite
oceanic basalts
it
i
coals
salt _—_——_
—_

dolomitas
= =
(chalks) limestones

sandstones

shales

4
a
2
5
a
6
7
VELOCITY km/second
Figure 8.11 The average velocity ranges of common

lithologies compared. The considerable amount of overlap
indicates that velocity alone is seldom diagnostic of lithology
{values are for depths typical of oil exploration wells). See
velocities.
i Table 8.4 for matrix velocities and Table 8.5 for mineral

Figure 8.10 The effect of gas on the sonic tog. The sonic
velocity in this porous sandstone is lowered by about 8%. Table 8.4 Some typical sonic matrix velocities (see also
Figure 8,11) (from Schlumberger, 1972; Serra, 1979;
Gearhan, 1983).
(Table 8.4, Figures 8.11, 8.12). However, such is the nat-

ural occurrence that high velocities are more likely to be
Ar (uift) Vins) V(fvs)
associated with carbonates, middle velocities with sands

and shales and jow velocities with shales (Figure 8.11).
Sandstones 55.5-$1 5490-5950 18,000-19,500
Velocity, nonetheless, is diagnostic of coals, which
{compacted)
have unusually low values (high interval transit times)
Quartz 55.] 5530 18,150
(Figure 8.13). It is also diagnostic of the evaporites,
which are essentially chemically pure substances with Limestones 5347.6 5800-7000 19,000-23,000
predictable physical properties, such as halite (rock salt), Calcite 46.5 6555 21,500
gypsum and anhydrite (Table 8.5). Moreover, halite Dolomites 45-38.5 6770-7925 22,200-26,000
velocities do not vary with depth. Dolomite 40 7620 25,000
Shale 167-62.5 1600-5000 5000-16,000
Texture

Although sonic response may not be diagnostic in terms
of lithology, it is very sensitive to rock texture, even
subtle changes. The way in which sound travels through Table 8.5 Some diagnostic (mineral) velocities (from Serra,
a formation is intimately associated with ‘matrix, matrix 1979; Gearhart, 1983; Schlumberger, 1985).
materials, grain size distribution and shape, and cementa-
At(usift) Velocity*(m/s) Velocity*(fUs)
tion’ (Wylie et al,, 1956), in other words texture (Figure

8.7). This is true for most lithologies and in fact extends
Water (saline) 189-200 1610-1525 5290-5000
upwards in scale to include also structural characteristics
Halite 66.7-67 4550 15,000
such as bedding. For example, the sonic has a very
Anhydrite 50 6100 20,000
distinctive response to slumped, debris flow intervals in
the Upper Cretaceous chalks of the North Sea. Over a Gypsum $2-53 5860 19,000
cored interval where the chalk bedding characteristics can Anthracite 90-120 3050 = 10,000
be detailed, intervals of debris flow, which have chaotic Lignite 140-180 2000 ~ 6500
textures and no bedding, show distinctly higher interval casing (steel) 57.8 5270 17,300
transit mes (lower velocity) than normal chalk which is
generally thin-bedded (Hatton, 1986). * Averages
99