Page 110 - The Geological Interpretation of Well Logs

~ THE GEOLOGICAL INTERPRETATION OF WELL LOGS -

CALIPER laches INTERVAL TRANSIT TIME It is probable that the sensitivity of the sonic to bed-
v1 21
a f_» 4. LiTH.
(a) 140s moilliseconds 40 ding, as well as to texture at a smaller scale, is because

OGAWA RAY fPI_100 pa 2800

the detected signals physically travel up (and down)
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> through the formation. Any horizontal feature, such as
bedding, must be crossed and will affect the response.
This is well illustrated in shales where finely laminated
intervals have a different response to massive intervals.
The example (Figure 8.14) shows a shallow marine shale
23
sé
oe & L
cycle in which the laminated section at the base shows
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= @ higher interval transit times (lower velocity) than the
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an r more massive, upper section.
hK » a0 : L br 2850 sonic log response may not be known and the difficulty
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>
In many cases the exact textural effects causing the
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in calculating porosity from the sonic is a demonstration
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of this. An example of textural changes in turbidites is an
oS
a r ~ = Q a illustration (Figure 8.15). The consistently low gamma
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oh
rue
=
Tay response suggests that there are no compositional
changes and yet the sonic shows distinct variations.
- O Undoubtedly porosity change occurs, but there are also
changes in grain size and bedding (sedimentary struc-
tures). It is not possible to extract, separate and identify
(bo)
each individual influence.
sand més) Correlation — sonic log character
4350
velocity The sonic transit time of a formation is a very distinctive
bit si « characteristic although, as indicated, the precise textural
< and lithological (compositional) causes are difficult to
shale m/s} define. Rather like colour, it is not diagnostic of a partic-
ular lithology but in some formations it is very typical
yelocity (3300 and slight changes indicate subtle formation changes.
The sequence illustrated (Figure 8.16) is entirely shaly:
3000
cuttings and side-wall cores find only shale. The sonic log,

however, picks out small variations, probably in texture,
Figure 8.12 Sonic log in sand-shale sequences. (a) The sands
carbonate content and quartz content, to show a very
have a lower sonic velocity (about 3385m/s) than the shales
distinct stratigraphic interval despite the depth differ-
(3900m/s). (6) The reverse, where the sands have higher
ences. It is this characteristic which makes the sonic
velocity (about 4350m/s} than the shales (3300m/s). Sonic
velocities are therefore not diagnostic of lithology. log excellent for correlation, and even for identifying

GR (API) =x DT (US/F}
0 4 150 5 140 1 30 ' 40

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LL eo
tom - |
a
20m

40m -
~
30m
Figure 8.13 Distinctive sonic log response in coals. The interval transit time is characteristically very high (low velocity)
(cf. Figure 8.11).
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