PERMFILE128524

• Permit No: M1999002
• IBM Index Class Name: Permit File
• Doc Date: 6/18/1999
• Doc Name: GAMMA RAY AND SPECTRAL GAMMA RAY LOG
• Media Type: D
• Archive: No

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- TIIE GAIIJIA RA5' AND SPECTRAL GA)I\IA RAY LOGS -
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LOGS CORRECTED TC TVD
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50
SCALE
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Figure 7.26 Correlation using the gamma ray log. Baronia field. Sarawak. (From Scherer, 1980).
and often leads to false correlations (e.g., Figure 15.20).
Log shapes in carbonates are generally related [o shale
distribution and as such are more reliable for corelation.
However, the shapes must be sufficiently consistent to
ensure that they are not related to uranium concentration,
as discussed above (see 'Radioactivity in Carbonates'
above).
Although i[ has many advantaees for correlation, the
gamma ray log also has disadvantages. The fine detail on
the logs is generally noise and statistical variation (pars
7.4). A comparison between any log and a repeat section
shows to what extent [his has an effect. Fine peaks
therefore cannot be used for correlation. The second
disadvantage is that the gamma ray cannot be calibrated
(cf. Chapellier, 1992). Although absolute values are
given on the logs they are relative both to hole size and
tool, (Section 7.4, Figure 7.8). Logs, to be entirely
comparable, must be 'normalized' (see Chapter I I ).
7.9 Quantitati~'e use of
the spectral gamma ray' log
The spectral gamma ray log, like the simple gamma ray,
is used to calculate shale volume. It can also be used [o
.calculate the volume of radioactive minerals.
Shale volume
In the description of shale radioactivity given previously
(Section 7.6), it was shown that the three naturally
radioactive elements are not distributed regularly in
shales. Some spectral logs are therefore plotted with a
computed potassium + thorium radioactivity tune as a
better shale indicator (Figure 7.6). However, as described,
potassium tan occur in detrital minerals such as micas
and feldspars so that thorium can be considered as the
best shale indicator (Fertl, 1979; Schenewerk er nl..
1930). The shale volume calculated from the spectral
gamma ray log therefore may be based entirely' on the
thorium values.
The mathematical relationship between thorium value
(in ppm) and shale volume is taken as linear, the same
relationship as between the simple gamma ray and shale
volume. The equation becomes
VV,n (r) _
Th (log value)-Th (min)
Th (max)-Th (min) (4)
Th (min) =thorium value in clean formation (ppm); Th
(max) =thorium value in pure shale (ppm), and Ve(t) _
shale volume from thorium values.
As with [he simple gamma ray, an empirical, exponen-
tial relationship to clay volume may be used instead of
the simple linear one shown above (Fenl, 1979), i.e. for
consolidated and Mesozoic rocks -
VV,h = 0.33(2""t't -1.0) (5)
85
SW BN-14 BN-17 BN-10 BN-6 BN-11 NE
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