Page 83 - The Geological Interpretation of Well Logs

- THE GAMMA RAY AND SPECTRAL GAMMA RAY LOGS -~

In practical terms, the compromise is that the simple The spectral tool is sensitive to speed effects, essen-
gamma ray too] should not travel more than 30 cm in the tially because the count rate of the individual detection
time constant. Table 7.5 sets out the limitations of time- windows is very small. Because the time constant is
constants and logging speeds. However, even following generally fixed, the significant variable is the logging
these constraints, gamma ray log repeatability in the speed. Lower speeds allow higher counts per formation
minor variations is poor. This is caused by a combination interval. Tools in which the low energy spectrum is used
of the logging method, the computing method and the are as a rule run faster than tools in which the low energy
natural statistical variation of gamma ray emissions. A spectrum is not used. Table 7.6 gives recommended
comparison between a main log and a repeat section or speeds. Despite these recommendations, many, if not
between separate runs of the gamma ray tool, gives an most spectra] logs have poor repeatability (Hurst, 1990
empirical indication of the extent of the problem. It only and Figure 7.9), which may be a result of logging speed
involves the smal] scale variations. rather than any inherent error. It is good practice to run a
repeat section with every spectral log so that the variability
Table 7.6 Spectral tool logging, time constants.
can be judged. Moreover, interpretation methods must

take the quantitative level of variability into account.
Time constant Logging Formation Company
(seconds) speed logged in Unwanted borehole effects
time constant Simple gamma ray — The simple gamma ray log is rela-
(m/h) (cm) tively unaffected by small-scale borehole irregularities,

but is affected by large caves (Table 7.7). The effect is
4 275 30.5 Schlumberger due to the increased volume of drilling mud between the
6-8 183 30.5-40.6 Western Atlas formation and the gamma ray detector which causes
increased Compton scattering and a consequent diminu-
depth related 275 nf/a B.P.B.
tion in the gamma ray log value, as described previously.

Most logging companies publish charts to correct for
SPECTRAL GAMMA RAY
borehole size with mud weight consideration.

Uranium A quite different effect is caused by the use of the
m
radioactive mud additive KCl. The potassium radio-

L
10
pp
_?
i.
activity of the KCl is detected by the gamma ray tool and
x Thorium Potassium the usual result is a marked increase in the absolute

- ppm

GAMMA RAY API
160

(m)

depth 25m-

metres

50m
— main run
--- repeat
Figure 7.10 The effect of KCI in the drilling mud on gamma
Figure 7.9 Repeatability of the spectra] gamma ray. Precise ray values. Well 1, with ordinary mud, well 2 with KC! mud.
repeatability is generally poor but it should be noted that the The formation values should be the same. A, is the difference
quantities being detected are very smat). created by the KCI content. The wells are 3km apart.

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