Page 89 - The Geological Interpretation of Well Logs

- THE GAMMA RAY AND SPECTRAL GAMMA RAY LOGS -

2. RADIOACTIVE ELEMENTS
80
1. NON-RADIOACTIVE ELEMENTS
clays
100

detrital grains {

0
x
§ 9

= 40
radioactive detrital
8
/
grains
E °
Qa
3

feldspar
=
0.031 0.0863 0.125 0.25 050 1.0 mM

SILT Vv. FINE FINE ME CSE
. . 0.031 0.063 0.125 0.25 0.50 1.0 mm
median grain size ==
SILT Vv. FINE FINE ME CSE
median grain size —~
Figure 7.16 Radioactive elements in detrital rocks. Typical sandstone compositian shown against grain size for the Reindeer
Formation, Mackenzie Deita, Beaufort Sea. Radioactive elements vary with grain size. It is a deltaic sand of Lower Tertiary age.
(Redrawn from Nentwich and Yole, 1982).
Table 7.15 Potassium content of some common detrital materials (from Serva, 1979;
Edmundson ef? ai., 1979; Dresser Atlas, 1983; Schlumberger, 1985).

% potassium Average Gamma ray value
Mineral species by weight % (API)

4 Glauconite* 3.2 -5.8 4.5 75' - 90'
$ Muscovite 7.9-9.8 9.8 140' - 270
Botite 6.2 - 10.1 8.7 90° - 275

Microline 10.9- 16 16 220 — 280'
ww
xs Orthoclase 11.8-14 14 220 — 280'
cm

*Detrital or authigenic
‘For 8in hole, 1.2g/cm? mud, 3%in Nal scintillator

7.8 Qualitative use of the Radioactivity of sandstones and
simple gamma ray log other arenaceous rocks
Quartz, the principal component of the coarse-grained
Lithology
detrital rocks, shows no radioactivity. Sandstones conse-
As a first indicator of lithology, the gamma ray log is
quently usually show low gamma ray values (Figure 7.1).
extremely useful as it suggests where shale may be
However, associated detrital minerals are radioactive. The
expected (Figure 7.1). Moreover, as shown above, the
most common of these are feldspars, micas, heavy
higher the gamma ray vajue, the higher the percentage of
minerals and lithic fragments (Figure 7.16). The first two
shale (Figure 7.15). But the log is only a first indicator.
groups contain potassium (Table 7.15), the third thorium
The radioactivity of some typical lithologies other than
(Table 7.12) and the last contains shale. These all cause
shale is now considered. This shows that any lithology
sandstones with high to moderate gamma ray values.
indicated by the simple gamma ray log must be con-
firmed by other logs.
79