Page 97 - The Geological Interpretation of Well Logs

- THE GAMMA RAY AND SPECTRAL GAMMA RAY LOGS -

100% illite ‘point’
possible 100% kaolinite ,
20 —
montmorillonite, illite ‘clay ling’ —_—
—_— —_—_
—_—_

kaolinite

+— 70% illite
\ oe
STONEY .
Bins 40% micas

ae mic bs

feldspar line

80% glauconite

3
K {%)
Figure 7.27 Graph of the theoretical distribution of clay minerals, heavy minerals and evaporites, in terms of potassium and
thorium content. (Re-drawn from Quirein ef al., 1982).

ui .
E & ~< —_ >

£ © = ThiK ratio Thorium ppm| Potassium % | Uranium ppm
on a
Lfu22, Jeol, 12 0
0
Gin Lauda ,
lg
5m Pa
2
o 5 8
«&
<2

é
Ls
s ©
o8
s 3
23
tz
Figure 7.28 Thorium/potassium, Th/K ratio changes in shales, associated with climatic variation. High ratios are associated with a
humid climate (abundant kaolinite) low values with an arid climate (abundant illites}. Westphalian and basal Permian, central UK.

to Cretaceous of central Kansas (Doveton, 1991), shows effect a progressive change in clay mineralogy). The
that low Th/K ratios (high potassium) are typical of the example (Figure 7.28) shows two sections from the same
aeolian Permian shales and silts, where the high potas- well over a 300 m interval. The Th/K ratio decreases pro-
sium content comes from feldspars, rock fragments and gressively upwards in the shales. The lowest section is
illites, while high Th/K ratios (low potassium) occur in from the deltaic coal-bearing Westphalian which had a
the marine, Lower Cretaceous because the shales are humid climate. The top section is from the Permian with
dominated by kaolinite and illite with some chlorite, an arid climate (Figure 7.28). Within a sandstone
smectite and mixed layer clays, all generally low in sequence, similar progressive changes are more likely to
potassium content. indicate mineralogical variations than climatic ones and
This example contains an often observed aspect of for instance, channel lags often show a high Th/K ratio
Th/K ratios: when they change progressively in shale because of the heavy minerals they contain.
sections, it is an indication of climatic change (it is in
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