Page 96 - The Geological Interpretation of Well Logs

- THE GEOLOGICAL INTERPRETATION OF WELL LOGS -

and for Tertiary clastics from uranium. The evidence suggested that the uranium
source was principally smectite, its presence being
V = 0.083(2°7— 1.0) (6) caused by the exchange of the uranyl ion from the forma-
tion waters. Uranium radioactivity was therefore related
where V, = shale volume. to the presence of smectite. Almost exactly the opposite
was found in the analysis of shales around the North Sea
Radioactive mineral volume (Dypvik and Enksen, 1983). The authors found that
Attempts to quantify the presence of radioactive minerals potassium and thorium were the dominant contributors
such as feldspars or mica are based on two assumptions: to gamma ray activity with uranium being of minor
(1) all thorium radioactivity is from shale, and (2) importance (cf. Table 7.14).
radioactive detrital minerals show only potassium A complex quantitative approach to clay-mineral
radioactivity. identification has been proposed (Quirein ef al., 1982).
For the quantification, the potassium values are normal- The authors suggest that clay mineral! species, along with
ized for shale volume using the maximum and minimum feldspar and evaporites, can all be identified relatively
method as for thorium. The normalized potassium value simply by their Th/K ratios (Figure 7.27). There is
will give shale volume + radioactive minerals volume. certainly a tendency for this behaviour (cf. Tables 7.8,
Subtracting the shale volume derived from the thorium 7.13) and it is the basis for using just thorium as a shale
log will leave the volume of radioactive minerals indicator (see ‘Quantitative uses’}. However, individual
{Schenewerk er ai., 1980). clay minerals do not fall into such a simple classification.
Such a classification demands a strict chemical control
Volume of radioactive minerals for the distribution of the elements. As was indicated,
potassium is chemically involved in the clay lattice, but
_ K(log value) - K(min)-V,,[(X (max) - (min))] the exact behaviour of thorium in terms of clay-mineral

= a (7) composition is not clear. This method has no experimen-
tal justification and the precision for the identification of
where K(min) = potassium % in clean formation; K(max) specific clay minerals is not justified (Hurst, 1990).
= potassium % in pure shale and a = empirical factor for Local variations, complexity of clay-mineral mixtures
the formation concemed. and many other contributory variables allow no convinc-
The two strictly quantitative methods outlined above ingly clear picture as yet for precise clay-mineral
are essentially used in petrophysical applications. Other, identification. The use of the spectral gamma ray log for
geologi¢ally applicable, qualitative and semi-quantitative this purpose is not yet available. Qualitative uses are,
uses of the gamma ray spectral Jog are described betow. however, available (see below).

Dominant clay mineral and detrital mineral content:
7.10 Qualitative and semi-quantitative
use of the Th/K ratio
uses of the spectral gamma ray log
The method described previously for quantifying
Shale and clay minerals radioactive mineral volume (Section 7.9) was based on
A certain amount of literature exists on the possibility of the proposition that thorium occurs effectively only in
identifying individual clay minerals using the spectral clays and is thus a clay volume indicator, while potassium
gamma ray log. Most results have local significance occurs in both clays and radioactive minerals. The
only, are inconclusive or unsuccessful. As was shown method was applied quantitatively to sandstones but may
previously (Geochemical behaviour, Section 7.5) the be used semi-quantitatively for both sandstones and mud-
potassium content of the clay minerals varies consider- stones: the lithologies should be interpreted separately.
ably between species but is moderately constant within That is, the Th/K ratio will be largely a function of detri-
species (Table 7.8). Thorium, too, varies with each tal mineral content in sands, but of clay mineral content in
species but with slightly less consistency (Table 7.13). shales (in that these are potassium rich). In both lithoto-
The intent is to find if these variations enable the individ- gies, the usual value for the Th/K ratio is 4-6 (Myers,
ual species to be identified qualitatively, and eventually pers comm), deviations from this band will be the result
quantitatively. of certain detrital mineral or clay mineral abundances.
The interval of the Muddy ‘J’ formation of Eastern For instance, a sandstone with a low Th/K ratio (of less
Wyoming has been studied by Donovan and Hilchie than 4), will generally be dominated by feldspars, micas
(1981). They found a fairly good correlation between or glauconite: with high ratios, (greater than 6), it is likely
potassium radioactivity and illite content. However, they that heavy minerals dominate.
also found that while there was no correlation between In mudstones, a low Th/K ratio (of less than 4),
clay mineral content and total gamma radiation, there was probably indicates that illites dominate the clay minerals,
a strong correlation between total counts and uranium while high ratios (more than 6) probably indicate that
content. The essential radiation was therefore coming kaolinite dominates. For example a study of the Permian
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