Page 98 - The Geological Interpretation of Well Logs

- THE GEOLOGICAL INTERPRETATION OF WELL LOGS -

TOTAL COUNTS Th ppm ThikK U ppm
5
9 (CPS) 56 100 lithology 9 10 5 10 15 o us 10
EE |
K% 2 4

tc
‘|
20m Tao
j=
qe
|2
15
Ve

10m
PRO-DELTA

o

Figure 7.29 Thoriumpotassium, Th/K ratios in silts and sands associated with change in grain size. Thorium is relatively more
abundant in coarser grained fractions when sediment source is constant (Namurian outcrop, Co. Clare, Ireland, from Myers, 1987).

The interpretation of the Th/K ratio must be carefully content), lacustrine source rocks have no gamma ray
controlled and as a general rule, sandstones and shales signature and are not uranium enriched (Meyer and
should be studied separately. This is necessary because the Nederlof, 1983). This means that gamma ray response
ratio is often seen to be related to grain size. Thus, in coars~ and uranium content are unreliable source-rock indicators
ening-up sequences the Th/K ratio will change between the (cf. Figure 7.12). It appears that lakes do not have
fine grained clays and silts and the coarser grained sand- reserves of dissolved uranium available to be adsorbed by
stones, generally increasing upwards into the sandstones organic matter, while oceans do. Using high gamma ray
(Figure 7.29). Another factor which must be considered is values to indicate source intervals should only be used in
that potassium distribution changes during diagenesis. If certain cases.
there is dissolution of potassic feldspars and consequent
Depositional environment and condensed sequences:
redistribution of the potassium, the present-day Th/K ratio
use of the Th/U ratio
will not be related to original mineralogical content.
Efforts to relate depositional environment to radioactive
Organic matter and source rocks: uranium content mineral content are generally based on thorium and
The theory of uranium adsorption by organic matter has uranium content and their inter-relationships. The affinity
already been discussed (see Uranium, Section 7.5) and of uranium for shales of marine origin can be demon-
illustrated (Figures 7.11, 7.12) and explains why source- strated (Koczy, 1956) as can the affinity of thorium for
rocks may be identified by their uranium content and terrestrial sediments (Hassan ef al., 1976). Consequently,
consequent overall high gamma ray value. This is the the contrast between thonum and uranium content
case with the Upper Jurassic, Kimmeridgian of the North should indicate the relative marine or relative continental
Sea area, where organic matter levels are high (TOC = influence (Adams and Weaver, 1958). Analysis of a wide
5%+) and so are gamma ray values as a result of uranium range of mainly mudrocks, shows that the ‘normal’
content (Bjgrlykke et a/.,1975; Figure 7.11, Table 7.10). Th/U ratio is 3-6 with higher ratios (higher thorium) in
Although marine source-rocks, like the Kimmeridgian, ‘continental’ environments and lower values (higher
generally have high gamma ray values (high uranium uranium) in more marine environments (Figure 7.30).
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