PERMFILE128524

• Permit No: M1999002
• IBM Index Class Name: Permit File
• Doc Date: 6/18/1999
• Doc Name: GAMMA RAY AND SPECTRAL GAMMA RAY LOG
• Media Type: D
• Archive: No

- .II t. f, F.O LOf.ICAI. I::\TLKI'K LTATIO\ OF 11hLI. hO4J -
"fable 7.10 The abundance o(the radioact~lements and
their relative contributions to the overall radioactivity o(the
black. shale example o(Figure 7.1 I (values calculated for the
peak at 40m). ._
Element Content *gamma tar % gamma ray
API equivalent value
Uranium I I ppm 89.OAP1 41.OSr
Thorium I S ppm 70.7 API 32.6'7c
Potassium 35% 57.1 API 26A%r
'using the multipliers given in the text (section 7.4)
Probably a more common way of introducing uranium
into sediments is in association with organic matter. I[
has been established zxperimentall}~ that carbonaceous
material can extract uranium from solution very effi-
cizntl}', zspzcially over the range of pH 3.~-6.0 (acidic)
(Durrancz. 1986). Organic-rich shales often (but not
al+cays) contain large amounts of syngznetic uranium (i.e.
extracted locally), in which case the}' arz associated with
high gamma ray log values (e.g. Schmoker and Hester.
1983) (Figurzs 7.1, 7.22). It is the large size and high
charge density of the uran}•le ion which allows this, and it
is thought that the process eventually involvzs an ionic
bonding. The urano-organic complexes produced may
form coatings on organic or inorganic particlzs or be dis-
setninatzd through the sediment mass. However, the exact
relationship betwezn organic matter and total uranium
content is not easy to establish, since high organic matter
content is not always related to high uranium content (cf.
Meyer and Nederlof, 1934) (Figure 7.12). Empiricall}•,
[he constant presence of organic ma«er in shales (Table
7.11) suggests that uranium adsorbed by organic matter is
an impottant contributor to o+•erall shale radioactivity
(see Section 7.6).
The third way of introducing uranium into sediments
concerns principally phosphates and associated deposits.
The uranium present in phosphatic rocks is generally
syngenetic and is found within [he phosphates. Primary
uranium minerals are absent. The very variable valence
bzhaviour of uranium means that under the right condi-
tions it (omts complex ions with carbonate, phosphate,
hydroxide and others and it is assumzd that U" substi-
tutes (or calcium in the carbonate-fluorapatitz generally
found in marine phosphorites (Durrance, 1986). The
correct chemical conditions for this type of reaction may
be very localiszd, such as exist in hardgrounds.
[n general, uranium behaves as an independent
constituent: it is not chemically combined in thz principal
inoleculzs of rocks like potassium, but is looszly associ-
ated with secondary components. For this reason it has a
ver}~ heterogeneous distribution in sediments. Moreover,
its continued solubility even in thz subsur(acz, which is a
function of its loose attachments, rzndzrs it susceptible to
Izaching and redeposition, making its distribution even
morn irregular.
Table 7.11 Average weighl•of organic matter in sediments
(kom Shaw, 1980).
~ 3
i
t y.
i
Sediment Average weight %
Shales 2.90
Carbonates 0.29
Sandstones 0.05
'o
o
^ 0
o c+
e~~~
0
0 0
0
0 0
0
0
~o ao ao
ura ~~~n o~m
Figure 7.12 Organic carbon content compared to uranium
content: there is wide dispersion. (Source o(data. Adams and
\veaver, 1955).
Typically, on the logs, uranium is shown by irregular,
high peaks corresponding to its uneven distribution. Due
to the unusual requirements of its original deposition,
these peaks are associated with unusual environments
such as are found in condensed sequences or at unconfor-
mities (i.e. Figure 7.31).
Thorium
Like uranium, thorium has its origin principally in acid
and intermediate igneous rocks. However, i[ is exvemely
stable and, unlike uranium, will not go into solution. For
this reason it is found in bauxites (residual soils).
Although there is a possibility that thorium is adsorbed
onto clay minerals (Durrance, 1986), it is generally
transposed to sites of sediment deposition as clay fraction
detrital grains. These are of heavy minerals such as zir-
con, thorite, monazite, epidote and sphene (Table 7.12)
which are all very stable.
Because of its detrital nature and current vansport in
Table 7.12 Thorium-bearing heavy minerals (Serra er ul., 1980)
Composition Thor content (%)
Thorite Th, Si, O, 25~i3
Monazite Ce, Y, La, PO, 4-12
Zircon Zr. Si. O, less than I
Uranium ppm Thorium pm
Zircon 300.3000 100-2500
Sphene 100-700 IOO~i00
Epidote 20.50 50.500
Apatite 5-I50 2-150
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