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

- TIIE GEOLOGICAL INl'ERPRET,ITIO~\' OF {tiELL LOGS -
Table 7
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Factor Effect on log Sererity
Simple Taol
caving lowers values, bigger the common
cave, lower the value
barite in mud lowering o(value common
in thick mudcakes
KCI mud significant increase present
in 'background'
Spectral Tao!
caving lowers value in caves: common
tool eccentred so
effect much reduced
barite in mud increase in calculated common
thorium and uranium
KCI mud increase in calculated present
potassium and
uranium
values (Figure 7.10). It is sometimes proposed that this
is simpl}• a 'base line shift', because the mud volume
through the hole is relatively constant so there will only
be a constant increase in the background: relative ampli-
tude changes will remain unaffected. This is not always
the case, especiall}' so when there is invasion and KCI-
rich mud enters into the formation. Such a situation will
cause an invaded resen•oir to show too high a gamma ray
reading (see also the spectral log below).
Spectra( gamma ray -The spectral gamma ray log is
run held near the borehole wall by a bowspring to reduce
the borehole effects which occur when a tool is centred.
However, this does not eliminate mud effects entirely and
Table 7.8 Potassium in clay minerals: chemical content. From
Serta (1979). Dresser Atlas (1983).
'Potassium content
A1ineral Sc by weight Average 9o Construction
Illite 3.51-8.31 5.20 K, AI.Silicate
Glauconite 3.20.5.80 4.50 K, Mo, Fe, AI,
Silicate
Kaolinite 0.00.1.x9 0.63 AI, Silicate
smectite 0.00-0.60 0?2 Ca, Na. Mg, AI
Silicate
Chlorite 0 0 Mg, Fe. AI,
Silicate
'Averaee shale = 29c - 3.59c potassium
the spectral log is affecte the mud additives barite and
KCI (Table 7.7). The effects vary depending on tool
design (Company), and the algorithms used to derive
abundances. If only the tlvee energy windows around the
high energy gamma ray emission peaks are used (Figure
7.4), barite does not affect the result while KCI will only
affect the potassium result and can be corrected for. But
when the low energy part of the gamma ray spectrum is
used, the barite effect on this pan of the specwm causes
an increase in thorium and decrease in uranium. KCl
causes an increase in the potassium (as to be expected)
but also a decrease in the uranium. Chars and computer
algorithms are available to correct for these errors but are
not entirely adequate since they are non-linear.
7.5 Geochemical behaviour of
potassium, thorium and uranium
and natural radioactivity
The old tenet that the gamma ra}' log is a 'shale log' was
based on its use as a black box, not understanding what
was inside. In modem interpretation an understanding of
the mineralogy and geochemistry leading to radiation is
used. Described below are the natural cecutrences of the
radioactive minerals and their geological significance.
Potassium
Potassium is both chemically active and volumetrically
common in naturally occurring rocks. Because of its chem-
ical activity it is generally chemically combined. In the
clay minerals, for example, it (and invariably its radioactive
isotope) occurs in the clay silicate structure. In evaporates it
occurs chetttically az a salt, and in rock-forming minerals,
such az the feldspars, it is again chemically combined in
the silicate structure. The behaviour of potassium can
therefore be considered in terms of chemical composition,
as can its contribution to radioactivity.
The potassium content of the clay mineral species
varies considerably. Illites contain by far the greatest
amount, while kaolinite has very little or none (Table
7.8). The consequence of this is that clay mixtures with
a high kaolinite or high smectite content will have lower
potassium radioactivity than clays made up essentially
of illite (mica) (Figure 7.1 ). However, since most clays
are mixtures of several clay minerals, the differences
discussed above are muted. The averaee shale has a
potassium content of about 2% - 3.5% (Table 7.8).
Potassium is present in man}' rock-forming minerals
besides the micas, considered above as clay minerals.
The most important of these are the feldspars. Microcline
contains approximately 16% potassium by weight, and
orthoclase approximately 14%; such percentages render
the feldspars highly radioactive in geological terms (see
Table 7.15). Fzldspathic sediments ma}' therefore be
detected by their radioactivity.
Finally, potassium is found in some of the less commonly
occurring evaporates but in sufficient quantities to have an
74
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