Page 100 - The Geological Interpretation of Well Logs

- THE GEOLOGICAL INTERPRETATION OF WELL LOGS -

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xr excess’ & ———$—$<$<—$ > ~
L l

Thorium pom
5 GAMMA RAY API 240 ThiU ratio 106]20 ' ojo K% 5/10 Uranium ppm
3
pitinet 7 1 ) 1 1 ) 4 L 1 b ) 1
10

mS
cdE
ay, rus, 2
OQ Q
10m
20m

30m
td = condensed sequence (marine band}
Figure 7.31 Condensed shale sequences showing very low thorium/uranium, Th/U ratios due to ‘excess’ uranium content. Most
shales have a THU ratio of 3-6, the ‘norm’. In condensed sections, where organic matter is concentrated, uranium values are
abnormally high and cause Th/U ratios of less than 3. The low ratio can be diagnostic.

analysis (Chapter 15). Condensed sequences in marine Fracture localization
areas arise when the detrital influx into an environment is The mobility of uranium and its presence in formation
low. This causes a relative increase in the in-situ material waters is considered to be the cause of high uranium
over the externally derived detritus. In-place material radioactivity in fractures and faults (Fertl, 1979; Fertl and
typically consists of the shells of pelagic macro- and Rieke, 1980). Using the spectral gamma ray log, zones of
micro-fauna and locally derived organic material, alt of high uranium radiation can be detected, and other logs
which simply fails to the depositional surface to form 4 may be used to confirm that fractures are present.
slowly accumulating (organic rich) sediment. Provided
that there is a low rate of dissolution, (i.e. that the envi- Comment
ronment is oxygen depleted or deficient = anoxic) the Because a precise, quantitative, petrophysical use for the
amount of organic maiter in the condensed sequence will gamma ray spectral log has not been found, it is frequently
be high, and in a marine environment, will have 2 high not included in a logging suite. This is short-sighted. As
adsorbed uranium content. On the gamma ray spectral methods are developed, such as those outlined above (but
log, this high uranium content will show up as a low which only represent a start), spectral logs in old wells will
Th/U ratio. That is, using the methodology proposed be sought in vain. There is no doubt that geologically, a
above, condensed sequences will show ‘excess’ uranium knowledge of the distribution of the three naturally
(Figure 7.31). Using this ratio based method is more radioactive elements leads to a much more refined interpre-
accurate and selective than simply using high overall tation of depositional environments, mineral content and
gamma ray (i.e. gamma sptke) or even high uranium val- fluid locatisation. The use of these logs in sequence stratig-
ues (Chapter 15). raphy and all that it implies is particularly significant.

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