Page 153 - The Geological Interpretation of Well Logs

- THE NEUTRON LOG -

1974). The same result is obtained on the admixcure of
any materia) with a very low hydrogen index with clays.
This may be quartz as above, limestone, dolomite or
COMPACTION DISPLACEMENT
certain evaporites: the lower the clay volume the lower deep <}-—-(1 100m) shallow

the neutron log vatue. Qo
oO
Entirely the reverse effect is caused by the admixture fe
204 on
of organic matter with clays: it causes an increase in the
hydrogen index. This is because organic matter has a
shallow
higher hydrogen abundance per unit volume (hydrogen
index around 0.66) than clay (Table 10.8) (Figure 10.20). 4 28 5m
Rx
The increase in neutron log values with organic matter is >
Q
notable, especially when combined with the attendant Zz
5
decrease in butk density (see Figure 9.18). Oo 10-
wi
One aspect in the evaluation of shales with neutron x
mh
tools which does not seem to have been evaluated
sufficiently, is the effect of thermal neutron absorbers. ] )
Capture and absorption in pore space liquids are mainly
due to hydrogen and chlorine (NaCl) which are corrected 0
for in too] response. Capture in the solid matrix is
negligible unless it contains elements with unusually high
1 30 46 60
capture cross-sections, such as boron or gadolinium NEUTRON POROSITY UNITS %
(log values)
(Table 10.3). Boron content has been examined in the
Gulf Coast and is said have a noticeable effect in certain
Figure 10.17 The effects of shale compaction on neutron log
shales and shaly sands (Dunlap and Coates, 1988). The
values in a sand-shale sequence, Histograms of the log values
geological significance of this and of the distribution of show the diminution in sand porosity, but only a smal] change
boron and gadolinium is not known. in the shales.

TRANSIT TIME NEUTRON POROSITY UNITS %
LITH. 240 140 BS/ft 40/60

-15
9
30
600m 600m
,
=
4
q
700m r 700m

800m - 800m

300m soon

1000m 1000m

Figure 16.18 Compaction in a shale sequence shown by a decrease in interval transit time (increase in velocity) while the neutron
log shows constant values.

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