Petrophysics MSc Course Notes The Neutron Log
Dr. Paul Glover Page 165
The values of apparent porosity
in shales varies considerably, but is usually higher than the apparent
porosity in carbonate or sandstone rocks (i.e., 45 to 75%). This clearly high and unrealistic porosity is
a partial indicator of shale, and can become diagnostic when combined with the gamma ray log. One
may see a slight decrease in the apparent porosity
in shales due to compaction, but only over large
depth intervals.
The apparent neutron limestone porosity is affected by the amount of shale and sand, or shale and
limestone in a mixture of the two. Hence we can recognize coarsening-up
and fining-up sequences in
the neutron log (Fig. 15.11). While one can calculate a shale volume from the neutron log directly, it is
not recommended because of the effect of hydrocarbon gasses which may be present to disturb the log.
Figure 15.11
The neutron log response to mixtures of shales and sandstones.
Organic mater, often present in shales may cause an even higher apparent neutron limestone porosity
than the bound water in the shales alone (Fig. 15.11).
The neutron log can detect evaporites by either their waters of crystallization. The most common
example is gypsum (CaSO
4
.2H
2
O),
but also include kainite, carnalite and polyhalite. The first three of
these give apparent limestone porosities of about 60%, while polyhalite give about 25%. The apparent
limestone porosities for halite, anhydrite, and sylvite are very low (-3, -2, and –3 respectively) as they
contain no water. Note that there is no effect of the chlorine atom in halite or sylvite (Fig. 15.12).
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