Chapter 15. pdf

Figure 15.9
The Hydrocarbon gas effect in the neutron log.
The Shale Effect.
Shale contain clays that have a significant amount of bound water molecules on
their surfaces. This increases the hydrogen index of the formation. Even very low porosity shales can
give erroneously high porosity readings due to the presence of these bound waters.
The Chloride Effect.
Chlorine is a good absorber of neutrons, and can lead to overestimations of
porosity if present either as formation fluid or mud filtrate.
15.10.2
Determination of Lithology
The direct use of the neutron log to identify lithologies depends upon the recognition of which
lithologies may contain hydrogen atoms (Table 15.2).
Table 15.2
Lithological and fluid indicators for the neutron tool.
Type
Examples
Fresh and salt water
Formation water
Drilling mud
Mudcake
Mud filtrate
Hydrocarbons
Gasses
Oils
Oil-based drilling mud
Oil-based mud filtrate
Oil-based mudcake
Coal
Organic-rich deposits
Bound water
Shales
Water of Crystallization
Evaporites
Hydrated minerals
Igneous and metamorphic rocks

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Petrophysics MSc Course Notes The Neutron Log
Dr. Paul Glover Page 164
Figure 15.10 shows typical log responses in common lithologies.
Figure 15.10
Typical neutron log responses in common lithologies.

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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).