Chapter 15. pdf

Figure 15.13
The density and neutron log responses for clean formations on a compatible scale (after
Rider, 1997).
One can see from the cross-plot that for density and neutron logs plotted on compatible scales, there
will be a 
separation
of the density and the neutron logs for sandstone and dolomite, but no separation
for limestone (Fig. 15.15). The sandstone separation is called 
negative
and the dolomite separation is
in the other direction and slightly larger, and is called 
positive
.
The separations are caused by the relative positions of both logs.

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Petrophysics MSc Course Notes The Neutron Log
Dr. Paul Glover Page 168
Figure 15.14
The density-neutron crossplot.
15.11.2
Clean Formations
•
There is no separation for pure limestones, and the porosity value that the log gives is accurate.
•
There is a small negative separation for clean sandstones.
•
There is a moderate positive separation for pure dolomites.

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Petrophysics MSc Course Notes The Neutron Log
Dr. Paul Glover Page 169
Figure 15.15
The density-neutron combination idealized responses.
15.11.3
Shaly Formations
If shale is present in the formation, the neutron log reads much higher porosities than it would
otherwise do because of the effect of bound water. This gives a large positive separation.
This large positive separation is diagnostic of shales.
If the shale volume decreases due to the intermixture of sandstone, the large positive separation
decreases, crosses-over and becomes eventually the small negative separation associated with pure
sandstone.
Thus, a sequence of clearly defined sand and shale formations shows switching between positive and
negative separations in the logs.
Often the beds will not be well defined, but coarsen-up or fine-up gradually. This can be seen by a
gradual switch from one separation to another. Hence the characteristic patterns (reviewed in the
chapter on the gamma ray log) can be recognized in the separations, and conclusions may be drawn
concerning the depositional environment.

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Petrophysics MSc Course Notes The Neutron Log
Dr. Paul Glover Page 170
The size of the separation is actually THE BEST quantitative estimator or shale volume. It is better
than the shale volume derived from the gamma ray log. We can write an equation for shale volume
based on separations
(15.8)
where 
Ω
stands for the separation measured in 100% sandstone, 100% shale, and at the point of
interest in the log. Note that the separations may ALL be measured on the limestone porosity scale,
ALL on the density scale, or ALL using a simple ruler on the log! Each is valid, but you must keep it
consistent for the whole equation.