Chapter 15. pdf

Sand
Shale
CNL 
(limestone p.u.)
0
60

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Petrophysics MSc Course Notes The Neutron Log
Dr. Paul Glover Page 166
Figure 15.12
The neutron log response
to evaporites (after Rider, 1997).
Hydrated minerals are not common in
basins. The only occurrence is
occasional chamosite in Liassic shales
in the North Sea. Hydrated minerals
may be seen in neutron logs used for
non-hydrocarbon purposes, such as
geothermal reservoir assessment in
metamorphic and igneous provinces.
However, volcanic and intrusive rocks
show high apparent neutron porosities
die to hydrated minerals, and these
horizons in reservoirs can be very
useful for correlation purposes.
15.11 Lithological Identification using the Neutron-Density Combination
15.11.1
Compatible Scales and the Density-Neutron Cross-plot
This is a hugely important technique, and together with the litho-density log forms the best downhole
lithological identification technique available to the petrophysicist.
Both the formation density log and the neutron log give a direct measurement of TOTAL porosity.
Hence, if they are plotted on compatible scales, they should overlie each other (Fig. 15.13).
Note that the compatible scale here is Density (1.95 to 2.95 g/cm
3
) and Neutron (-15 to 45% limestone
porosity units). This is the most commonly used scale range, although other wider or more restricted
ones are possible.
Note also that good superimposition will ONLY occur for clean limestone formations 100% saturated
with fresh water.
To construct a compatible scale we follow the following steps:
•
For limestone with 0% porosity, the density log reads 2.71 g/cm
3
and the neutron log reads zero.
•
For limestone with 100% porosity, the density log reads 1.00 g/cm
3
and the neutron log reads 100.

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Petrophysics MSc Course Notes The Neutron Log
Dr. Paul Glover Page 167
These two points can be plotted on a graph of density (from the density log) on the 
y
-axis against
neutron porosity (from the neutron log) on the 
x
-axis. We can join up the two points with a straight
line, and calibrate the line for porosity (Fig. 15.14). The graph is called the 
density-neutron crossplot
and the line is called the 
clean limestone line
.
The clean limestone line only works for clean limestones because of the effect of non-hydrogen
elements in the matrix on the neutron porosity values, and the different grain densities. However, we
can take account of the different densities and correct the neutron porosity units using Fig. 15.8, to
obtain lines of the cross-plot for clean sandstones and clean dolomite, the 
clean sandstone
and 
clean
dolomite
lines.