Page 172 - The Geological Interpretation of Well Logs

- THE GEOLOGICAL INTERPRETATION OF WELL LOGS -

"
g/cm?

Density La en 4 eae pire ye. | | mee

lig!
“| ae bs, PAS
t
}
1 Pe Neen Ft ae mi > ; ae pte

Neutron black
Neutron porosity %
Density red
Figure 11.15 Interactive investigation of a neutron-density ¢cross-plot in order to define lithology empirically but quantitatively.
1. Magenta: interval interactively selected on the logs (bar on left margin of log track), plots as magenta coloured points on the
cross-plot. The lithology is calcareous shale.
2. Blue: points in the area outlined on the cross-plot come from the {og intervals indicated by blue bars on the right-hand margin
of the log wack. The lithology is clean, porous sand (with gas). (TerraStation Software)

LITHO-POROSITY PLOT
(SALT MUD) cross-plotting in the vain hope of finding a unique ‘shale
1.37
point’ or ‘mineral point’. These points rarely exist in
SALT 6
nature. Rather than use complex numbers to cross-plot,
25
the variables can be kept simple but the cross-plot itself
1b made more sophisticated. For example, a third dimension
GYPSUM GYPSUM~ANHYDRITE -DOLOMITE
can be added to produce the so called z-plot, such as a
“LITHOLOGY TRIANGLE’
1.0 - typical envelope for polals
gamma ray added to a neutron-density plot. This third
from shale-lrae evaporite zone
axis is best manipulated with the computer when, on
90.9
veut ® SILICA (2) screen, values may be indicated using a colour scale or
M SILICA (1)
0.8 when plotted, number values added to the points.
OM . Alternatively, the plot may be represented in three
Kk 4), (2), (3 ‘
6. \ gacOg-DOLOMTE-SILICA dimensions as some form of isometric presentation
“LITHOLOGY TRIANGLE ”
os > for points Irom shale-trae {Figure 11.17) or a 3-D surface projection made of an
impressive but not always easy to use.
0.6 we ¥ no secondary porcsity envelope of the points. These plots are generally visually
carbonate zone wilh
a
0.5
—
‘
1
.
.
.

0.4 0.8 06 7 1 0.8 0.9 1.0 Cross-plotting incompatible logs
a
The cross-plotting of incompatible logs is usually done to
Figure 11.16 Cross-plot of cross-plots. The M and N plot quantify lithology. Incompatible logs are those which do
from Schlumberger. (From Burke ef ai., 1969). not, in the first instance, measure or indicate the same
parameter. Resistivity and gamma ray Jogs are incompati-
ble, one gives the resistivity, the other natural radioactivity
Nis defined by a density-neutron cross-plot, M is defined and, by inference, shale volume. However on cross-plot-
by a density-sonic cross plot, and M is plotted against NV ling, compatibility will become evident (there usually is
(Figure 11.16). Theoretically, minerals and shale become compatibility), The resistivity logs, for instance, will show
separated into fields, and porosity is eliminated. However, a consistent set of values in shales, as will the gamma ray
the geological value of the logs is lost and a mineral] iden- log; this will become evident on cross-plotting (Figure
tification more reliably done using mud log and simple 11.18). In fact in almost al] cross-plots, it is rare not to find
cross-plots. In fact there is a tendency to rather obscure a consistent relationship of some sort (cf. Heslop, 1974).
162