15.11.4
Distinctive Lithologies
Certain minerals
have unusual combinations
of density and neutron porosity. These
unusual combinations can be used to identify
them on a combination log. Examples are
given in Fig. 15.16.
Coals are a good example of a distinctive
combination of density and neutron porosity
values. They
have unusually low densities
combined with unusually high apparent
neutron porosity values associated with their
hydrogen content.
15.11.5
Evaporites
Remember from the section about the
formation density tool that evaporites can
often be recognized
because their often pure
nature leads to well constrained density
values. Evaporites have apparent neutron
porosity values which are either very high or
very low. The combination of well
constrained density
values and very high or
very low apparent neutron porosity values is
a characteristic and diagnostic combination
for evaporites (Fig. 15.17).
Fig 15.16
Distinctive minerals on a density-neutron log.
sand
sh
sand
log
sh
V
Ω
−
Ω
Ω
−
Ω
=