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around the borehole. Since virtually all the hydrogen in a sedimentary rock
is confined to the pore fluids, the Neutron log will detect porosity in general,
and distinguish between gas and liquids in particular. The differences of hy-
drogen concentration between oil and water are too small to be detected
(but these can be distinguished by electrical logs). Deep invasion of a gas
reservoir by mud filtrate may mask the effect of gas.
The Neutron log deflexion is inversely proportional to the logarithm of
porosity, to a close approximation, because in large porosities, neutrons are
slowed down and captured over a shorter distance than in small porosities,
so fewer gamma rays of capture reach the detector. However, a neutron
“does not know” if the hydrogen it has collided with is in free water or in
the crystal lattice of a clay mineral such as smectite (montmorillonite), or of
gypsum, so the effective porosity may well be less than that indicated by the
Neutron log, and care must be taken with dirty sands and lithologies in which
such minerals may occur. The depth of penetration is usually less than 0.5 m,
and this rock may not be representative of the natural material due to drill-
ing effects.
The Neutron log can be run in cased holes, but it cannot be used reliably
to determine porosity quantitatively behind casing. It usually indicates
changes in porosity, so it may be useful qualitatively.
Formation Density log (Fig. 6-11). Gamma rays emitted from a sonde that

’ ‘ mud cake
Fig. 6-11. Formation density logging device (Schlumberger F.D.C.). A = long spacing de-
tector; B = short spacing detector; C = source. (Courtesy of Schlumberger Seaco Inc.,
Sydney.)