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affect the electrical logs obtained, and so must be taken into account for
quantitative evaluation. This treatment is beyond the scope of this book, and
reference should be make to the manuals prepared by the electrical logging
companies, and to specialized books (see References). Particular care must
be taken with beds thinner than the Long Normal but thicker than the Short
Normal spacing because these affect the two Normals differently.
The third resistivity device is usually a Lateral or Inverse of large spacing
(the Inverse is essentially the Lateral with the roles of the electrodes inter-
changed, on the principal of reciprocity). These differ from the Normals in
measuring the potential drop between two measuring electrodes, M and N,
that are close together relative to their distance from A. The effective mea-
suring point is the mid-point between M and N (or A and B in the Inverse),
usually labelled 0. Thus the spacing is A0 in the Lateral. All spacings are re-
corded on the log heading.
The Lateral and Inverse logs belong almost exclusively to the realm of the
specialist well-log analyst because there is little that can be deduced from
them qualitatively, and their fluctuations usually bear little obvious relation-
ship to the beds penetrated by the borehole.
The standard electrical log will also include the SP or a gamma-ray log,
which will be discussed shortly.
The contamination of formation fluids by drilling fluids is both a help and
a hindrance: in any case, it is unavoidable. Drilling mud in the borehole is given
a larger pressure gradient than that in the formation fluids in order to contain
them, so at any given depth there is a pressure gradient, or fluid potential
gradient, outwards from the borehole into the rocks. The resulting flow of
fluid is insignificant into the relatively impermeable beds such as mudstones,
marls, silts, and some evaporites, during the time interval between drilling
through them and logging them. But it is significant into the more permeable
beds, such as sands, sandstones, limestones, and dolomites.
The wall of the borehole in a granular permeable bed acts as a filter to the
mud, with the result that a filter cake (also called a mud cake) forms on the
wall of the borehole. Through this filter cake, mud filtrate passes to the pore
spaces of the permeable rock. This contamination ranges from nearly 100%
in the water-bearing sands in the immediate vicinity of the borehole (virtual-
ly complete flushing of the original fluids) to nil at some distance from it. In
oil-bearing sandstones, the flushing is not complete and residual oil remains
in the pore spaces. The proximal zone is known as the “flushed zone” (and
parameters relating to it are given the suffix xo, as in Rxo) and the conta-
minated zone beyond this and including the flushed zone is known as the
“invaded zone” (suffix i; Fig. 6-8). The dimensions of these zones are not
only variable from bed to bed, but they are also likely to be variable at dif-
ferent depths within the same bed. In representing them as circular in plan,
one makes this assumption from ignorance of their true geometry. There is
also the dimension of time involved in the invasion of the rock unit by mud
filtrate.