Page 47 - The Geological Interpretation of Well Logs

- SELF-POTENTIAL OR SP LOGS ~

Using the anchor chain produces a similar effect. 5.4 Quantitative uses
So far, the logging companies seem to have given little
Methodology
thought to the offshore SP problem, perhaps because the
Quantitatively, the SP Jog is used mainly for the evaluation of
SP is not an expensive hi-tech tool. However it produces a
formation-water resistivities but it can also be used for shale-
cheap, useful log and should be requested.
volume calculation The quantitative use of the SP requires a
special methodology which is described briefly below.
5.3 Log characteristics
SP values for caiculation — shale baseline
Bed boundary definition and bed resolution
and static SP
The sharpness of a boundary will depend on the shape
With no absolute values, the SP is treated quantitatively
and extent of the SP current patterns. Generally when
and qualitatively in terms of deflection, that is, the
there is considerable difference between mud and form-
amount the curve moves to the left or the right of a
ation resistivity, currents will be spread widely and the SP
defined zero. The definition of the SP zero is made on
will deflect slowly: definition will be poor (Dewan,
thick shale intervals where the SP does not move: it is
1983). The contrary also applies. When resistivities are
called the shale base line (Figure 5.8). All values are
similar, boundaries are sharper. In general, boundaries
related to this line.
should not be drawn using the SP. If the log has to be
The theoretical maximum deflection of the SP oppo-
used, the boundary should be placed at the point of max-
site permeable beds is called the static SP or SSP. It
imum curve slope (i.e., maximum rate of change of the
represents the SP value that would be observed in an
SP - Figure 5.6).
ideal case with the permeable bed isolated electrically. It
SP bed resolution is also poor. For a full SP deflection
is the maximum possible SP opposite a permeable, water-
(i.e. SSP or static SP, see Section 5.4) and proper bed
bearing formation with no shale (Figure 5.8). It is only
resolution, as a rule of thumb, a bed should be thicker
the log-derived SSP that can be used for the quantitative
than 20 times the borehole diameter (Ellis, 1987). The
evaluation of R, (the formation-water resistivity).
exact minimum SP bed resolution will obviously depend
Frequently the SP does not show its full deflection, for
on depth of invasion and salinity differences, in the same
a number of reasons: the bed is not thick enough, there is
way as described above for bed boundary definition.
shale in the formation, the invasion is very deep, there are
adverse lithological effects (junction beds with high
tesistivity) or hydrocarbons are present (Figure 5.9) (Pied
Figure 5.9 Some conditions causing aberrant SP values when et al., 1966). These conditions must be considered when
the SSP is not attained. taking SP values for calculation.

(jsee Invasion) oftect of
Log
SSP
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(invasion) eg!
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1 THIN BED 2. INVASION 3,.SHALY SAND

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4. LITHOLOGY 5. HYDROCARBONS