Page 45 - The Geological Interpretation of Well Logs

- SELF-POTENTIAL OR SP LOGS -

SPONTANEOUS - POTENTIAL] & RESISTIVITY
millivolts z ohms — m?/m

~ <3) + 1/500

3

4s-P 3
oO
oO

i

|
Figure 5.5 SP log presentation. The SP is in track 1. There is no absolute scale, only relative deflection — negative or positive.
1 division equals 10 millivolts.

SPONTANEOUS POTENTIAL SPONTANEOUS POTENTIAL
10Mv
millivoits °
- k>|+
40 O° depth
-|<>|+ 3 constant,
550m
i
5
API
GR
om 2 150 . 4
GR

>
a
a
sp °
2 )

6
o
=

E
wR £ D
25m 4 @

°
y
a <q
CG
c
a
an
Figure 5.7 The SP log indicating sea state! For this test,
2
offshore Louisiana, the SP tool was grounded to the rig and
a
a
a stopped at 550m. The changes in SP potential occur with the
E passing waves, which were 2.5m — 3.0m (8’— 10°) crest to
e
a trough. The hole was cased (from Wallace, 1968).
50m = 3
=
wo contiguous shale (Figure 5.4). The flow of current is
an focused at the bed junction. It is only here that there is a
3 2 3 a change in potential. This is important since SP measure-
values. It is only at the bed junctions, then, that changes
ments are made not of absolute values, but of changes in
take place and will be recorded.
If a bed is not permeable, ions will not be able to move,
75m —| there will be no current flow and thus no potential
change: that means no SP. However, even the slightest
permeability will permit current flow and an SP change
will be recorded.
Figure 5.6 The bed definition and ‘character’ of the SP tog
The SP tool
compared to the gamma ray log. In most cases the gamma ray
log gives much more formation information and better bed The SP too] approaches the simplicity of the circuit
boundary definition than the SP. described (Figure 5.2) and consists simply of an electrode
(lead) mounted on an electrically isolated bridle on the
This couple works in a complementary sense and downhole tool. A 1.5 volt battery is included in the
creates a spontaneous current flowing between the bore- circuit to give a bucking current to bring the SP to the
hole (mud filtrate), the porous formation and the required scale. The tool’s galvanometer records only
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