Page 75 - The Geological Interpretation of Well Logs

- RESISTIVITY AND CONDUCTIVITY LOGS -

lo 4000 4000 CONDUCTIVITY trend below. The abrupt increase is probably related to
0 4000 = mmbho/m and caused by an increase in shale porosity which occurs
oO t 1 1 1

as the overpressured zone is entered (Schmidt, 1973).
Plotting shale conductivity with depth therefore brings
out normal] pressure and compaction trends, and abnor-
4
4 shale & mally pressured zones. Care must be taken with such
~~ fe
plots to ensure that the changes are not due to variations
: e in shale composition (cf. Figure 6.31}. [f enough data are
‘S
§000 4 s available in a particular region, tables can be constructed
to give quantitative estimates of over-pressure from
= shale conductivity / resistivity values (Ichara and Avbovbo, 1985). This
Ee 4 4 relationship has taken on considerable importance with
s yf fe
s | /"s the advent of LWD measurements since resistivity
3 3 ‘
(conductivity) anomalies can now be detected during
: ,*
19 000+ } na top of drilling (Rasmus and Voisin, 1990).
——--—__—_—
Normal conductivity depth trends may be used simply
| / Tt wa. / xv x, overpressure to indicate geological compaction. In rapidly-deposited
4 wy, £ / or stratigraphically contiguous zones, compaction will be
persistent and regular. When there is a break, either in
‘.
4 / See sedimentation or, more importantly, an unconformity,
a
a
TT
15 000 compaction trends wil] be interrupted and hence so will
10 44 20 26 resistivity trends. Breaks in resistivity trends can there-
SHALE POROSITY % fore be used to diagnose geological breaks (see Chapters

14, 15).
Figure 6.37 Indication of overpressure on a conductivity plot.
Conductivity changes correlate with changes in shale porosity.
(Data source, Schmidt, 1973). Source-rock investigation
The resistivity log may be used both qualitatively and
corresponding to persistent, normal compaction. The quantitatively to investigate source rocks.
reason for this trend seems to be a relationship between The effect a source rock has on the resistivity log
conductivity and shale porosity (Figure 6.37). The same depends on the maturity of the organic matter: it has little
relationship to shale porosity is shown by the sonic log, effect when immature, but causes a large increase when it
which also gives persistent trends with shale compaction is mature (Figure 6.38), The reason seems to be that it is
(Chapter 8). the pore fluid content that causes the increase and not the
In some wells, a reversal in shale conductivity with solid matter. A typical shale which is not a source-rock
depth is encountered: in such cases overpressure is diag- consists of a clay mineral matrix and a certain water filled
nosed. When a zone of overpressure is encountered, shale porosity. A source shale also contains both matrix and
conductivity increases abruptly and considerably (Figure porosity but typically 4%—12% of the matrix is organic
6.37) although possibly taking on a new diminishing matter (Figure 6.39). If the source is immature the pore

GR API TOC % GR API TOC %
9 125 RESISTIVITY |, 10 2525-42 200 RESISTIVITY], iS

3
112644 a \ 9 \
“5 q MS
= S 5” € mt —

= ~~ ot Q Se
=
2680-
S 11680- —_ Ya - ss
a
o —-— x Sef) SD
5
>
a =
6 = i
< “
r
—-
1175 2675
we OP
pa

IMMATURE (LOM 5) MATURE {LOM 10.5)
source interval
Figure 6.38 Resistivity log characteristics in source rocks. When a source rock is immature, no resistivily anomaly is seen. When it
is mature, high resistivities are measured. TOC = total organic carbon. LOM = level of maturity (re-drawn from Passey ef al., 1990).
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