Page 65 - The Geological Interpretation of Well Logs

- RESISTIVITY AND CONDUCTIVITY LOGS -

in Chapter 2 (see Table 2.3). Table 6.7 gives the approxi- induction Jogs give very poor bed-boundary resolution
mate bed resolutions for petrophysical purposes. For but, at the same time, they average all the bed effects in
beds thinner than the minimum resolution, correction such a way as to make lithology trends stand out, and
charts must be used to find real values (consult logging should be used for this purpose (Figure 6,21).
company bed-correction charts), or specialist tools used.
For geological purposes, the resistivity logs should be Depth of investigation
used knowing their resolution capabilities. The micro- The depth of investigation of the resistivity tools has
tool logs give too fine a resolution for practical, usable, been discussed previously as related to too] performance,
geological bed resolutions. The logs are best used for the capabilities being created for petrophysical needs
defining bedding characteristics (Figure 6.21: see also (Figure 6.18). Depth of investigation also has geological
Chapter 14). The laterologs resolve beds at the right scale significance.
for bed-boundary indications (Figure 6.21), but they The logs from deep-reading devices, especially
should be used in conjunction with the other logs. The the induction logs, are best used for gross formation

laterclog shallow cable tension,
back-up scale pounds
laterolog deep
(1) DUAL LATEROLOG back-up scale
scale chm m2/m

AT LUS (OHM) micro~
bit si 2000. 200000 sphericatly
ep BSN) Lt Coo (OHM) | TENS LB) | focused log
caliper | 12-8° Z0.00 € 2000 200000 |11000, 1000, | ———————
inches P=~— cet > laterolog
MSFL_COHMMH)
46.00 20.00 £ 0.2000 2000. # shallow
Bc ceceeceee eee tbs tony
oO 0, 2000 2000. Llaterelog
Pe LL! LLO (OHMM) | ee
. Z000 ~~ 2600

4; 4 4/500 =
4
buns
1

i | {
} {

t WIL
4 MSFL Ut]
!
tenslon
|
H
LLO—TT | Ne

size Ht caliper
bit SIZ vt 1 1
{Kon gauge) ‘ gc | 1 min,
i
;
+—
(12 1/44 \li.e. log speed
mesh
|
i. 1800 J ULS Pre i | |
1Om/min.)

} H . |] tI

J
and
Rt He
|

1

{|
\
LIL
_|

I
i]
\ sie 1 rTM 7]
é
Jog scale for
(2) ISF-SONIC resistivity
cable tensian
(pounds)
conductivity
CILD (MMHO) deap induction
2000. {miflimho)
DTL (US/F)
(other logs) sonic logs,
DT (US/F) long space and
short space
conductivity
ILO (
OHMM }
deap induction
deep induction
axpressed as
resistivity
a
spherically
focused
resistivity
inguction log
tension
—— log scale ————-»4—_ arithmetic scale
tor resistivity cond. and sonic
Figure 6.22 Typical resistivity log formats. (1) Dual laterolog combination; (2) induction, sphericatly focused log combination.
Both from Schlumberger.
35