Page 167 - The Geological Interpretation of Well Logs

- LITHOLOGY RECONSTRUCTION FROM LOGS -

GAMMA RAY API NEUTRON POROSITY %
100 eo,

CAL *| SP m¥V| INDUCTION ohm m BULK DENSITY g/cm3
+,-2", |- 20" ,|0.2 1 20] [1.7 2.72.9

‘ GR < ?
Ise 4 Lae ,
| é
' “i 2 4 SHALE
/ a IND '
oN —
: pec] i “
ce o SAND clean
, | ~, . shaly san
\ 4 ~
1 td 4
Se ‘ SHALE
in Se _S
= c SAND sl. shaly
T> = =
a kp < SAND clean
CAL. 7] pom $ ~~ A
1 . ts A
I 4 “=| a
1 : 3 == 2, ‘t SHALE
1 | r<S 4 ———. > _- } SAND shaly
i
a oe
SAND clean
ew
“tof

aA 4 ~~ 73 A
A
ct
a z c
alz| ¢é e S|O 53> 8
eiaein sla = 6 ole Oks
aad WwW *” WwW wo eo
c _ a 412 52 8
0 = < xa ww a
2 x x rz zo 8
o 6 ” “an
Figure 11.7 Vertical routine, manual method. Base lines and curve separations are used for the vertical analysis in this sand-shale
sequence.
Absolute values and lithology

For some of the more difficult, uncommon lithologies and
for beds with very high or very low readings, absolute ° Gh API 100 MSFL ohm/m woo
6 CAUPER a6 >
value tables can be useful. For example, evaporites are
generally pure enough in the subsurface to have distinct
densities and velocities — this is certainly the case with salt — 2 GR — 1775 =
(Chapters 8, 9). Abrupt peaks, which may be important in
CALI
stratigraphical interpretations or diagnostic of a particular
interval, are often best interpreted using absolute-value
mid-point
mid-point
tables, Coals, for example, will be distinct on logs, as will
Z {
*
be pyrite and other mineralizations (cf. Figure 9.18).
:
tangent \ J
Table 11.1 gives a résumé of some of the more useful
mud
BED
cake
absolute log values for lithology interpretation. Tables in
BOUNDARY
the individual chapters should also be consulted. . * tune s7a0
inflexions
Bed boundaries
Figure 11.8 Positioning a bed boundary. The mid-point of the
Bed boundaries should be drawn concisely. Moreover, the
tangent to the log curve between inflection points (i.e. the
correct log should be chosen to position a limit. The best
shoulder interval) is taken. Note this is thinner on the MSFL
geophysical logs for bed boundary definition are those
compared to the gamma ray log, but the mid-point is the
with a moderate depth of investigation (Chapter 2), in same. The caliper, being mechanical, shows an instantaneous
general the SFL (Chapter 7) and density logs (Chapter 9). response, in this case to mud cake limit.
The shoulder, where a log is responding to two different
lithologies simultaneously (Chapter 2), is generally As a general rule, a bed boundary should be drawn in
broader in logs with greater depth of investigation but the mid-point of the tangent to a shoulder (Figure 11.8).
thinner in shallow investigating logs (Figure 11.8). When This may not always be the rea] position as anisotropy
mud cake is present, an accurate limit may be taken from affects log responses (Chapter 2), but it is an identifiable
the caliper (Chapter 4) because it gives a mechanical method which can be applied consistently and corrected
response and has no shoulder effects (Figure 11.8). subsequently if necessary.

157