Page 134 - The Geological Interpretation of Well Logs

- THE GEOLOGICAL ENTERPRETATION OF WELL LOGS -
LITHOLOGY

CALIPER
ins BULK DENSITY g/cm
2-0 24 22 23 24 25 2-6 27 28 29 30
4
L
|
1
1
ll
—
1
—_|

PYRITE Shate
|
low peak
Sm 2.02 gyem3
AL
} with
zy Ti below
high peak
Bedded PYRITE
2.8 g/cm?
a
10m
| silt
|

13m
Figure 9.17 The identification of coal, with low density and pyrite, with high density, on the bulk density log. Lithology from
core analysis.
>
oO Table 9.7 Evaporite densities. Typica) values as seen on the

G BULK DENSITY NEUTRON @ density log (Schlumberger, 1989a).
2 a/oms %

EF 2.0 2.5 340 20
0
po 24 1 ? Evaporites Density g/em?
2

t Salt 2,04
_-t
Gypsum 2.35
Teen
Anhydrite 2.98
<-°
Carnalite 1.57
SHALE <=>
<=<-2_L Sylvite 1,86
sy
—-” Ployhalite 2.79
SS + 25m

en
5)
t diminishing rapidly to below 20% from about 600m
downwards (Figure 9.19) (Magara, 1978). The actual
7A
+ bh +
watt , < figures and gradients vary from one region to another
(Table 9.8), although the normal trend of a progressive
+ + + xv
tet ~~
porosity loss is universal.
a es
However, porosity may increase with depth and when
ii occurs there is overpressure. The general decrease in
erage &
SALT a4? ; shale porosity is accompanied by an expulsion of both
+ + # \
tote ape Salt é pore-water and interstitial water (Burst, 1969). The fluids
ote 2.04 g/m 5 are gradually squeezed out during burial. If the fluids
t + 4
7
+ ‘ cannot escape, once trapped they inevitably become
\
aan <) overpressured: they begin to support some of the over-
aay 1 r 75m burden pressure (see Chapter 2). This has the effect of
+ + __--4 { preserving porosity, It is this preservation which causes a

break in the compaction trend which is registered by the
Figure 9,18 Bulk density log over a salt-shale series. The density log. The density break therefore identifies zones
density log over the evaporite intervals tends to give constant
of abnormal pressure (e.g. Fertl, 1980) (Figure 9.20).
values. The neutron log assists in the identification of the
evaporite intervals. ( >N salt = -3). Fracture recognition
Numerous methods have been proposed for the identifica-
Overpressure identification tion of fractures (Schafer, 1980). One of these involves the
The general increase in shale density with depth of burial comparison of density-log porosity with sonic-log porosi-
was described under the heading of compaction. The ty. The density tool records bulk density, and as such will
principal cause for this gradual increase is a diminution include both intergranular porosity and fracture porosity.
in shale porosity with increasing overburden. Mud- For the sonic measurement, however, the sound waves
stone porosities may be as high as 50% near the surface, will take the quickest path from emitter to receiver. This
124