Page 135 - The Geological Interpretation of Well Logs

- THE DENSITY AND PHOTOELECTRIC FACTOR LOGS -

path should avoid the fractures. The sonic velocity will In practical terms, the two logs should be normalized
therefore give only intergranular porosity. When the density- to permit a proper comparison. This may be done with the
derived porosity is much more than the sonic porosity, the logs themselves or by cross-plotting core-verified values
difference is due to the fracture porosity (Schafer, 1980). to define ‘fracture fields’ (Et yre, 1981).
The example shows this effect in a compact, upper

Carboniferous sandstone and shale sequence (Figure 9.21), = | GAMMA RAY| BULK DENSITY
5 API gicm
° 200 2.9 2.2 24, 2.6 2.8)
Qo
| / Uonuma
A Group

VA Haizume -
2600m
: Formation
At
Ls oormal a"
500 compaction
a
- a "i
2700m
i
“ oe
a ° Nishiyama
if . Formation
ik
1000 2800m
overpressure
Tone ee,

2900m
= z i i
= 1800 a‘ Figure 9.20 Overpressure seen by a bulk density drop and

a g break in the normal compaction curve. The gamma ray shows
uu, the interval is shale.
>»
sf sha" Shiiya
x o? Formation
Table 9.8 Some shale porosity values (1, 3, 4, from ref. 2).
a
2000
“Kt.

i Approximate shale porosity
gtk
Area 500m 1000m 2000m 5000m
hs
“ye
~ ‘t Teradomari 1. Guif Coast 45% 28% 22% 13%
Plain 45% 29% 17% -
®
t
2500
2. Nagaoka
Nanalani
I
Formation
2° Japan
Shiva}
24%
12%
3. Venezuela
30%
-
3000 Tult

° 10 20 30 40 50 60 76
4. Oklahoma 23% 13% 4% (0)
MUDSTONE POROSITY %

Figure 9.19 Diminution of mudstone porosity with depth. |. Dickinson (1953) 2. Magara (1968)
Neogene, Japan. (Re-drawn from Magara, 1968). 3. Hedberg (1963) 4. Athy (1930)
GY
GAMMA RAY API BULK DENSITY g/cm? microsec/ft
SONIC
LITHOLO
CALIPER INCHES 2,2
4880m
fractured
sandstone 4900m
4910m
possible shale fractures
4920m
Figure 9.21 The contrasting effect of fractures on the density log and sonic log. The density log shows very low values over the
fractured sandstone interval while the sonic is not affected. The shale interval may also show a similar effect. The caliper is from
the density tool.
125