Page 132 - The Geological Interpretation of Well Logs

- THE GEOLOGICAL INTERPRETATION OF WELL LOGS -
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BULK DENSITY | £ avERAGE 3
S$ ¢& avert = SHALE wo a BULK DENSITY
3 = gicm?
4 a ps 20 25 30] @ DENSITIES
4
< 5 oijz.s 1:9 20 21 22 23 24
| oO
lL
1
4
A
1
L
L
i
«
<t
i $00 2.10 gfem? 10
kK
[4
w
- unconformity
|
+ 20

x 600
n
E > F 30
3
Wi w
e
oO
<
5 700
by w
= L 40
° o
al 50m
2.22 gic?

800

Figure 9.13 Tertiary shales unconformably overlying dipping,
eroded, Cretaceous shales. The abrupt change in density
marks the unconformity.
£ gog
(siderite) is involved (density when pure, 3.89g/cm%).
When organic matter is present, the reverse occurs and the

1000 2.22 g/cm?
density diminishes, organic matter having a very low den-
sity (of around 1.2g/cm*; Figure 9.22). This relationship
may be quantified (see ‘Source rock evaluation’ below).

: 1160
The density log in sandstones — composition
and diagenesis
Bulk density variations in sandstone generally indicate
8 4200 2.25 g/cm?
5 porosity changes. However, as explained above, this is
uu
not true when there are changes in grain density. Pure
2
Ee quartz sands are considered to have a grain density of
Ww
a
o 4300 2.65g/cm>, but in reality such sands are rare. Overall
ec grain density will change depending on the non-quartz
Ww
S constituents. Sands are commonly mixed with feldspars
af
(density 2.52-2.63g/cm*), micas (2.65-3.1 g/cm’) lignite
14900 2.40 gem? fragments (0.5-1.8g/cm?) and rock fragments (variable
5
density). Heavy minerals may also be a constituent
(2.7-5.0g/cm?). The well-known mica sands of the North
1500 Sea Jurassic reservoirs (as already discussed) contain up
to 30% muscovite (Figure 9.9). Fhe density of muscovite
(2.76-3.10g/cm*) increases the average grain density from
4 2.50 gicm?
2.65g/cm? to c. 2.82g/cm? and it varies with the mica
1600m
content (Figure 9.15). In sands without shale, therefore,
Figure 9.12 Shale compaction with depth seen on a bulk
grain density can give some idea of sand composition.
density log plotted at a compressed (small) vertical scale.
Changes in grain density in sands are generally gradual
and of a moderate order. Abrupt changes, especially in
It is very responsive to local lithological variations and a otherwise homogeneous beds, often indicate diagenetic or
usable average is often hard to obtain. secondary changes. The example shows a sand with zones of
secondary carbonate cement (Figure 9.16). In cores these
Shaie composition
zones are shown to have very abrupt limits. A similar phe-
Shale density changes due to compaction are gradual,
nomenon may also occur with secondary pyrite cement.
while small-order, local variations are more likely due to
changes in shale composition. For example, an increase Mineral identification
in carbonate content is generally accompanied by an Density becomes a criterion for lithological identification
increase in shale density (Figure 9.14). The increase in when it is either abnormally high or abnormally low (the
density is even more marked when iron carbonate average for sedimentary rocks in oi) wells being about
122