Page 72 - The Geological Interpretation of Well Logs

a! 20 shown that the separation is due to the presence of micro-

- THE GEOLOGICAL INTERPRETATION OF WELL LOGS -

= [5! mset, tis, od
|
ILD
Qm2im 10
20
mim 49
3
scopic permeability paths associated with bedding

Meer
lamination or small scale vertical fractures. These have a
v selective effect on the electrical behaviour of the different
ny tools (Pezard and Anderson, 1990). Deep reading devices
Wi
Om have current fields which use horizontal permeability
Nese] paths, such as bedding: shallow devices tend to use
7, vv vertical permeability paths such as vertical fractures or
Sat ¥ joints. Thus, in well bedded shales, deep devices tend to
a be the lower, following the bedding permeability. When
fF
4
4 f vertical joints os fractures are present the reverse is true,
a
ry MEAs Shallow devices are lower. The log values are similar in
25m homogeneous formations. In the example (Figure 6.31},
f t
Va we the shallow device (SFL) is consistently higher than the
>
deeper device (ILD). This effect can be caused by persis-
Figure 6.30 Siderite stringers in a shale sequence as seen on tent horizontal bedding.
resistivity logs. The SHDT dipmeter curve (2.5mm sampling)
shows that these are very thin, often concretionary layers.
Correlation
The sensitivity of the resistivity logs to subtle lithological
structureless, indicative of an anoxic environment. The changes is the basis for their use in correlation. Ideally,
overlying shales are slightly silty and also probably tami- logs which correlate well are those which are more
nated. They contain woody organic matter, typical of sensitive to vertical changes than to latera) variations.
more open, oxygenated waters, These then grade upwards Within a limited geographical extent, this is often the
into silty shale (Figure 6.31). case with the resistivity logs, especially in shale or silt
This example also illustrates how bedding character- intervals. Distinctive shapes, trends or peaks over shale
istics can affect the resistivity logs. In normal zones are related to subtle compositional changes reflect-
circumstances, when there is no permeability, shallow ing original patterns of sedimentation, (i.e. Figure 6.31)
and deep resistivity devices should show similar values and as such can be correlated. The best log for this pur-
related to the formation lithology (cf. Figure 6.23). It has pose is usually the deep induction log (Figure 6.32).
been observed, however, that in certain fine grained (as in Despite its frequent (and successful) use for correla-
the example Figure 6.31) or crystalline formations, the tion, mainly as a result of its availability, the resistivity
resistivities show significant separation, Research has log has drawbacks for this task. It is influenced by

GR API LITH Res. OQm2/m
° , 100 12 . 1,0 _ Ie 20
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dioturbated? i
fez SFL
'
ee Ny
ILd :
254
i
> bioturbated? NS
\
Lf

Figure 6.31 Subtle textural and composi- 50- bioturbated? S
tional variations in shallow marine shales _
indicated on the resistivity logs. £
Compositional changes are noted in the = Hi
organic matter content and in the amount 2
of silt. Textural variation is seen in the
fine lamination of the organic rich shales KEY
which causes distinctive, low resistivities.
Note the separation between the shallow shale eS silty shale pal laminated shale
(SFL) and deep (ILd) devices (see text). structureless organic matter