Page 50 - The Geological Interpretation of Well Logs

- THE GEOLOGICAL INTERPRETATION OF WELL LOGS -

Mineral identification is considered linear as discussed. In so far as shaliness is
The rare exceptions when the SP will deflect and the related to grain size, the SP is a good facies indicator. The
formation is not permeable are due to mineralizations. example (Figure 5.14) shows a well-marked channel
Pyrite is an example (Figure 5.13). It is also possible that sand; the coarse-grained base is clean while the finer-
the SP reacts to excessively reduced and excessively grained top is shaly. The SP is, therefore, following
oxidized beds (shales or sandstones) which are not in grain-size change (see also Chapter 14).
subsurface electrical equilibrium (Hallenburg, 1978). The SP has now been largely replaced by the gamma
However, coals, which are extremely reduced, give a ray log for facies identification: the gamma ray log has
large negative SP deflection (Figure 5.13) or no deflection more character and is more repeatable (Figure 5.6).
at all, although the reasons are obscure. The SP should be
Correlation
used with caution for mineral or redox identification —
Previously, the SP log was one of those used for
other logs are much more diagnostic.
correlation but, for the reasons given above, has now been
Facies replaced especially by the gamma ray log. The SP is still
When it was introduced the SP was to become one of the useful for correlation, however, in areas of varied water
first logs to permit correlation in sand-shale sequences, salinities. If wells are quite close (and drilling mud fluids
principally because certain intervals had typical log are similar), correlation should only be made between
shapes. This shape, in sand-shale sequences, is related to sands with similar salinity values (Figure 5.15). For this
shale abundance, the full SP occurring over clean inter- the SP is the only log that can be used as a guide.
vals, a diminished SP over shaly zones. The relationship

SEDIMENTOLOGY

SPONTANEOUS intervals
POTENTIAL LITHOLOGY
cored

10

+

SYMBOLS
shale - mudstone
Silt

vf-fi sandstone

me

cse

laminations
microx laminae

VY cross beds

3 ripples
S disturbed beds
U burrows

Figure 5.14 Facies identification using the SP log. A typical fining-upwards, channel sandstone giving a bell-shaped SP curve.
From the Carboniferous, UK. (After Hawkins, 1972).