Page 265 - The Geological Interpretation of Well Logs

— SEQUENCE STRATIGRAPHY AND STRATIGRAPHY -

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ALBO-APTIAN
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= pay PE

BARREMIAN f wht = | Z |
=
=

PETROREP WELLS = wt
stratigraphic marker:
top of Greensand
Barramian - Aotian transition (thology)
lovation
This traditional way of correlating logs is now tied to detailed biostratigraphy:

Figure 15.19 Detailed correlation of log markers across the Paris Basin, France.
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some markers are time transgressive. (From Serra, 1972).

15.4 Traditional correlation methods to find that the liquids that they contain are incompatible
for the structure, and that, in reality, they are not related.
Marker horizons
The correlation was of like facies. It is essential to take
The simplest and most evident form of log correlation uses
account of the facies being correlated. In sand-shale
log markers. This marker may be a distinctive peak, a dis-
sequences, although sand bodies are the reservoirs and
tinctive shape or a distinctive lithology with unique log
therefore important, they should, nonetheless, nor form
responses (this obviously has connotations in sequence
the basis for correlation.
stratigraphy). The Lower Cretaceous of the Paris Basin, for
In the example (Figure 15.20), correlation was based
example, shows the behaviour of marker shapes over a sta-
essentially on coals and electrosequences. From their
ble intra-cratonic basin and illustrates traditional
position and log shape, the sandstone bodies appear to
correlation (Figure 15.19). Today, such correlations are
correlate, but in fact do not, as proved by fluid incompat-
stil] usual, but associated with a detailed biostratigraphy
ibitities (Figure 15.20). The sands were deposited in
and an explanation is now required as to why they exist,
channels and the similarities in log shape only indicate
usually in sequence stratigraphic terms (Section 25.2). It is
similar facies. This is an area of active deltaic deposition
far better to know what causes a particular log marker, peak
with channels forming and being abandoned; individual
or trough, than to correlate uniquely on appearances.
channel reservoir correlation is extremely complex but
False correlations — facies the channel facies is frequently repeated.
In sand-shale sequences, correlations often become The rules for correlating only certain facies is universal,
extremely complex. It is exceedingly easy to correlate as has been demonstrated in the discussion of sequence
two similar-looking sand bodies between two wells, only 255 stratigraphy (Section 15.2).