Page 242 - The Geological Interpretation of Well Logs

- THE GEOLOGICAL INTERPRETATION OF WELL LOGS -

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Figure 14,13 The database for electrosequence analysis, a full set of logs through reservoir and non- reservoir. The fine-grained
section shows a progradational sequence from coal to organic shale to shale with an increasing quartz content: the sand, on the
other hand, shows no well-defined trends. Note the varying reactions of the individual logs.

stratigraphic principles to suit modern attitudes. These examination of both log- and drill-derived data (Figure
modifications have come out of both practice and discus- 14.14). It is an essential and fundamental first step.
sions with oil industry colleagues during many industrial
courses. Throughout, the system has shown itself to be Step 2; Electrofacies annotation
simple to apply, adaptable and capable of bringing out a Once the lithology is established, the logs can be
great deal of geologically significant information. It also examined for characteristics seen on the traces which
forms a base for an eventual sequence stratigraphic may or may not have geological significance. These
analysis (Chapter !5). characteristics, baselines, trends, shapes, abrupt breaks
The first principle of the system is that reservoirs and and anomalies will be discussed individually below.
non-reservoirs are equally important: geology is not Together, they make up what is called (in this book at
restricted to reservoirs. In a sand-shale sequence, for least) an ‘electrofacies’, which can be defined as: a suite
example, the environmental information contained in the of wireline log responses and characteristics sufficiently
fine-grained, non-reservoir intervals, equals or even distinctive to be able to be separated from other electro-
exceeds that in the reservoir zones. The second principle facies. Facies in the usual geological sense, may not be
is that no one log can characterize a formation: each log- identical to the electrofacies.
ging tool examines the same formation but from different The principal objective of an electrofacies annotation is
aspects, and all are equally characteristic (Figure 14.13). to prepare the log set for an ultimate interpretation for
The essential steps for a proper electrosequence whatever geological information can be obtained, such as
analysis are laid out below in moderate detail. A house log sequence, depositional environment or facies. For
is built from the foundations upwards: the roof comes example, an interval may be interpreted lithologically as
last. Electrosequence analysis musi follow a route of entirely shale; an electrofacies annotation will bring out
construction; an interpretation for facies, depositional the fact that there are two distinct types of shale within
environment or sequence stratigraphy comes last, a litho- this interval and that they are in fact separated by an
logical interpretation comes first (Figure 14.14). unconformity. There is much more in the logs than just
lithological information and the electrofacies annotation
Step I: Interpretation of lithology will show this.
The electrosequence analysis is undertaken using a The annotation should be done in a somewhat ‘un-
document on which all the well logs are plotted, depth- thinking’ manner. That is, an immediate explanation
correlated and at the same scale. Since modem log should not be sought for the annotations that are being
acquisition is digital, a re-plot of all the logging nuns in marked; the explanation is intended to be extracted in the
one well is a fairly simple affair and can be requested from final interpretation when all the information has been
the service company or made in-house. The re-plotted amassed. Also, it is a job for coloured pencils; dashes,
data should, however, be checked for computer (actually dots and symbols are not enough. Although this is a ‘low
human!) errors in scale, depth matching and even log tech’ approach, it is highly effective. Chapter 15 describes
label. The ‘merged data’ docurnent can then be completed sophisticated ‘high tech’ methods.
with a detailed lithological interpretation, using the The annotations that should be marked on the logs are
methods previously discussed in Chapter 11 (Figure described below in a logical order of simple to more and
14.13). The lithological interpretation will come from an 232 more complex.