Page 246 - The Geological Interpretation of Well Logs

- THE GEOLOGICAL [INTERPRETATION OF WELL LOGS -

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Figure 14.18 Examples of abrupt breaks. A, channel erosion, sand overlying shale; B, flooding surface, shale overlying sand;
C, unconformity, shale on shale; D, fault, shale on shale.
unusual minerals at unconformities or in hardgrounds. Step 3: Electrosequence analysis for log-based
for instance, will often create a large gamma ray peak sequences, facies and depositional environment
(Figure 14.19), a high density value and so on. Having established a lithology and built up an annotation
Anomalous log values are often either ignored, not iden- of electrofacies, the only remaining preparation before
tified as anomalous or wrongly interpreted lithologically. undertaking the final, grand interpretation, is to add to
They will contain unusual minerals, often in smal] quan- the document all other relevant data such as stratigraphic
tities and indeed the lithotogy may be very complex, as is ages, core sedimentology, thin section information,
the case with most hardgrounds. The example (Figure dipmeter interpretations and so on (Figure 14.14).
14.19) shows an unconformity which spans a gap of Frequently, an enormous amount of data is available
approximately 15 Ma marked by an anomalously high which is not exploited: all should be added to the com-
gamma ray peak. The mineralogical concentrations at the posite, annotated document.
unconformity are not known but quite probably include a The entire suite — lithology interpretation, electrofacies
high concentration of uranium possibly associated with annotation, sample analysis data, can now be subjected to
phosphatic nodules (Chapter 7). the ‘Electrosequence Analysis’ from the base of the well
Anomalies should be annotated with an asterisk by the upwards. All the data are used (Figure 14.20). In this step
side of the anomalous log response. 236 of the interpretation, the objective is to build the vertical