Page 245 - The Geological Interpretation of Well Logs

- FACIES, SEQUENCES AND DEPOSITIONAL ENVIRONMENTS FROM LOGS -

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8 Neutron Neutron Neutron 8
5 60 45 1 30 45 L 30 4 i 45 L 30 3
J
- =
a
a
——
O datum
ma
30
c. —

200m

basinward
—<————_—_—__—

SIMPLIFIED
LOG SHAPES

Figure 14.17 Complex log shapes indicating depositional patterns. Log shapes in neutron logs are persistent over thick shale
intervals which represent at least 30 Ma of deposition and basin filling. The shapes are caused principally by changes in quartz
content, carbonate and organic matter: they indicate basin filling conditions, The two outside wells are over 50km apart.

The following abrupt breaks are identifiable: and will need to be substantiated subsequently using
additional data. If the lithological interpretation shows
1) Lithologically related Erosion that the abrupt break is entirely within a shale sequence,
breaks Flooding shale resting on shale, then neither erosion nor flooding is
(catastrophe) an appropriate initial proposition and a fault or unconfor-
mity can be suspected (Figure 14.18C,D). Clearly, more
2) Non-lithological Unconformity data are required to differentiate a fault from an uncon-
breaks Fault formity; dipmeter and seismic possibly to substantiate the
(diagenetic change) fault hypothesis, dipmeter, seismic and faunal dating to
(fluids change) identify an unconformity. A symbo! is used to indicate the
possibility selected.
(The items in brackets should be eliminated from the Many abrupt breaks will have a banal explanation and
analysis as soon as possible as they do not contribute should be eliminated - simply rubbed out (hence the
directly to the eventual geological interpretation). pencil). For instance, the limits of most coal beds are
All abrupt breaks should be marked (in pencil) on the sharp, as are some diagenetic contacts. Other breaks may
side of the lithological interpretation, initially by a thick be more subtle and kept in for further thought as is the
horizontal] line. Next, an attempt is made to annotate the case with shale on shale contacts. During the interpretation
break by examining it with a ‘geological logic’. For phase, the important abrupt breaks will normally come to
instance, if the abrupt log break separates a sandstone stand out. It should of course be added here that when a
above from a shale below, this may be an erosional] con- sequence stratigraphic interpretation is attempted, the
tact and can be annotated as such on the log using the breaks that have been marked will take on a new and
appropriate symbol (Figure 14.18A). If, on the contrary, quite specific significance (Chapter 15).
the abrupt break shows a shale resting sharply on a sand-
stone, then erosion is unlikely and a flooding event can be Anomalies. Anomalous log values are important ~ exces-
proposed, again by inserting the appropriate symbol sively high or excessively low peaks (values) may have
(Figure 14.183). These are only reasonable propositions great stratigraphic importance. The concentration of
235