Page 194 - The Geological Interpretation of Well Logs

- THE GEOLOGICAL INTERPRETATION OF WELL LOGS -

will not be differentiated from random, background The outcrop example (Figure 12.21) is from a deltaic
spread, which is typically around 5°. As a rule of thumb, sequence in which both tabular and trough cross-bedded
the optimum size of sedimentary structure resolvable by structures are present (Cameron er al., 1993). Bed-set
the dipmeter (including internal structures), assuming a thicknesses are generally 1 m (3’) or Jess. That is, one
correct and appropriate processing, is 30 cm to 50 m bedform with a consistent set of cross-beds is normally
(1ft-160ft) (Figure 12.20). There are exceptions. less than | m in thickness. This is a typical size (thick-
ness) and has implications for the choice of processing
Cross-beds and palaeocurrents parameters as discussed above. Only logs with small
Dipmeter response to cross-bedding is central to sedi- correlation intervals, in this example 20 cm (simulated),
mentary dipmeter interpretation. All the basic principles will contain foreset data. When the correlation interval is
are involved. Response depends on textural (electrical) too large, the set boundaries, which have a high textural
contrast, size of structure and processing parameters. contrast and hence create a strong resistivity anomaly,
Moreover, interpretation of cross-beds requires a knowl- will dominate the correlations (Figure 12.19). A correla-
edge of the sedimentology of bed-forms and an tion interval of no more than half the bedform thickness is
understanding of the manipulation of grouped dip and needed to bring out the internal structure.
azimuth data. These principles will be ijlustrated using With a correct correlation interval, the outcrop work
dipmeter-type logs (dip-logs) measured at outcrop as well demonstrates that a typical cross-bedded interval shows
as subsurface examples. dip groupings or ‘clumps’ related to the cross-bed dips

HISTOGRAM
i
Log 22 re e- ange ae d directh 50° a) TABULAR SETS I 4 N=404
hdd

o :
o *
6m
Ex
" 2 -
— ow *®

* z badfarm 18
Im - a
t ,
%
a v
a «
2m “= o-

4)

‘ ° b gncsin
¥
NI
surtace”
®
3m 5 *|© ® 4

NY « foresets
N t Tabular sets, planar foresets

4m JN ? . AZIMUTH

HISTOGRAM
- b) TROUGH SETS -

j N=-512
0 ® i
mM
KO “ly dedtorm .
NS , 2
re
t
Im S&S 2
Nw 2
RR ! .
RS ;
2m AT 3
*
Pa ‘ N
@
°
=
[+-—@ | Trough sets, curved foresets ‘oresels !

4m +—— ® ‘ mum measure for palaeocurrant
-
3m 4.
—

@® set boundary dips f£ foresets AZIMUTH
Figure 12.21 Dipmeter cross-bed characteristics illustrated by outcrop measured diplogs. a) Tabular sets show a bi-modal dip
histogram (set boundaries and cross-beds) and tight azimuth rose: b) trough sets show a uni-modal dip histogram (set boundaries
and cross-beds combined) and a broad azimuth rose. Simulated 20cm correlation interval {from Cameron et af., 1993).