Page 193 - The Geological Interpretation of Well Logs

- THE DIPMETER -

ian) SIZE > (4°) 300m (1"? 16") (1607) (3000")
O-1¢em 106m Sm 50m 1000m
small 7 \ medium fi large v.large

DIPMETER
CURVES
DIPMETER
PROCESSED indirect affect --— at
cont WTB ==

FIELD
LLL VLiodnEnm
OUTCROP
LLL
7
SEISMIC — se Z AZOLE)

Jaminae/beds foresets dunes
; delta foreset
lenses lateral accretion tinof,
1
STRUCTURES bioturbation slumps clinoforms
.
(size approx.) . continental
ripples channel fill slope
HCS drape
_I

usually visible
sometimes visible
very occasionally visible
Figure 12.20 Dipmeter resolution of sedimentary structures. Very smal! scale features can be detected (not identified) by the dip-
meter curves, Processed dipmeter logs are quite different. Sedimentary structures with vertical thicknesses from about 30cm -— 50m
can be measured and recognized. The minimum depends on processing parameters, the maximum on presentation format. (HCS =
hummocky cross-stratification).
The minimum size of sedimentary structure with inter- thickness required for at least nwo separate, processed
nal dips which can be resolved by the dipmeter is in the measurements (fixed interval processing). For example, a
region of 30 cm (ft). Ripples, for example, will not be minimum thickness of 40 cm (16") with a 20 cm (8")
seen (Figure 12.20). The limiting factor is processing correlation interval (10 cm (4") step) is required (Table
resolution. To show how the dipmeter derives its dip, take 12.3). From theoretical considerations and empirical
this book to any outcrop with sedimentary structures and observations, cross-bed foreset structures (subaqueous
lay it on the rock surface. Now interpret the structure only) exist with heights between 10 cm and 10 m (aeolian
with the part behind the book covered. This is what the go up to 100 m) with typical values being (subaqueous)
dipmeter does. [t assumes that if there is a correlation in between 10 cm (4") and 2 m (6’). At the smaller scale, the
the curves, it is due to a planar surface crossing the entire structures will not generally be seen (Figure 12.20).
borehole (Figure 12.19), If the minimum size of structure resolved is dictated by
To have the potential for being resolved, a structure processing, the maximum size is dictated by the structure
must be present, and consistent along bedding, for at least itself. Generally structures which have a vertical
the across-hole displacement of internally dipping sur- thickness of around 50 m (160°) (provided they cause a
faces. For foresets, the cross-bedding structure must be recognisable anomaly), are clear on the logs. Partly this
quite large (i.e. thick in dipmeter terms). A foreset with is a question of display format. Compressed scale logs
a 25° dip (bedding horizontal) has an across borehole (Figure 12.30) can show much larger scale structures than
displacement of 10 cm in an 8.5" borehole and 14.5 cm in normal 1:500 scale logs. However, in most large scale
a 12.25” borehole (Table 12.3). For the foreset laminae to structures, such as drapes and sedimentary depositional
be measurable, they must be present over the vertical slopes, the angles involved are very small, 2°-4°, and

Table 12.3 Across hole displacements of dipping surfaces.

Hole size 6" 8.5" 12.25" 17.5"

Dip 10° 2.7¢em (1.1") 3.8m (1.5") 5.5¢m (2.2") 7.8¢m (3.1")
Dip 20° 5.5cm (2.2") 7.9cm (3.1") 11.3cm (4.5") 1§.2cm (6.4")
Dip 30° 8.8cm (3.5") 12.4em (4.9") 18.0em (7.i") 25.7em (10.1")
Dip 40° 12.8¢m (5.0"} 18.1em (7.1") 26. lem (10.3") 37.3cm (14.7")

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