PERMFILE128524

• Permit No: M1999002
• IBM Index Class Name: Permit File
• Doc Date: 6/18/1999
• Doc Name: GAMMA RAY AND SPECTRAL GAMMA RAY LOG
• Media Type: D
• Archive: No

- TIIE GEOLOGIC,tL I\'TERPR ETATIOV OF N~ELL LOCS -
a mix contributes 24.27 API units, anon. The SGR is
therefore the sum of these API contributions (and can be
remembered as the_Sum Gamma Rays). The CGR, or
computed gamma ray curve, represents the contributions
of only the thorium and potassium in API units. Hence,
the difference between the SGR and the CGR is the con-
tribution, in API units, of uranium. For reasons explained
below (Section 7.10), the CGR is considered to be an
improved clay volume indicator to the total (SGR) API
count (and can be remembered as the Clay Gamma Ray).
In formats not described here, curves of the different
elemental ratios can be displayed.
~~ x
F
API ~
tool
centred
O
26
Depth oJinres«ga«on
The depth from which radiations can be detected by the
simple gamma ray tool is generally small but difficult
to be precise about. One experiment found that 759c of
radiations detected came from a 14 cm radius and 25 cm
verticall}• above and below the detector. This was for
gamma rays with a sinele energy of 1.76 MeV and the
detector centralised in a IS cm diameter hole filled with
1.? cm' density mud (Rhodes and Mott, 1966). Clearly,
natural conditions vary greatly from this specific case.
However, as a rough guide the volume of investigation
A. AVERAGE INVESTIGATION DEPTH
Resolution:
} vertica140cm
depth 10cm
20cm
gamma ray
detector
120em
1
B. INTEGRATED RESPONSE
00
3.0 2.0
u 1.0 ._ Formation
N
c density gcm'
a
d
0 10 20 40 60 a(
radius (cm)
Figure 7.7 Depth of investigation of the gamma ray tool. a.
average solume from which radiations are detected. b. depth
of investigation shown to be dependent on formation density.
Investigation depth is less in dense formations (graph, B,
re-drawn from Hallenberg, 1992).
c
a
a ~~~
eccentred
tool
so """

hole size 12'/."
Figure 7.8 Comparison of a gamma ray log from a hole
centred tool (DLL-MSFL-GR logged at IOm/min) and an
eccentred tool (LDL-CNL-GR logged at 4m/min). The
eccentred tool shows higher values and greater sensitivity.
can be considered to be approximately 20 cm vertically
above and below the detector (along the borehole) and 10
cm radially (Figure 7.7a). Because of Compton scatter-
ing, [his volume will vary with formation density: it will
be smaller in dense formations (cf. Hallenburg, 1992)
(Figure 7.76). Moreover, readings will be commonly
smeared, since the presented gamma ray log value is
generally an average of three contiguous raw values.
The simple gamma ray sonde can be combined in many
tools; it is run both centred in the borehole (sonic and resis-
tivity tools) or against the borehole wall, that is eccentred
(density and neutron tools). Because of Compton scatter-
ing in the drilling mud, the log made against the borehole
wall with duet[ contact to [he formation, will always show
a hieher reading and higher amplitude than the borehole
centred version entersed in the mud (Figure 7.8).
Logging speed
Because gamma radiations are discrete events and, as
described, are measured in the gamma ray tools by
`counting', there are restrictions on logging speeds.
Radiations are 'counted' by the tool over a fixed period of
time, called [he «nte consronr. Because the number of
individual emissions is not high, to have as large a count
as possible, the time constant should be long. However,
since a borehole tool is constantly moving, too long a
time-constant will blur bed boundaries and mix several
lithologies (Figure 2.12). With a rapidly moving tool, the
rock being 'counted' at the beginning of a long time-
constant will not be the same as the rock being 'counted'
at the end (for a discussion of this see 'Bed boundary
definition' Chapter 2).
72