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 GF~OGIC:\L I\TE
To summarise: as shale indicators, thorium, may be
used in most cases, potassium may be used in many cases
but uranium should not be used at all. This obviously
has implications for the simple gamma ray log: it is not
necessarily a good 'shale indicator'.
7.7 Quan[itati~'e use of the
simple gamma ray log
Tite gamma ray log may often be used quan[itatively, and
although the gamma ray value for shales varies enor-
lnousl}', in any one area or well, the values for pure shale
tend to be constant (Figure 7.14). Thus, if one considers
25
50
E
L
a
75
100
125
D.CAL GAMMA RAY
inches 0 50AP~ 100
zo tao 10
bit
Size
,
i
w
i = z
~ j J
~ w
~ = a
x
~ N
,\
1
,
i
_, ~
Figure 7.14 Sand line and shale line defined on a gamma ray
log. These 'baselines' are for the quantitative use of the log.
and ma}' be reasonably constant in any one zone.
a
0
0
x
J
RPRF.TAT10\' OF KELL LOGS -
the maximum average gaa ray log value to be pure
IOOr9° shale (i.e., shale line, Figure 7.14), and the lowest
value to indicate no shale at all (i.e., sand line, Figure
7.14), a scale from 0 - 100~7o shale can be conswcted. U
the scale is considered to be lineaz, any value (GR) of the
gamma ray log will give the volume of shale from the
simple calculation
volume of shale % = GR value (log) - GR (min)
GR(max)-GR(min) (I)
GR (max) = 100% shale, GR (min) = 0°/a shale, i.e, clean
formation.
Generally the value is not very accurate and tends to give
an upper limi[ to the volume of shale (Vsh or Vclay).
Moreover, there is no scientific basis for assuming that
the relationship between gamma ray value and shale
volume should be linear. Thus, a modification of the sim-
ple linear relationship used above has been proposed as a
result of empirical correlation (Dresser Atlas, ] 982). The
relationship changes between younger (unconsolidated)
rocks and older (consolidated) rocks (Figure 7.15):
for pre•Tertiary (consolidated) rocks,

for Tertiary (unconsolidated) rocks,
wan = 0.083(2"`•" _ 1) (3)
where Vsh =shale volume from these formulae
(see Figure 7.15) and
_ GR-GR(min)
W~ GR(maX)-GR(min)
as shown previously in (I).
vo
-^ o.e
Ee
¢ ~
O,
a' °6
uE
[d
E a O.a
¢_'
u_
¢a
~ ~ 0 1
° J z0 ao 6° ao IOpi
.oiume o~ s~aie (VS~I
Figure 7.15 Graphical representation of the relationship
between relative gamma ray deflection and shale volume.
(From Dresser Atlas, 198?).
78