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Gamma Ray and Spontaneous PotentialSchlumberger
2-1
GR
Gamma Ray Corrections for Hole Size and Mud WeightFor 33⁄8-in. and 111⁄16-in. SGT wireline gamma ray tools GR-1
0 5 10 15 20 25 30 35 40
10.0
7.0
5.0
3.0
2.0
1.0
0.7
0.5
0.3
t (g/cm2)
Cor
rect
ion
fact
or
33 ⁄8-in. to
ol, centered
111⁄16-in. tool, centered
111⁄16-in. tool, eccentered
33⁄8-in. tool, eccentered
© Schlumberger
Log interpretation Charts GR-1 and GR-2, replacing Chart Por-7,are based on laboratory work and Monte Carlo calculations toprovide improved corrections for 33⁄8- and 111⁄16-in. SGT gammaray tools. The corrections normalize the response of both tools toeccentered positions in an 8-in. borehole with 10-lbm mud. ChartGR-2 provides a correction for barite mud in small boreholes.
Although these charts are more difficult to use than the onesthey replaced, the results are more exact since they are normal-ized to current tools, no interpolation is required, and the rangesare extended.
The input parameter, t, in g/cm2, is calculated as follows:
The correction for standoff is
CF′m is the correction factor for centered tools, while CF′o is thecorrection factor for eccentered tools. Both are corrected forbarite if it is present in the borehole. S is the actual standoff, andSm is the standoff with the tool centered.
Example: GR reads 36 API units, dh is 12 in., and mud weightis 12 lbm/gal. The tool is 33⁄8 in. and centered.
Therefore, t = 15.8 g/cm2, resulting in a correction factor of 1.6.
The corrected GR = 58 API units.
CF CF CF CF′ = ′ + − ′−
m o m
m
m
S S
S( ) .
2
tW d dmud hole sonde= −
8 345
2 54
2
2 54
2.
. ( ) . ( ).
Crossplots for Porosity, Lithology and SaturationSchlumberger
4-30
CP
Radioactive minerals often occur in relatively small concentra-tions in sedimentary rocks. Even shales typically contain only30 to 70% radioactive clay minerals.
Unless there is a complex mixture of radioactive mineralsin the formation, Chart CP-19 can be used to identify the morecommon ones. The ratio of thorium to uranium activity—the
thorium/potassium ratio, Th/K—does not vary with mineralconcentration. A sandstone reservoir with varying amounts ofshaliness, with illite as the principal clay mineral, usually plotsin the illite segment of the chart, with Th/K between 2.0 and 2.5.Less shaly parts of the reservoir plot closer to the origin, andmore shaly parts plot closer to the 70% illite area.
Mineral Identification fromNGS* Natural Gamma Ray Spectrometry Log CP-19
0 1 2 3 4 5
Potassium (%)
25
20
15
10
5
0
Tho
rium
(pp
m)
Mixed layer clay
IlliteMicas
Glauconite
Potassium evaporites, ~30% feldspar
~30% glauconite
~70% illite
100% illite point
~40% mica
Mon
tmor
illoni
te
Chlorite
Kaolinite
Possible 100% kaolinite, montmorillonite, illite “clay line”
Th/
K: 2
5
Th/K
: 12
Th/K: 3.5
Th/K: 2.0
Th/K: 0.6
Th/K: 0.3Feldspar
Hea
vy th
oriu
m-b
earin
g m
iner
als
*Mark of Schlumberger© Schlumberger