BASIC WELL LOGGING ANALYSIS –
THE GAMMA RAY (GR) LOG
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Hsieh, Bieng-Zih
Fall 2009
THE GAMMA RAY (GR) LOG

Gamma ray logs measure natural radioactivity in
formations and because of this measurement, they can be
used for identifying lithologies and for correlating
zones.
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THE GAMMA RAY (GR) LOG

As shale content increases, the gamma ray log response
increases because of the concentration of radioactive
material in shale.

Shale-free sandstones and carbonates have low
concentrations of radioactive material, and give low
gamma ray readings.
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GR LOG
THE GAMMA RAY (GR) LOG (CONT.)

However, clean sandstone (i.e. low shale content ) may
also produce a high gamma ray response if the
sandstone contains potassium feldspars (鉀長石), micas
(雲母), glauconite (海綠石), or uranium-rich (高含量鈾的)
waters.

In zones where the geologist is aware of the presence of
potassium feldspars, micas, or glauconite, a Spectralog
can be run in addition to the gamma ray log.
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THE GAMMA RAY (GR) LOG (CONT.)

The Spectralog breaks the natural radioactive of a
formation into the different types of radioactive material:
(1) thorium (釷), (2) potassium (鉀), and (3) uranium (鈾).

If a zone has a high potassium content coupled with a
high gamma ray log response, the zone may not be shale.
Instead, it could be a feldspathic (長石質的), glauconitic
(海綠石質的), or micaceous (雲母質的) sandstone.
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THE GAMMA RAY (GR) LOG (CONT.)

Besides their use with identifying lithologies and
correlating zones, gamma ray logs provide information
for calculating the volume of shale in a sandstone or
carbonate.
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GR LOG CURVE


The gamma ray log is
recorded in track ＃1,
usually with a caliper.
Tracks ＃2 and ＃3 often
contain either a porosity
log or a resistivity log.
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SHALE VOLUME (VSH) CALCULATION

Because shale is more radioactive than sand or
carbonate, gamma ray logs can be used to calculate
volume of shale in porous reservoirs.

The volume of shale can then be applied for analysis of
shaly sands.
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CALCULATION OF THE GAMMA RAY INDEX (IGR)


Calculation of the gamma ray index is the first step
needed to determine the volume of shale from a gamma
ray log (the following formula from Schlumberger, 1974).
IGR=
GRlog  GRmin
GRmax  GRmin
Where: IGR = gamma ray index

GRlog = gamma ray leading of formation

GRmin = minimum gamma ray (clean sand)

GRmax = maximum gamma ray (shale)

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FIND GRMAX, GRMIN FOR SAND FORMATION (SAND-B)
SAND-B
GRmin
GRmax
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CHART FOR CORRECTION THE GAMMA RAY INDEX
(IGR ) TO THE VOLUME OF SHALE (VSH).

Curve 3


Curve 2


unconsolidated,
Tertiary rocks
consolidated, older
rocks
Curve 1

Vsh=IGR
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PRACTICE – VSH CALCULATION FROM GR LOG

Step 1


Setup shale base line
Step 2

Analyze SAND intervals
(formations)
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PRACTICE – VSH CALCULATION FROM GR LOG

Step 3

Calculate IGR between
13680 -13720 ft
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PRACTICE – VSH CALCULATION FROM GR LOG

Step 4
Calculate Vsh between
13680 -13720 ft
 Note: this formation is a
kind of consolidated rocks
(you should use curve 2)

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REVIEW – GR LOG

Gamma ray logs are lithology logs that measure the
natural radioactivity of a formation.

Because radioactivity material is concentrated in shale,
shale has high gamma ray readings. Shale-free
sandstones and carbonates, therefore, have low gamma
ray readings.

Gamma ray logs are used to: (1) identify lithologies; (2)
correlate between formations; and (3) calculate volume
of shale.
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END OF THIS CHAPTER
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