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Gamma Ray Log in Petroleum Engineering

The document discusses gamma ray (GR) logs, which measure natural gamma radiation in rock formations. A GR log records the intensity of gamma rays emitted from the formation over time. Key applications of GR logs include estimating shale content, identifying lithology, determining bed boundaries, and correlating formations. GR readings are influenced by naturally radioactive elements like potassium, uranium, and thorium concentrated in clay-rich shale. Spectral GR logs can identify specific emission sources to aid lithology analysis. Formulas are presented to calculate shale volume from GR readings.

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137 views15 pages

Gamma Ray Log in Petroleum Engineering

The document discusses gamma ray (GR) logs, which measure natural gamma radiation in rock formations. A GR log records the intensity of gamma rays emitted from the formation over time. Key applications of GR logs include estimating shale content, identifying lithology, determining bed boundaries, and correlating formations. GR readings are influenced by naturally radioactive elements like potassium, uranium, and thorium concentrated in clay-rich shale. Spectral GR logs can identify specific emission sources to aid lithology analysis. Formulas are presented to calculate shale volume from GR readings.

Uploaded by

rayan alkurdi
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© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Lecture 7

Gamma Ray (GR) Log


Usama Alameedy
Petroleum Engineering Dept.
University of Baghdad
Ph.D. Candidate
Gamma Rays and GR Log (GR)

• A GR: is a high-energy electromagnetic wave which is emitted by


atomic nuclei as a form of radiation.
• Gamma rays are emitted by nuclei in their transition from an
excited state to a lower energy state. Gamma rays have
characteristic energy levels which can be used to identify the
parent substance.

• GR log: is the continuous recording of the intensity of natural


GRs emitted from the formation.
GR Log Applications
1. Shale content estimation.
2. Lithology identification.
3. Estimation of bed boundaries & formation thickness
4. Identification of radioactive minerals such as potash, uranium, &
nonradioactive minerals like coal.
5. Stratigraphic/ formation correlations
6. Monitoring movement of injected radioactive materials
7. Perforating depth control---cased hole.
Natural GR emission

• Potassium K40
• Uranium
• Thorium

SPECTRAL GAMMA RAY LOGS IDENTIFY EMISSION SOURCES BASED ON THEIR ENERGY LEVELS.
Principle of Gamma Ray Logging

The gamma ray log is a passive measurement of


the naturally occurring radiation in the earth.

• Principal isotopes emitting radiation are


Potassium-40, Uranium, and Thorium (K40, U, Th).
• Isotopes concentrated in clays; thus higher
radioactivity in shales than other formations.
• Sensitive detectors count the number of gamma
rays per unit of time
GR in common
minerals
GR log
• In sedimentary rocks radioactive elements
tend to concentrate in clays, which in turn
concentrate in shales.
• Sandstone, limestone, dolomite have low
radioactivity.
• Black shales and marine shales exhibit
highest radioactivity.
• Thus: radioactivity can be used to distinguish
shale from non-shale formations and to
estimate shale content.
• Qualitative lithology log
Gamma Ray 4 Feet
5 1/2” J55 Csg
Neat Portland
Unit of Cement
measurement:
24
What is an API unit ? 200 Uranium 13 ppm
Ft. Thorium 24 ppm
API
Units Potassium 4 %

Neat Portland
Cement

Since there was no obvious standard for scaling gamma rays, one was created. A test pit was
built with a cement made to have twice the gamma activity of a “normal” shale.
GR API unit - 1/200th of the difference between that and a typical cement.
Versatility of GR log
GR log can be run under the following conditions:
a. Water-Based mud
b. Gas or air mud
c. Oil based mud
d. Cased hole
e. Open hole
Comparison of SP and
GR Logs
Estimation of Shale Content: Vsh
• Vsh : is the bulk volume of shale (precisely the volume of silt, dry clay, and
bound water) to bulk volume.
• Calculate shale index, IGR , by:
GRlog  GRclean
I GR 
• where GRshale  GRclean
• GRclean is clean zone reading
• GRsh is shale zone (max) reading
• GRlog is reading in zone of interest
• GR correlations based on:
• shale distribution type
• age of shale (tertiary or older)
• local area
• Disadvantage: Contamination from non-shale radioactive sources.
Shale Volume Estimation
GRlog  GRclean
I GR 
GRshale  GRclean
EQUATIONS:

Larionov (1969), Curve 1(index), Vsh = IGR

Curve 2 (older rocks), Vsh = 0.33(22+IGR - 1.0)

Curve 3 (Tertiary rocks), Vsh = 0.083(23.7×IGR - 1.0)

Steiber (1970): Vsh = IGR /(3.0 - 2.0IRA)

Clavier et al (1971): Vsh = 1.7 - [3.38 -(IGR +


0.7)2]1/2
Spectral Gamma Ray:
Identify radioactive minerals such
as potash, uranium, and
nonradioactive minerals such as
coal.
Spectral GR

• Example:
• Zone looks like shale.
• K40 low  Uranium rich sand

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