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Muhammad Shamshad GadiInstitute of Geology
University of Punjab, Lahore
Presented in Schlumberger, Islamabad.
Gamma Ray Log
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ContentsSection 1 - Geophysical
logging⁻ Definition⁻ Use of well logs⁻ Information obtained
from logsSection 2 - Gamma Ray
logs⁻ Principle and theory⁻ Tools⁻ Log presentation⁻ Horizontal and
vertical resolutionShamshad
⁻ Factors affecting the log value
⁻ Log interpretation and applications
⁻ Conclusions
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⁻ Continuous recording of a geophysical parameters (density, temperature etc) along a borehole
⁻ -Wireline logs or simply Well logs.
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Geophysical Well Log
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⁻ Drilled cuttings leaves an imprecise record of formation drilled.
⁻ Cores are expensive & time consuming
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Use of geophysical well logs
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⁻Rock Types (Formation) Lithology⁻Resistivity.⁻Geometry of Bore Hole⁻Saturation (Water & Hydrocarbon)
⁻Porosity and Permeability.⁻Bulk Density⁻Temperature in hole
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Information we can obtain
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⁻ Method of measuring naturally occurring gamma radiation to characterize the rock or sediment in a borehole
⁻ Uranium, Thorium and Potassium
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Gamma ray log
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Figure1 : The radiations from rocks surrounding boreholeShamshad7
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⁻ Different types of rock emit different amounts and different spectra of natural gamma radiation.
⁻ Amongst sediments, shales emit strongest radiation.
Principle
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Occurrences of u, th and k
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⁻ Acidic rocks contain 4.65 ppm⁻ Soluble salts in uranyle form U⁻ Uranous form UO
Occurrence of u
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2+
6+
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From solution Uranium is deposited in three ways:1) chemical precipitation in acidic and reducing environment2)Adsorption on organic matter3) Chemical reaction in phosphorites
⁻ Primary source is acidic and intermediate igneous rocks
⁻ It is not soluble in water⁻ Transported as clay fraction detrital grains
so more common in terrestrial environment than marine
⁻ Heavy minerals such as thorite, monazite zircon contain thorium
Occurrence of t
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h
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⁻ Potassium is chemically active ⁻ Found in clay minerals, micas, feldspars and
k-salts such as KCl
Occurrence of
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Figure 2. Gamma rays from sandstone, shale and coal (schematic)
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⁻ The radiations from three elements (K,U,Th) in combination.
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Simple Gamma Ray Log
Spectral Gamma Ray Log⁻ The amount of each individual element
(K,U,Th) contributing to this radioactivity
⁻ By the wavelengths of their characteristic gamma emissions.
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Energies of gamma rays
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Figure 3 Energies of gamma rays . (Rider,1996)
⁻ Scintillation counter made up of a large sodium iodide crystal
⁻ Produces a flash of light⁻ Converted to an electric pulse by a
photoelectrical cell
GR Logging Tool
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Figure 4: Scheme of a gamma ray tool
(re-drawn from Serra, 1979 after a Lane Wells documents)
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Compton scattering
http://www.ndt-ed.org
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Figure 5. Compton scattering
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Log presentation (Simple gamma ray log)
Figure 6: Presentation of Gamma Ray Log.
From Dr. Paul Glover
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Log presentation (Spectral gamma ray log)
Figure 7: Presentation of Spectral Gamma Ray Log.
From Dr. Paul Glover
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1ppm U=8.09 API1ppm Th=3.93 API1% k=16.32 API
Figure 8: Volume Of Investigation.
(Rider, 1996)
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Logging speed ⁻ Radiations are counted by a tool over a fixed
period of time, say two seconds, called the time constant.
(Rider, 1996)
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Unwanted borehole effects
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Factor Effects on log
Figure 9: Effect of KCl mud on log value From Dr. Paul Glover
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⁻ Rocks of low radioactivity include primarily clean sandstones, limestones, and dolomites.
⁻ Anhydrite, halite, lignite and coal have also a low radioactivity. Their radioactivity increases when they are shaly.
⁻ Ordinary shales have a much higher radioactivity than the rocks listed above. The radioactivity of sandy shales is less than that of shales.
Interpretation of Gamma Ray Logs
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Figure 10. Effect of different lithologies on the gamma ray log.(schematic)
From Dr. Paul Glover26
Evaporites
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Figure:11 Potassium Salts giving very high peaks of radioactivity in a evaporate sequence. (Permian North sea)
( Rider, 1996)27
Figure 12. Gamma ray characteristics of coal.Shamshad
Coal And Source Rock Radioactivity
(Rider,1996)28
It is based on relationship between grain size and shale content.
Facies and grain size
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( Rider, 1996)29
Gamma ray shapes and facies sequences
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From Dr. Paul Glover
Figure 15. Gamma ray shapes and facies sequences
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⁻ Most frequently used for correlation because it is lithology indicator.
Correlation
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{
Figure 16. Correlation using the gamma ray log. Baronia field, sarawak,
(From Scherer, 1980)32
⁻ U has affinity for marine environment
⁻ Th has affinity for continental environment
⁻ Normal Th/U ratio of shales is 3-6⁻ Shales with higher
ratios=continental environment⁻ Shales with lower ratios show
more marine envionmwnts
Th/U ratio – Depositional environment of shale
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(From Rider,1996)
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Th/U ratio – Depositional environment of shale
Figure 18.Use of Th/U ratio to indicate environment of deposition
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Shale Volume
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= GR log – GR minGR max – GR min
𝑽=𝑻𝒉 ¿¿
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⁻ Clean sandstones, limestones, and dolomites give low value of gamma ray log
⁻ Anhydrite, halite and gypsum have low radioactivity.
⁻ lignite and coal show very low values on gamma ray log.
⁻ Gamma ray log is good shale indicator.⁻ Organic rich black shales give very
high value on log.
Conclusions
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