The unique art of Gamma Log

7/23/2019 The unique art of Gamma Log

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Gamma Ray Logging


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GR Principles


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GR response to lithology

The GR log records the abundance of the radioactive isotopes of K, Th andU

K, Th and U are usually concentrated in shales and less concentrated insandstones and carbonates (owing to differences in mineralogy)

Common GR readings, in API units*, are:

Limestones and anhydrites, 15-20 API Dolomites and clean (shale-free) sandstones, 20-30 API

Shales, average 100 API, but can vary from 75 to 300 API

Other lithologies: coal, salt (halite, NaCl) and gypsum usually give low readingswhile volcanic ash and beds of potash salts (sylvite, KCl) give high readings

Therefore, the GR log is a good first-pass indicator of lithology

*1 API unit = 1/200th of the response generated by a calculated standard thathas 2x the average radioactivity of shale with 6ppm U, 12ppm Th and 2% K


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Factors affecting tool response

Radiation intensity of the formation

Counters efficiency

Time constant (TC) Logging speed

Borehole environment


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Time constant/logging speed

Gamma Ray Logs never repeat

exactly! The minor variations are

statistical fluctuations due to the

random nature of the radioactive

pulses reaching the detector.

Typical ranges are 5 - 10 API Units

in shales, and 2 - 4 units in clean

formations

Reduce statistical fluctuations by

optimizing the time constant and

logging speed.


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Effect of Time Constant

and Logging Speed on

bed resolution


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The faster the logging speed, the less time the tool can sufficiently react andproperly count the radiation intensity.

Two effects:

The tool response is shifted in the direction the tool is moving. This lag or critical

thickness (Hc) is given by Hc = LS*TC; where LS is logging speed (ft/sec) and TC is

the time constant (sec). The log cannot properly respond when H < Hc


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LS (ft/hr) TC (seconds)3,600 1

1,800 2

1,200 3

900 4


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Borehole Effects


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Borehole Effects

Examples:

A GR-CNL-LDT combination is run eccentered. What is the

corrected response if the log response is 40 API units in a 9

hole with 8.3 ppg mud? ...16 hole ...?

A GR - BHC combination is run centered. What is the

corrected response if the log response is 40 API units in a 9

hole with 16 ppg mud? ....16 hole....?


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Spectral GR

Eagleford Fm: Shale and sourcerock. High U content associatedwith high TOC (organics)

Buda Fm: Limestone. Very lowradioactivity (


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Gamma Ray Emission Energy Spectra

Intensity of radiation per gram per second

U-Ra series 26000 photons per gram per second

Th series 12000 photons per gram per second

19K40 3 photons/g/s


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Scattering and Attenuation

Gamma rays with energy >3 MeV. These

interact with the nucleus of the materials that

they are travelling through and are converted

into an electron and a positron in the process(pair production). The efficiency of the process

is low, so these gamma rays may be measured

by a sensor. However, they contribute onlysmall amounts to the overall signal.


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Gamma rays with energy 0.5 to 3 MeV. Thesegamma rays undergo compton scattering,where a gamma ray interacts with the

electrons of the atoms through which they arepassing, ejecting the electron from the atom,and losing energy in the process. A gamma rayin this range may undergo several of these

collisions reducing its energy from its initialvalue to an energy of less than 0.5 MeV in astepwise fashion.


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Gamma rays with energy


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Determination of Shale content

IGRis Gamma Ray Index


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Other uses

Depth matching

Cased hole correlations

Recognition of radioactive mineral deposits Radio-isotopes tracer operations

Facies and depositional environment


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The unique art of Gamma Log