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Chapter 7

Gamma Ray (GR) log

Lecture notes for PET 370

Spring 2012

Prepared by: Thomas W. Engler,Ph.D., P.E.

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GR Log Uses

Estimate bed boundaries, stratigraphic correlations

Estimate shale content

Perforating depth control

Identify mineral deposits of potash, uranium, and coal

Monitor movement of injected radioactive material

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GR Log Background

The Gamma Ray log is a continuous measurement of

the natural radioactivity emanating from the formations.

Principal isotopes emitting radiation are Potassium-40,Uranium, and Thorium (K40, U, Th)

Isotopes concentrated in clays; thus higher radioactivityin shales than other formations.

Sensitive detectors count the number of gamma rays perunit of time

Recorded in API Units which is 1/200th of the

calibrated, standard response.

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GR Log General GR Response

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GR Log Factors affecting tool response

(1). Radiation intensity of the formation

(2). Counters efficiency

(3). Time constant (rc)

(4). Logging speed

(5). Borehole environment

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GR Log

Gamma Ray Logs never repeatexactly! The minor variations arestatistical fluctuations due to therandom nature of the radioactivepulses reaching the detector.

Typical ranges are 5 - 10 API Units inshales, and 2 - 4 units in cleanformations

Reduce statistical fluctuations byoptimizing the time constant andlogging speed.

Time constant/logging speed

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GR Log

Effect of Time Constantand Logging Speed

on bed resolution

Time constant/logging speed

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GR LogThe faster the logging speed, the less time the tool can sufficiently react and

properly count the radiation intensity.

Two effects:1. The tool response is shifted in the direction the tool is moving. This

lag or critical thickness (hc) is given by hc = n tc; where n is loggingspeed (ft/sec) and tc is the time constant (sec).

2. The log cannot properly respond when h < hc

Time constant/logging speed

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GR Log

The time constant and logging speed are regulated so that the GR logis representative of the formation radioactivity.

1. By experience, hc = 1 ft., avoids excessive distortion of the GRcurve.

2. Common combinations are:

n (ft/hr) tc (seconds)

3,600 1

1,800 2

1,200 3

900 4

Time constant/logging speed

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GR Log Borehole effects

Correction typically ignored except for quantitative analysis suchas shale volume calculations.

Function of tool type, borehole size, mud weight, eccentricity

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GR Log

Examples:

(1). A GR-CNL-LDT combination is run eccentered. What is thecorrected response if the log response is 40 API units in a 9 hole

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

(2). 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 ppgmud? ....16 hole....?

Borehole effects

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GR Log Vsh estimation

Vsh is the bulk volume of shale (precisely the volume of silt, dry

clay, and bound water) to bulk volume. Calculate shale index, IRA , by

where

RAmin is clean zone reading

RAsh is shale zone (max) reading

RA 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.

minRA

shRA

minRARA

RAI

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GR Log Shale Distribution

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GR Log Vsh estimation

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Radioactivity Index, IRA

ShaleVolume,Vsh

laminated

Clavier, et al

Larionov (older rocks)

Stieber

Larionov (tertiary rocks)

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GR Log SP/GR Comparison

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GR Log References

Bassiouni, Z: Theory, Measurement, and Interpretation of Well Logs, SPETextbook Series, Vol. 4, (1994)

Chapter 2, Sec 2.1 2.5

Chapter 7, Sec all

Schlumberger, Log Interpretation Charts, Houston, TX (1995)

Western Atlas, Log Interpretation Charts, Houston, TX (1992)