Chapter 7

Gamma Ray (GR) log

Lecture notes for PET 370

Spring 2012

Prepared by: Thomas W. Engler,

Ph.D., P.E.

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

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 radioactivity

in shales than other formations.

• Sensitive detectors count the number of gamma rays per

unit of time

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

calibrated, standard response.

GR Log General GR Response

GR Log Factors affecting tool response

(1). Radiation intensity of the formation

(2). Counter’s efficiency

(3). Time constant (rc)

(4). Logging speed

(5). Borehole environment

GR Log

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.

Time constant/logging speed

GR Log

Effect of Time Constant

and Logging Speed

on bed resolution

Time constant/logging speed

GR Log The 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 logging

speed (ft/sec) and tc is the time constant (sec).

2. The log cannot properly respond when h < hc

Time constant/logging speed

GR Log

The time constant and logging speed are regulated so that the GR log

is representative of the formation radioactivity.

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

curve.

2. Common combinations are:

n (ft/hr) tc (seconds)

3,600 1

1,800 2

1,200 3

900 4

Time constant/logging speed

GR Log Borehole effects

Correction typically ignored except for quantitative analysis such

as shale volume calculations.

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

GR Log

Examples:

(1). 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 ...?

(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 ppg

mud? ....16” hole....?

Borehole effects

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

GR Log

Shale Distribution

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

Sh

ale

Vo

lum

e, V

sh

laminated

Clavier, et al

Larionov (older rocks)

Stieber

Larionov (tertiary rocks)

GR Log SP/GR Comparison

GR Log References

Bassiouni, Z: Theory, Measurement, and Interpretation of Well Logs, SPE

Textbook 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)