Education engineering well logging

1. WELL LOG ANALYSIS
BASIC LOG INTERPRETATION
CONCEPT- GR LOG
3RD STAGE
PETROLEUM ENGINEERING DEPARTMENT
BAGHDAD UNIVERSITY
Dr. Rwaida Kaiser

2. GAMMA RAY PRINCIPLES
▪ The Gamma Ray log is a measurement of the formation's
natural radioactivity. Which results from the natural decay
of radio active element surrounding the borehole.
▪ Gamma Rays are high-energy electromagnetic waves
which are emitted by atomic nuclei as a form of radiation
▪ Gamma ray log is measurement of natural radioactivity in
formation verses depth.
▪ Gamma ray emission is produced by three radioactive
series found in the Earth's crust.
- Potassium (K40) series
- Uranium series
- Thorium series.
▪ Gamma rays passing through rocks are absorbed at a rate
which depends on the formation density.
▪ Less dense formations exhibit more radioactivity than more
dense formations even though there may be the same
quantities of radioactive material per unit volume.
▪ Shale-free sandstones and carbonates have low
concentration of radioactive material and give low gamma
ray readings.
▪ Thus, as shale content increases, the gamma ray log
response increase because of the concentration of
radioactive material in shale.
▪ It is also known as shale log.
▪ GR log reflects shale or clay content.
▪ Clean formations have low radioactivity level.
▪ GR log can be run in both open and cased hole
▪ Appear in the left track ( first track) on a standard log.
▪ Measured in 𝐴𝑃𝐼 units.
▪ Linear scale (0 – 150 𝐴𝑃𝐼)
▪ Usually recorded with Caliper log
▪ The natural gamma ray log can be run in almost any logging
situation including cased wells, or in open holes drilled with air
or Gas mud, Water based mud (salt mud or fresh mud) and
Oil based mud

3. • 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
❖ Uses of Gamma ray log (GR–Log)
o Gamma Ray (GR) logs can be used for identifying lithologies
and for correlating zones.
o GR logs can be used not only for correlation, but also for the
determination of shale volume.
o Identify mineral deposits of potasium, uranium, and coal.
o Estimate bed boundaries & formation thickness.
o Perforating depth control in cased hole
❑ GR Log Applications :
A.cased hole Application : including depth control in all operation
during well completion and workover
B.Open hole Application :
Correlation; Lithology identification ( sand and shale count ); Depth
control

4. • Because shale is more radioactive than sand or
carbonate, gamma ray logs can be used to calculate
volume of shale in porous reservoir.
• The volume of shale expressed as decimal fraction or
percentage is called 𝑉𝑠ℎ𝑎𝑙𝑒.
• Calculation of the gamma ray index 𝑰𝑮𝑹 is the first step
needed to determine the volume of shale from a
gamma ray.
• GR has a linear and non linear relationship based on:
Shale distribution type ; Age of shale (tertiary or
older); Local area
• The non linear responses are based on geographic
areas or formation age. All non linear relationships
produce a shale volume value lower than that from a
linear equation.
• For a first order estimation of shale volume, the linear
response 𝑉𝑠ℎ = 𝑰𝑮𝑹 should be used.
𝑰𝑮𝑹 =
𝑮𝑹𝒍𝒐𝒈 − 𝑮𝑹𝒎𝒊𝒏
𝑮𝑹𝒎𝒂𝒙 − 𝑮𝑹𝒎𝒊𝒏
Shale Volume Calculation Where:
IGR = Gamma ray index
GRlog = Gamma ray reading of
formation
GRmax = minimum gamma ray
(clean sand or carbonate)
GRmin = maximum gamma ray
(shale).

5. ▪ The nonlinear responses are calculated as
follow
▪ Larionov (1969) for Tertiary Rocks:
𝑉𝑠ℎ = 0.083 × (23.7×𝐼GR − 1)
▪ Steiber (1970):
𝑉𝑠ℎ =
𝐼𝐺𝑅
3 −2∗𝐼𝐺𝑅
▪ Larionov (1969) for older rocks:
𝑉𝑠ℎ = 0.33 × (22×𝐼GR − 1)
▪ Clavier (1971):
𝑉𝑠ℎ = 1.7 –[3.38 – (𝐼𝐺𝑅-0.7)2 ] 0.5
▪ Note: we can use the above eqs. Or the
figure.
Nonlinear Relationship
Chart for correcting the gamma ray index to the shale
volume (Vshale).

6. ❖ In general depending on depth, the
relation can be as:
➢ For very hard compacted formation at
depth of 8,000 ft or more, gamma ray
index is considered equal to shale
volume: Vsh= IGR
𝑰𝑮𝑹 =
𝑮𝑹𝒍𝒐𝒈 − 𝑮𝑹𝒎𝒊𝒏
𝑮𝑹𝒎𝒂𝒙. − 𝑮𝑹𝒎𝒊𝒏
➢ For tertiary sediment rocks;
unconsolidated at depth of less than
4,000 ft, the shale volume is:
𝑉𝑠ℎ = 0.083 × (23.7×𝐼GR − 1)
➢ For Order rocks; consolidated at depth
of 4,000-8,000 ft, the shale volume is :
𝑉𝑠ℎ = 0.33 × (22×𝐼GR − 1)
• Solution:
1) Pick a clean GR response (GRmin), here GR min = 35 API
2) Pick a shale GR response (GRmax), here GR max = 135 API
3) Scale between (i.e. the GR log reading at the depth of interest), here
GR log = 90 API
Then 𝑽𝒔𝒉 =
𝑮𝑹𝒍𝒐𝒈 − 𝑮𝑹𝒎𝒊𝒏
𝑮𝑹𝒎𝒂𝒙. − 𝑮𝑹𝒎𝒊𝒏
=
𝟗𝟎 −𝟑𝟓
𝟏𝟑𝟓 −𝟑𝟓
so Vsh = 55%
Calculate Vsh at the
depth of interesting
(red circle) .
Example

7. COMPARISON OF
SP AND GR LOGS
• For the same well, on the left side
of the figure, the gamma ray log
(or GR) has a structure similar to
the SP trace: a low reading in the
clean zone and a high reading in
an apparently shaly zone

8. SPECTRAL GAMMA RAY (SGR)
• In some cases, might also produce a high
gamma ray response, because of the presence
of potassium feldspars, mica or uranium-rick
waters.
• In such a zone, if the geologist is aware of that,
spectral gamma ray can be run in instead of the
standard gamma ray.
• The spectral gamma ray log records not only the
number of gamma rays emitted by the formation
but also the energy of each and processes that
information into curves representatives of the
amount of Thorium (𝑇ℎ), Potassium ( 𝐾 ), and
Uranium ( 𝑈) present in the formation.
❖ Important uses of the spectral gamma
ray log include:
1- Determining shale (clay) volume (Vshale) in
sandstone reservoirs that contain uranium
minerals, potassium feldspars, micas, and/or
glauconite
2 - Differentiating radioactive reservoirs from
shales
3- Source-rock evaluation
4- Evaluation of potash deposits
5- Geologic correlations
6- Clay typing.
7- Fracture detection .
8- Rock typing in crystalline basement rocks

9. Symbols:
SGR : Total gamma ray in API (dashed
curve, track 1)
CGR : Total gamma ray minus uranium
in API (solid curve, track 1)
POTA or K : Potassium 40 in weight
percent (tracks 2 and 3)
URAN or U : Uranium in ppm (tracks 2
and 3)
THOR or TH: Thorium in ppm (tracks 2
and 3)
Typical Spectral Gamma Ray Log

10. LOG PRESENTATION