6.1.

Lithology Determination

1/12/2006

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6.1.1

GR, Density and PEF as Lithology Indicators

• Conceptual Interpretation Line #1

• Objectives
– Physics of Measurement
– Most relevant Bore-Hole Effects
– Applications
– Interpretation

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6.1.1

Lithology Indicators

• GR (Gamma Ray)

•Density & PEF (Photo Electric Factor)

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Gamma Ray Logging

Voice Over Text

To measure the natural Gamma rays emitted from the formation, the Gamma ray tool
is lowered in the borehole. The Gamma ray tool consists of a detector and associated
electronics to measure the gamma radiation originating in the volume of formation
near the tool.
Screen Text
To measure the natural Gamma rays emitted from the formation, the Gamma ray (GR)
tool is lowered in the borehole. The GR tool consists of a detector and associated
electronics to measure the gamma radiation originating in the volume of formation
near the tool.

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Natural Gamma Ray Energy Levels

Voice Over Text

Of the three particles generated during natural radioactive decay, the gamma ray is
the only one that can penetrate a rock formation for any appreciable distance and as a
result it is the only one that can be measured. For this reason Gamma rays are of
interest to us. In nature, isotopes of potassium, thorium and uranium are the three
main radioactive elements. Each element is capable of producing gamma rays that
can be measured. The figure shows the different energies of the gamma rays
produced by these radioactive materials.
Screen Text
Of the three particles generated during natural radioactive decay, the gamma ray is
the only one that can penetrate a rock formation for any appreciable distance and as a
result it is the only one that can be measured. In nature, potassium (K 40 ), thorium
(Th 232 ) and uranium (U238) are the three main radioactive elements. Each element is
capable of producing Gamma rays that can be measured. The figure shows the
different energies of the Gamma rays produced by these radioactive materials.

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Natural Gamma Ray Spectrometry (NGT) Tool

Counts Rate

Energy Windows

Voice Over Text

The standard Gamma ray tool measures the total number of Gamma rays coming to
the detector, irrespective of the energy of the gamma ray. The Natural Gamma Ray
Spectrometry tool measures both the number and the energy level of gamma rays.
This permits the determination of the concentrations of radioactive potassium, thorium
and uranium in the formation rocks.
Screen Text
The standard GR tool measures the total number of gamma rays coming to the
detector, irrespective of the energy of the Gamma ray. The Natural Gamma Ray
Spectrometry (NGT) tool measures both the number and the energy level of Gamma
rays. This permits the determination of the concentrations of radioactive K40, Th232 and
U238 in the formation rocks.

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Gamma Ray Tools

Depth of Investigation
10" - 15" (varies with formation density)

Vertical Resolution
24" (varies with logging speed)

Voice Over Text

The depth of investigation of the Gamma ray tool varies from 10 to 15 inches. As the
formation density increases, the probability of the Gamma rays getting absorbed by
the formation before reaching the detector also increases. The vertical resolution of
the Gamma ray tool varies with logging speed but at the standard logging speed it can
be considered to be around 24 inches.

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6.1.1

Logging Speed
Statistical variations vs. Logging speed

Accuracy

actual formation value

tool reading at given
logging speed

Voice Over Text

All radioactive detectors record statistical variations associated with the decay of
radioactive materials. As these random fluctuations can mask the true reading and
affect its accuracy, it is important to log at speeds slow enough that averaging
functions can reduce these fluctuations. The trade-off is that slower logging takes
longer and costs valuable rig time. In response, Schlumberger has set logging speeds
for tools so that accuracy is maintained and logging speed is maximized. The
appropriate logging speed can be found in the log quality control manual.
Screen Text
The radioactive decay is a random process. Because of the random nature of the
process, it is important to log at speeds slow enough that averaging functions can
reduce these fluctuations. Schlumberger has set logging speeds for tools so that
accuracy is maintained and logging speed is maximized. The appropriate logging
speed can be found in the Log Quality Control manual.

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6.1.1

Gamma Ray Applications

• Correlation
– Well to Well correlation.
– Depth matching between separate trips in the well.
– Positioning of open -hole sampling tools.
– Providing the depth control needed for cased hole perforation.

• General lithology indicator

– Discriminate between reservoir & non-reservoir (Net/Gross)

• Quantitative shaliness evaluation of the reservoir rock

Voice Over Text

The main application of the standard Gamma ray tool is correlation. Shales usually
have signatures on the Gamma ray log that can be recognized field wide and hence
the log is used for well to well correlation. It also permits logs made on one trip into the
borehole to be depth matched with those made on another trip, to position open-hole
sampling tools and to provide the depth control needed for cased hole perforation. In
areas where certain lithology aspects are already known, the Gamma ray log can be
used as a lithology indicator which is then used to discriminate between reservoir and
non-reservoir. The log often reflects the proportion of shale and, in many regions, can
be used quantitatively as a shale indicator.

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Natural Gamma Ray Spectrometry Applications

• Lithology identification.
• Study of depositional environments.
• Investigation of shale types.
• Correction of the GR for clay content
evaluation.
• Identification of organic material and
source rocks.
• Fracture identification.
• Geochemical logging.
• Study of a rock's diagenetic history.

Voice Over Text

The Natural Gamma ray spectrometry tool has the ability to evaluate the radioactive
potassium, thorium and uranium content of the formation. This results in a number of
important uses in formation evaluation.

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Corrections to GR logs
– Hole Size
– Stand-off of the tool from the bore hole wall
– Barite content of the mud
– Potassium content of the mud (only NGS data
can be corrected)
– Cased hole operations

Voice Over Text

One of the biggest features of the Gamma Ray log is its wide range of operating
environments. It can be run in almost any logging situation including cased wells, or in
open holes drilled with air, salt mud, oil-based mud or fresh mud. But certain
corrections need to be applied to the Gamma ray log as it is affected by :
- the size of the borehole
- the distance of the detector from the borehole wall
- the barite content of the mud
- the presence of potassium in the mud
- and the presence of casing.
Only the natural gamma ray spectrometry log can be corrected for potassium content
in the mud.

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6.1.1

Gamma Ray Corrections

Chart GR-1

Voice Over Text

The corrections can be made using the Schlumberger Charts GR-1, GR-2 and GR-3.
These charts provide the corrections for the log acquired under various open-hole and
cased-hole conditions using Wireline tools. This figure presents the correction chart for
the Wireline Gamma ray tool run in non-barite mud. These corrections are usually not
a part of the standard log data and have to be applied before using the data for
formation evaluation.
Screen Text
Schlumberger Charts GR-1, GR-2 & GR-3 contain the corrections for the log acquired
using the Wireline tools under various open-hole and cased-hole conditions. These
corrections are usually not a part of the standard log data and have to be applied
before using it for formation evaluation.

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6.1.1

LWD - Gamma Ray Corrections

Chart GR-4

Voice Over Text

The Schlumberger Chart GR-4 represents the correction for the Gamma ray log
acquired via logging while drilling technology. The corrections illustrated by this chart
are routinely applied to the logging while drilling data before delivery and therefore
care should be taken not to duplicate the correction.
Screen Text
The Schlumberger Chart GR-4 represents the correction for the LWD-GR log. The
corrections illustrated by this chart are routinely applied to the LWD data before
delivery and therefore care should be taken not to duplicate the correction.

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6.1.1

GR Log Interpretation

Voice Over Text

In sedimentary formations, the Gamma ray log reflects the clay or shale content. This
is because the radioactive elements tend to concentrate in clays and shales, causing a
high GR log reading. Clean formations, such as sandstones or limestones, usually
have a very low level of radioactivity and, consequently, a low Gamma ray log reading.
Making distinctions between these high and low Gamma ray log readings is the basis
of the Gamma ray log interpretation.
Screen Text
In sedimentary formations, radioactive elements tend to concentrate in shales, causing
a high GR log reading. Clean formations, such as sandstones or limestones, usually
have a very low level of radioactivity and, consequently, a low GR log reading. Thus,
the GR log reflects the shale content.

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6.1.1

0 GR (API) 100
Identify Reservoir rocks
GR max Shale

GR min

Sand
GR - GR min
Vsh =
GR max - GR min
Shale

Vsh : Shale volume Shaly sand

GR : GR Log reading
Sand
GRmax : GR Log reading in Shale zone
GRmin : GR Log reading in clean Sand zone
50 %
shale

Shale
0% Shale

100% Shale

Voice Over Text

To discriminate reservoir and non-reservoir rock first a Gamma ray level in thick shale
beds is identified. This reading is assumed to represent 100% shale. Similarly a sand
line is constructed by reading the average Gamma ray level of thick clean sands which
will be the sands with the lowest Gamma rays. Then a vertical line in the middle of the
shale line and the sand line, called the cut-off line is constructed for an initial quick-
look. All intervals where the Gamma ray log is on the left of this cut-off line are
assumed to be reservoir. The actual Gamma ray level within the reservoir interval is
the measure of shaliness.
Screen Text
To discriminate reservoir and non-reservoir rock :
- a GR level in thick shale beds is identified. This reading is assumed to represent
100% shale and is called shale-line.
- a sand line is constructed by reading the average GR level of thick clean sands
(sands with the lowest GR)
- a vertical line in the middle of the shale line and the sand line is constructed for an
initial quick-look (cut-off line).
- all intervals where the GR log is on the left of this cut-off line are assumed to be
reservoir.
The actual GR level within the reservoir interval is the measure of shaliness.

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GR Log Evaluation : Radioactive Sand

Radioactive sands

Total GR counts Potassium Uranium Thorium

Voice Over Text

Sometimes sands themselves contain radioactive minerals, like uranium. In case of
such radioactive sands, the Gamma Ray Log measuring the total formation Gamma
Ray will be misinterpreted as a shaly-sand. In such cases the Natural Gamma Ray
Spectrometry Log is used.
Screen Text
Sometimes sands themselves contain radioactive minerals, like uranium. Using the
GR log, such “radioactive sands” will be misinterpreted as a shaly-sand. In such
cases, the Natural Gamma Ray Spectrometry Log (NGS) is used.

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6.1.1

NGS Log Interpretation

Radioactive Sand

SGR Th U K
CGR

Voice Over Text

The NGS log provides the concentrations of potassium, thorium and uranium in the
formation and the total formation Gamma ray. Uranium can be present in both clean
and shaly formations. Thus, a corrected Gamma ray curve is also provided which is
the total formation Gamma ray with the effect of uranium removed and is known as the
CGR curve. This curve should be used for identifying reservoir and non-reservoir rock
and for the shale volume computation in the presence of radioactive sands.
Screen Text
The NGS log provides the concentrations of K40, Th232 and U238 in the formation and
the total formation GR (SGR). Uranium can be present in both clean and shaly
formations. Thus, a corrected Gamma ray (CGR) curve is also provided which is SGR
with the effect of uranium removed. This curve should be used for identifying reservoir
and non-reservoir rock and for Vsh computation in the presence of radioactive sands.

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Mineral Identification from NGS Log

Voice Over Text

The ability of the NGS log to resolve the potassium, thorium and uranium content also
helps in determining the clay type. Unless there is a complex mixture of radioactive
minerals in the formation, the Schlumberger Chart CP-19 can be used to identify the
common minerals. The ratio of thorium to uranium activity and the thorium to
potassium ratio, does not vary with mineral concentration. As an example a sandstone
reservoir with varying amounts of shaliness, with illite as the principal clay mineral,
usually plots in the illite segment of the chart, with thorium to potassium ratio between
2.0 and 2.5. Less shaly parts of the reservoir plot closer to the origin, and more shaly
parts plot closer to the 70% illite area.
Screen Text
Unless there is a complex mixture of radioactive minerals in the formation, the
Schlumberger Chart CP-19 can be used to identify the common minerals. As an
example a sandstone reservoir with varying amounts of shaliness, with illite as the
principal clay mineral, usually plots in the illite segment of the chart with Th/K between
2.0 and 2.5. Less shaly parts of the reservoir plot closer to the origin, and more shaly
parts plot closer to the 70% illite area.

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Gamma Ray Summary

• Correlation

• Shale Indicator (Reservoir / Non-Reservoir Identification)

• Shale Typing

• High GR is not Necessarily High Volume of Shale

• Spectral GR: Thorium, Uranium, Potassium

Ø Additional Rock Identification Applications
Ø Uranium-free Volume of Shale

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Lithology Indicators

• GR (Gamma Ray)

•Density & PEF (Photo Electric Factor)

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Gamma Ray Interactions

Voice Over Text

When the Gamma rays pass through matter, they experience a loss of energy due to
collisions with other atomic particles. These collisions can be divided into three basic
categories : pair production, compton scattering and photoelectric absorption. For the
determination of lithology the photoelectric absorption is the interaction of interest to
us.
Screen Text
When the Gamma (γ ) Rays pass through matter, they experience a loss of energy due
to collisions with other atomic particles which can be divided into three basic
categories :
- Pair production
- Compton scattering
- Photoelectric absorption.
For the determination of lithology the photoelectric absorption is the interaction of
interest to us.

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Photoelectric Absorption

Voice Over Text

Photoelectric absorption is the disappearance of a low energy gamma ray as it collides
with an atom, causing the ejection of an orbital electron. This interaction is a factor at
gamma ray energies below 100 kilo-electron-Volt and predominates at energies below
75 kilo-electron-Volt.
Screen Text
Photoelectric absorption is the disappearance of a low energy γ -ray as it collides with
an atom, causing the ejection of an orbital electron. This interaction is a factor at γ -ray
energies below 100 keV and predominates at energies below 75 keV.

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Energy Spectrum

Photoelectric Absorption

Compton Scattering

Voice Over Text

The electrical pulses are analyzed and converted to the gamma ray count rate versus
their energy. The spectrum represents the energy lost by gamma rays as they interact
with the formation. The number of gamma rays in the high energy region “B” is related
to the amount of Compton scattering taking place in the formation, and the number of
gamma rays in the low energy region “A” is related to the amount of photoelectric
absorption taking place in the formation.
Screen Text
The electrical pulses are analyzed and converted to the gamma ray count rate versus
their energy. The number of Gamma rays in the region :
- “A” is related the amount of photoelectric absorption
- “B” is related to the amount of Compton scattering
taking place in the formation.

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Energy distribution function

High Z
Medium Z
Changing
Low Z
Matrix
Counts per second

Constant density
(hence, changing porosity)

0 200 400 600

Gamma Ray energy [keV]

Voice Over Text

As can be seen in this figure, with the density held constant as the photoelectric
absorption factor is increased, the spectrum shows a decrease only in the low energy
area. This shows that the photoelectric factor of the formation is only related to the
number of gamma rays in the lower energy area.
Screen Text
With the density held constant as the PEF is increased, the spectrum shows a
decrease only in the low energy area. This shows that the PEF is inversely
proportional to the number of Gamma rays in the lower energy area.

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Window A Window B

Plot 1

Effect of Bulk Density

on the Spectrum

Window A Window B Plot 2

Effect of Lithology
on the Spectrum

Voice Over Text

Plot-1 shows the effect of changing bulk density on the spectrum. As the density is
increased, the spectrum drops in both the windows. Plot-2 shows the effect of
changing the lithology on the spectrum. Here the effects of changing the lithology can
be seen in the low energy window “A”, but there is no change in the high energy
window “B”. By comparing the two spectrums you can see that the low energy window
“A” changes in both cases. As a result of this, the low energy window “A” must be
normalized against the high energy window “B”. This will then assure that changes in
the formation density will not give the false impression that the lithology has changed
and a density independent photoelectric absorption factor can be obtained.
Screen Text
Plot-1 shows the effect of changing bulk density on the spectrum. As the density is
increased, the spectrum drops in both the windows. Plot-2 shows the effect of
changing the lithology on the spectrum. Here the effects of changing the lithology can
be seen only in the window “A”. By comparing the two spectrums you can see that the
window “A” changes in both cases. As a result of this, the window “A” must be
normalized against the window “B” to obtain the density independent PEF.

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PEF Applications

• Lithology Indicator for :

– Mono-mineral simple matrix (alone)
– 2-mineral matrices (in combination with density)
– 3-mineral matrices (in combination with density & neutron log)
– clay mineral identification (in combination with NGS log)

Voice Over Text

The Photoelectric factor alone gives an excellent indication of the lithology in the case
of simple mono-mineral matrix. In combination with the density log it is used to analyze
formations with two minerals. It also provides solution to the formations with three
minerals when combined with the neutron and the density log. The Photoelectric factor
along with the natural gamma ray spectrometry log can also be used for clay mineral
identification.

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Detector Spacing and

Volume of Investigation

Long Spacing LS
detector

Volume of
Investigation
Short Spacing SS
detector

Back-Scatter
BS
detector

Source

Formation density

Mudcake density
Density radial profile
Mud density

hmc
(Mudcake thickness or standoff)

Voice Over Text

The depth of investigation and resolution of the logging tool is dependent on the
spacing between the source and the detector. This source to detector spacing must be
great enough to allow gamma rays to have multiple interactions with the formation
electrons and must not be so great that all the gamma rays lose their energy prior to
reaching the detector. The tool incorporates more than one detector and each detector
spacing results in a different depth of investigation and enables compensation for the
effects of mud-cake. The Litho-Density tool has only two detectors while the Three-
Detector Lithology Density tool has an additional back-scatter detector very close to
the source which improves the mud compensation.
Screen Text
The source-to-detector spacing must be great enough to allow Gamma rays to have
multiple interactions with the formation electrons and must not be so great that all the
Gamma rays lose their energy prior to reaching the detector. The tool incorporates
more than one detector. Each detector spacing results in a different depth of
investigation and enables compensation for the effects of mud-cake. The LDT has only
two detectors while the TLD has an additional back-scatter detector very close to the
source which improves the mud compensation.

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Density-PEF Logging

Voice Over Text

A tool with a chemical gamma ray source and gamma ray detectors is placed in front
of the formation. The energy of the source is such that the gamma rays emitted from
the source undergo photoelectric absorption and Compton scattering in the formation.
They are then scattered back to detectors. The probability of photoelectric absorption
occurring is known as the photoelectric absorption cross section of the target atom.
Screen Text
A tool with a chemical Gamma ray source (662 KeV) and Gamma ray detectors is
placed in front of the formation. The Gamma rays emitted from the source interact
(photoelectric absorption and Compton scattering) with the formation and are
scattered back to detectors. The probability of absorption occurring is known as the
photoelectric absorption cross section of the target atom.

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Logging while Drilling Tool

ADN (Azimuthal Density Neutron Tool)
Safety Feature Measurements taken
in all quandrants

(Gravity Tool Face)

(Gravity Tool Face)

Voice Over Text

The Schlumberger’s Azimuthal Density Neutron tool houses a cesium source and two
detectors. Since the tool is a part of the rotating drilling string, it allows the recording in
all quadrants of the borehole while it is being drilled. The tool also houses an
ultrasonic detector to record the sensor stand-off from the borehole wall to make
appropriate corrections to the measurements. The tool incorporates an important
safety feature, that the radioactive sources can be retrieved if the drill string becomes
stuck.
Screen Text
The Schlumberger’s ADN tool houses a cesium source and two detectors. Since the
tool is a part of the rotating drilling string, it allows the recording in all quadrants of the
borehole while it is being drilled.

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Comparison of
WL & LWD Data

Voice Over Data

This figure compares the wireline data with the average of ADN quadrant data.

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Comparison of
WL & LWD Data

Voice Over Text

This figure illustrates the advantage of ADN quadrant data which is able of identify the
heterogeneous interval.

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Lithology identification using PEF

Atomic
Elements and Atomic Photoelectric
Number
Compounds (Z) Weight (A) Index (PEF)

Hydrogen 1 1.008 0.00025

Carbon 6 12.011 0.15898

PEF as a function of F, fluid content & Lithology
Oxygen 8 16 0.44784

Sodium 11 22.99 1.4093

Magnesium 12 24.32 1.9277

Aluminium 13 26.98 2.5715

Silicon 14 28.09 3.3579

Sulphur 16 32.07 5.4304

Chlorine 17 35.46 6.7549

Potassium 19 39.1 10.081

Calcium 20 40.08 12.126

Quartz 11.78 60.09 1.806

Calcite 15.71 100.09 5.084

Dolomite 13.74 184.42 3.142

Anhydrite 15.69 136.146 5.055

Sylvite 18.13 74.557 8.51

Halite 15.3 58.45 4.169

Gypsum 14.07 172.18 3.42

Anthracite 6.02 --- 0.161

Voice Over Text

The Photo-electric factor curve is a good matrix indicator and most of the commonly
occurring minerals have Photo electric factor of unique value. As can be seen from this
figure, the Photo-electric factor responds mainly to the lithology and has very little
effect due to changes in porosity or fluid content. Hence a safe interpretation of matrix
lithology can be made when dealing with simple lithologies.
Screen Text
The PEF is a good matrix indicator. As can be seen from this figure, the PEF responds
mainly to the lithology and has very little effect due to changes in Φ or fluid content.
Hence a safe interpretation of matrix lithology can be made when dealing with simple
lithologies.

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Mineral Identification from PEF & NGS

Voice Over Text

The Schlumberger Chart CP-18 provides clay mineral information using natural
gamma ray spectrometry and the photo-electric factor. Since the porosity and the
composition of many clay minerals may vary, the minerals plot on these cross-plots
not as unique points but as general areas.

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Limitations

- Presence of Barite in the mud

- Presence of thick mud-cake
- Poor quality data in rugose holes

Voice Over Text

Barite is often added to the mud to increase the mud weight. Barite has a photo-
electric factor of over 250 compared to that of the formation which has less than 6.
This results in a reduced gamma ray count in the low energy part of the spectrum. A
little amount of barite in the mud and hence the mud-cake will make the reading
incorrect. The photo-electric factor measurement is made using the short spacing
detector and therefore is more affected by the presence of mud-cake. The density
tools are pad type tools and the data quality degrades with hole rugosity. This is a
result of the poor pad contact with the bore-hole wall. Consequently, the mud enters
the volume between the pad and the borehole wall, affecting the log reading.

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Density & PEF Summary

• PEF: Photoelectric (Low Energy GR)

• Presence of Barite in the mud
• Presence of thick mud-cake
• Poor quality data in rugose holes

• Density: Compton Scattering (High Energy GR, 75kev/10Mev)

• Will be discussed further in the Porosity Density Section

Now, Let’s Do a Line #1 Exercise!

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