Last Time
UNIVERSITY OF SOUTH ALABAMA
Electric Logs 2
GY 305: Geophysics
Lecture 8:
Electric (Wire Line) Logs 3:
Porosity Logs (Neutron and Density)
Radioactive Decay
Three modes of decay
1) Alpha Decay
Loss of alpha particle
•
Convert parent into element
that has nucleus containing
two fewer protons
2) Beta Decay
Loss of beta particle
•
But gamma ray emissions occur in all
types of decay
Convert parent into element
whose nucleus contains one
more proton by losing an
electron
•
•
•
Radioactive decay types
Gamma Ray Emission
Gamma Ray logs
Gamma Ray Emissions
Each isotope has a
specific emission
energy…
• Potassium: 1460 KeV
• Thorium series: 2620 KeV
• Uranium-Radium: 1760
KeV
3) Gamma Decay
Capture of beta particle
•
Convert parent into element
whose nucleus has one less
proton
Gamma Ray Logs
•Gamma ray logs record the
radioactivity of a formation.
Gamma Ray Logs
But like so much in geology,
one type of information is
seldom sufficient to
completely characterize
lithology.
•Shales (or clay-minerals)
commonly have a relatively high
gamma radioactive response, and
consequently gamma ray logs are
taken as good measures for grain
size (and subsequently inferred
depositional energy).
Gamma ray logs are
commonly paired with
Resistivity or Neutron Logs
•Sand, which contains little mud,
will have low gamma ray value,
while a fine mud will have a high
gamma ray value. The values range
of gamma ray is measured in API
(American Petroleum Institute)
units and range from very few units
(in anhydrite) to over 200 API units
in shales.
http://www.geomore.com/Electric%20Logs.htm
www.kgs.ku.edu
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Porosity
Today’s Agenda
Electric Logs 3
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•
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One of the most important parameters in petroleum geology and
one of the hardest to directly measure (especially in limestones)
Porosity Logs
Neutron Logs
Density Logs
Note: Resistivity can sometimes
do it, but this log is ultimately
controlled by water salinity
1 mm
Plan polarized light
http://www.spec2000.net/07-eslog.htm
Porosity Logs
Neutron Logs
Neutron Logs have a
radioactive source
(tritium?) that generates
neutrons. As they pass
into the rock, they lose
energy (especially if
they hit nuclei of
similar weight (e.g.,
hydrogen).
A term applied to 3 basic log types:
1) Neutron
2) Density
3) Sonic (next time)
http://www.spwla.org/library_info/glossary/reference/glossd/images/glsp33f1.gif
Neutron Logs
Neutron Logs
...Neutron Logs also
distinguish hydrogen
concentration.
Neutron Logs use
shallow penetration
(6”) and mostly see
mud filtrate, but...
http://www.spwla.org/library_info/glossary/reference/glossd/images/glsp33f1.gif
The more H, the more
adsorption.
http://www.spwla.org/library_info/glossary/reference/glossd/images/glsp33f1.gif
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Neutron Logs
Neutron Logs
...Neutron Logs also
distinguish hydrogen
concentration.
...The more hydrogen
you detect, the more
porosity you have
The more H, the more
adsorption.
Notes:
1) Shales absorb neutrons
because of high (bound)
water content
2) Water and oil have
similar H content and thus,
similar neutron absorptions
3) gas has very low H
content
4) coal shows enormous
absorption of neutrons
High counts=low
adsorption (Low
porosity)
Low counts=high
adsorption (high
porosity)
http://www.spwla.org/library_info/glossary/reference/glossd/images/glsp33f1.gif
Neutron Logs
http://www.spwla.org/library_info/glossary/reference/glossd/images/glsp33f1.gif
Neutron Logs
•Neutron logs measure the porosity
of a formation, indicating in its
response the quantity of hydrogen
present in the formation.
•The log is calibrated to limestone.
•Neutron logs can be used to
resolve lithology (usually in
combination with Density Log).
•The linear limestone porosity units
are calibrated using the API
Neutron pit in 19% porosity, waterfilled limestone is defined as 1000
API units.
Chalk Board
http://www.geomore.com/images/new%20pa8.jpg
Density Logs
http://www.geomore.com/images/new%20pa8.jpg
Density Logs
Use a gamma ray
source attached to a
skid plate
•gamma rays interact with electrons clouds of atoms,
•the gamma ray “flux” decreases with increasing numbers of
electrons in the formation adjacent to the emitter
far detector
CH 100:
electrons = protons (Z)
Atomic number (A) = protons+neutrons
near detector
http://www.freepatentsonline.com/6738720.html
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Density Logs
Density Logs
•gamma rays interact with electrons clouds of atoms,
•the gamma ray “flux” decreases with increasing numbers of
electrons in the formation adjacent to the emitter
•The Density log is a measure of
the formation's bulk density and is
mostly used as a porosity measure.
CH 100:
electrons = protons (Z)
Atomic number (A) = protons+neutrons
if you assume Z/A = 0.5,
then you can then estimate apparent formation density
Reported in g/cm3 or kg/m3
http://www.resolve-geo.com/geophysical/index_clip_image004.jpg
Density Logs
Neutron and Density Logs
•The Density log is a measure of
the formation's bulk density and is
mostly used as a porosity measure.
•Different lithologies can also be
determined using Density log based
on returned density value. For
example, pure quartz will have a
bulk density (g/cm3) up to 2.65,
coal 1.2-1.8, halite 2.05, limestone
up to 2.75, dolomite up to 2.87,
anhydrite 2.98.
http://energy.cr.usgs.gov/OF00-200/WELLS/SBAR20/LAS/SB20LOG.JPG
Chalk board
http://www.resolve-geo.com/geophysical/index_clip_image004.jpg
Logs used in Combination
Upcoming Stuff
Thursday Lab:
SP Correlations Due at 2:00 PM
Gamma Ray Correlations issued
•The “crossover” effect as a gas
indicator
Next Monday Lecture
“Midterm” exam (Haywick’s part)
•CNL: Neutron Log
•FDC: Density Log
http://www.geo.wvu.edu/~jtoro/Petroleum/Review%203.html
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