Last Time
UNIVERSITY OF SOUTH ALABAMA
Electric Logs 1
GY 305: Geophysics
Lecture 6:
Electric (Wire Line) Logs 2:
Gamma Ray Logs
Electric Logs
•
•
•
Introduction (first electric logs)
Resistivity Logs
SP Logs (Spontaneous Potential)
Electric Logs
Formation Resistivity
•There are numerous types of
logs that provide a wide variety
of information.
Pelchelbronn, France
September 5, 1927
•We’ll look at the most
important
Consisted of a line
dropped down the
bore hole
1) Resistivity
2) SP
3) Gamma Ray
4) Neutron
5) Sonic/Density
Doveton, J.H., 1994. Geological Log Interpretation. SEPM Short Course Notes 29., 169p.
Resistivity Logs
Can be used to resolve
lithology, but they are
most useful for assessing
fluid composition.
Shallow versus deep
induction
Resistivity Logs
Ultimately resistivity of a
rock (Rt) is controlled by:
•Salinity of formation pore water
•Volume of pore space
•Geometry of pore space (Lab 2)
•Temperature of the logged zone
•Morphology and types of clays
•Phases of pore fluids
1
SP Logs
SP Logs
Sand line versus Shale Line
Spontaneous potential
is...
Beware of signal drift
…most useful as a
basic tool to distinguish
lithology (sandstone
versus shale)
http://www.kgs.ku.edu/Dakota/vol1/petro/gifs/fig08.gif
http://www.spwla.org/library_info/glossary/reference/glosss/images/glsp85f1.gif
SP Logs
Infiltration Concerns
http://www.gomr.mms.gov/images_opt/graphics/re/sp.gif
SP logs can also allow you to
resolve broad stratigraphic and
sedimentological transitions
(e.g., transgressions, regressions)
Today’s Agenda
Ion diffusion
Doveton, J.H., 1994. Geological Log Interpretation. SEPM Short Course Notes 29., 169p.
Introduction to Gamma Ray Logs
Electric Logs 2
•
•
•
Radioactive decay types
Gamma Ray Emission
Gamma Ray logs
2
Introduction to Gamma Ray Logs
Introduction to Gamma Ray Logs
• Radioactivity is common in the subsurface due to the
presence of unstable elements.
• Radioactivity is common in the subsurface due to the
presence of unstable elements
• Most useful elements are isotopes of U, Th and K
Introduction to Gamma Ray Logs
• Radioactivity is common in the subsurface due to the
presence of unstable elements
Radioactive Decay
Three modes of decay
1) Alpha Decay
Loss of alpha particle (He4)
•
• Most useful elements are isotopes of U, Th and K
Convert parent into element
that has nucleus containing
two fewer protons
• Radioactive decay can occur in one of three modes:
Radioactive Decay
Three modes of decay
Radioactive Decay
Three modes of decay
1) Alpha Decay
Loss of alpha particle (He4)
1) Alpha Decay
Loss of alpha particle
•
•
Convert parent into element
that has nucleus containing
two fewer protons
Convert parent into element
that has nucleus containing
two fewer protons
2) Beta Decay
Loss of beta particle (e)
2) Beta Decay
Loss of beta particle
•
•
Convert parent into element
whose nucleus contains one
more proton by losing an
electron
Convert parent into element
whose nucleus contains one
more proton by losing an
electron
3) Gamma Decay
Capture of beta particle
•
Convert parent into element
whose nucleus has one less
proton
3
Radioactive Decay
Three modes of decay
1) Alpha Decay
Loss of alpha particle
•
Gamma Ray Emissions
• The energy of gamma rays can tell you about the nature of
the radioactive isotope(s)
E = h<
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
E = energy in Joules
ν = frequency (hertz)
H = Planck’s Constant
Convert parent into element
whose nucleus contains one
more proton by losing an
electron
3) Gamma Decay
Capture of beta particle
•
Convert parent into element
whose nucleus has one less
proton
Gamma Ray Emissions
http://207.10.97.102
Gamma Ray Emissions
•… so it is possible to
resolve the percentage
of each radioactive
isotope in surface and
subsurface analyses
Each isotope has a
specific emission
energy…
• Potassium: 1460 KeV
• Thorium series: 2620 KeV
• Uranium-Radium: 1760
KeV
•This is a useful
technique for surface
analysis and remote
sensing.
•Requires a simple
Geiger counter
Gamma Ray Emissions
Gamma Ray Emissions
• It is rare to see spectral gamma ray logs
in the oil industry. They are most
interested in total gamma ray emissions.
• It is rare to see spectral gamma ray
logs in the oil industry. They are most
interested in total gamma ray
emissions.
www.spec2000. net
www.spec2000. net
• Why? Shales tend to have the highest
concentrations of K40 (clay minerals)
and clays also tend to absorb U/Th
isotopes.
www.odp.tamu .edu
4
Gamma Ray Logs (sub-surface)
Gamma Ray Emissions
•This is a more typical
Gamma Ray log as
used in the oil industry.
• It is rare to see spectral gamma ray logs
in the oil industry. They are most
interested in total gamma ray emissions.
• Why? Shales tend to have the highest
concentrations of K40 (clay minerals) and
clays also tend to absorb U/Th isotopes.
• But beware ringer rocks:
www.spec2000. net
•Arkose sandstone (K40)
•Evaporites (K40)
•Clay cemented sandstone
•Coals and rare dolostones absorb U
•Uranium-enriched sandstones
www.geomore.com
Gamma Ray Logs (sub-surface)
Gamma Ray Logs (sub-surface)
•This is a more typical
Gamma Ray log as
used in the oil industry.
•This is a more typical
Gamma Ray log as
used in the oil industry.
•Deflections to the right
mean higher counts
•Deflections to the right
mean higher counts
= Shale
= Shale
•Similar to the SP log
in that you can
distinguish a shale line
www.geomore.com
Gamma Ray Logs (sub-surface)
Like SP logs, Gamma Ray
log responses can be used
to identify depositional
conditions.
www.geomore.com
Gamma Ray Logs (sub-surface)
But like so much in geology,
one type of information is
seldom sufficient to
completely characterize
lithology.
Gamma ray logs are
commonly paired with
Resistivity or Neutron Logs
www.129.79.145.5
www.kgs.ku.edu
5
Upcoming Stuff
Thursday Lab:
SP/Resistivity Correlations (major oil project part 1)
Monday Lecture
Neutron/Density Logs
6