FORMATION EVALUATION
PETE 321
Lab 2
_______
GR
Spring 2005
Sections 501, 502
Dr. David Schechter
Lab: Charles Ozobeme
Sections 503, 504
Dr. James Russell
Lab: Kalwant Singh
Lab 3 objectives
o More information about GR
o Calculation of Vsh
o Geographix – PRIZM tutorial
Gamma Ray Log
• Natural gamma-ray log is based on the measurement of natural
radioactivity of formation rocks, which depends on the content of
radioactive isotopes of uranium, potassium and thorium and, also,
on the lithological composition of formation rocks.
• Types:
– Natural
– Spectral
• Thorium, Uranium, Potassium
• Applications:
–
–
–
–
–
–
Correlation.
Lithology indicator
Determining the shale content of rocks.
Introducing depth corrections to other well logs.
Detection of radioactive minerals.
Paleoenvironmental indicator
Gamma Ray Log Interpretation
The interpretation of gamma ray logs can be
summarized as follows:
• Clean sands, sandstones, limestones, and dolomites
have low radioactivity. Anhydrite, salt, lignite and coal
have also a low radioactivity.
• Ordinary shales have a much higher radioactivity
than the rocks listed above. Shales are sufficiently high
in radioactivity and can generally be easily
distinguished from the other rocks on a gamma ray
log.
Gamma Ray Log
Figure: Scintillation Gamma Ray Detector
Gamma Ray Log
Gamma Ray Log
Gr max
Shale
Gr min
Sand
Shale
Spectral Gamma Ray Log
•It is a borehole log indicating
the concentration of thorium
(Th), uranium (U),
and potassium (K) in the
rocks surrounding a borehole.
Applications:
•Estimate volume and type of clay
minerals
•Identify fractures
•Differentiate radioactive carbonates
from shales and clays
Spectral Gamma Ray
Spectral Gamma Ray Log Vs Gamma Ray Log
Spectral Gamma Ray Log
•It is able to detect and
identify the radioactive source
(gamma-rays from Th, U, and
K respectively.)
•It is displayed as three curves
from all sources of thorium,
uranium and potassium.
Gamma Ray Log
•It is not able to discern
between the gamma-rays from
Th, U, and K respectively; it
just "counts" the total number
of gamma-rays that hits the
detector per second.
•The "output" is counts per
second (cps) that also may be
converted to API units for
gamma radiation.
Gamma Ray Log
Gamma Ray Log
Gamma Ray Log
Shale Content from GR Log
Shale Content from GR Log
I SH
GR  GRMIN

GRMAX  GRMIN