Well Log Interpretation Gamma Ray Log

1
Well Log InterpretationGamma Ray Log
Earth Environmental Science University of
Texas at Arlington
2
Gamma Ray Log
The gamma ray tool measures natural radiation
from the rock which primarily comes from K, Sr,
U, Th. Because these elements are present in
shale but usually absent in siliceous ss and
carbonates, the log is used as an estimator of
shale content.
3
Gamma Ray Log
The shale content in reservoirs is important
because it reduces permeability (and porosity).
4
Gamma Ray Log
Potassium (K) is the most abundant radioactive
element and is present in micas, clays, feldspars
and glauconite. Glauconitic and arkosic
sandstones produce an elevated GR response in
addition to shale, and shaly sandstones.
5
Gamma Ray Log
Cementation and secondary alteration may
precipitate uranium and thorium bearing minerals
in sandstones and carbonates which could be
mistaken for shale and bypassed as a potential
reservoir.
6
Gamma Ray Log
Because different radioactive elements produce
particles with different energies, it is possible
to detect the elements producing gamma rays using
the GR spectral log. The example shows carbonate
with U and Th mineralization above Barnett shale
at 9606
7
Gamma Ray Log
The GR log is used to find the shale in a
reservoir assuming micas and clays are the only
source of radioactivity. GR is not affected by
resistivities.
8
Gamma Ray Log
The 1st step is to calculate the gamma ray index,
Igr, which is just a linear interpolation of the
GR reading between the max and min values
9
Gamma Ray Log
The minimum GR value should be in a unit without
internal radiation. The maximum value should be
100 shale.
10
Gamma Ray Log
Normally, Vsh Igr and that is a conservative
estimate. Other formulas can be used in special
circumstances (see eq. 3.2 3.5 in text)
11
Gamma Ray Log

• Create a spreadsheet that calculates Igr and
  three estimates of Vsh
• Assume Vsh Igr
• The Larionov eq. for tertiary rocks

• The Larionov eq. for older rocks