1. How To Easily Read A Well Log
in 5 Minutes or Less Using
These 'Six Ko Ko Rules'
Enter 'The Ko Ko Rules'
What you need:
1. Plot triple combo logs in this sequence GR, Resistivity, Density-
Neutron. Plot GR in the first track and Resistivity in the second track.
Both curves should be plotted in increasing values from the left to the
right. Plot Density-Neutron together on the same track. The values
2. should be decreasing as we go from the left to the right.
(image: Oilfield Review, Spring 2011)
2. Plot Density-Neutron in lithology compatible scale: Sandstone or
Limestone scales. We will be able to tell the reservoir types, the fluid
types and estimate porosity right away.
3. 3. Look at the directions of GR, Resistivity, Density and Neutron curves
deflections, either to the right or to the left.
4. By combinating the patterns of each curve deflections, we can tell the
difference between reservoir and non reservoir and/or the fluid types (oil
vs gas vs water).
Follow the Ko Ko Rules:
1. Look at the directions of the curve deflections (whether to the right or
to the left) in these following sequence: GR-Res-Density-Neutron.
1. Tight non-reservoir: Right-Right-Right-Right.
GR-Res-Density-Neutron all deflect to the right.
4. Why it works: If we are not in a reservoir zone, GR is higher due to
larger natural radioactivity from U, Th, K contents. Resistivity is higher
due to tightness. Density and Neutron read low porosity.
2. Shale: Right-Right-Right-Left.
GR, Res-Den deflect to the right. Only Neutron deflects to the left.
Why it works: GR is higher due to larger natural radioactivity from U,
Th, K contents. Resistivity is higher due to tightness. Density read low
porosity. Neutron falsely responds to the clay bound water resulting to
higher apparent porosity. You will see a reversed Density-Neutron
crossover.
Sometimes, the resistivity could deflect to the left as well, depending on
the properties of the shale itself. So it could read, Right-Left-Right-Left.
Note: You might see a reversed Density-Neutron crossover in dolomite.
Over a dolomitic zone, GR reads lower.
5. 3. Low porosity (tight) reservoir: Left-Right-Right-Right.
GR deflects to the left, GR-Res-Density-Neutron deflect to the right.
Why it works: GR reads lower due to lower radioactivity of Th, K and
U. Resistivity, Density-Neutron respond to the tightness of the reservoir
(rock quality), rather than to the fluid types.
4. Water bearing reservoir: Left-Left-Left-Left.
GR-Res-Density-Neutron all deflect to the left.
6. Why it works: GR reads lower due to lower radioactivity of Th, K and
U. Resistivity responds to saline formation water. Density-Neutron reads
higher porosity in reservoir.
5. Oil bearing reservoir: Left-Right-Left-Left.
GR and Density-Neutron all deflect to the left. Only resistivity deflects
to the right.
Why it works: GR reads lower due to lower radioactivity of Th, K and
U. Resistivity responds to non conducive hydrocarbon, giving higher
resistivity. Density-Neutron reads higher porosity in reservoir.
7. 6. Gas reservoir: Left-Right-Left-Right.
GR deflects to the left; Resistivity, right; Density left; and Neutron,
right.
Why it works: GR reads lower due to lower radioactivity of Th, K and
U. Resistivity responds to non conducive hydrocarbon, giving higher
resistivity. Density gives lower bulk density due to lower gas density.
Neutron reads low apparent low porosity in gas zone due to lower
neutron-hydrogen interactions in gas as compared to neutron-hydrogen
interactions in water.
8. So, there you go - the six Ko Ko Rules in well log interpretation.
The caveat is the rules does not work all the time nor will it make you a
qualified petrophysicist. It may not work for freshwater environment,
low resistivity pay zones, hot sands, and perhaps in some
unconventional reservoirs.
But it's a good starting point for any well log evaluations.
You never know unless you try. Thanks Ko Ko, you rule!
