Reading petrophysical well logging
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 should be decreasing as we go from the left to the right.
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.
For example, this is Density-Neutron scale for sandstone
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 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.
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.
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.
It may not even work for you next well that's half a mile away because the reservoir quality could change abruptly. That's just how nature works.
It's a good starting point for any well log evaluations.
The next step would be to look at all the available logs curves and other well data so you can do a proper quantitative log analysis. That's when petrophysics gets really interesting.
4. Water bearing reservoir: Left-Left-Left-Left.
GR-Res-Density-Neutron all deflect to the left.
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.
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.
So, there you go - the six Ko Ko Rules in well log interpretation.
It goes without saying that the rules won't work all the time. They are just rules, not a law. Even for gravity, it works on earth, but not so well in space. When it doesn't work, ignore these rules.
It may not work for freshwater environment, low resistivity pay zones, hot sands, and for unconventional reservoirs.