"The Butterfly Effect" is not all Hollywood.

In our oil and gas, our "Butterfly Effect" has nothing to do with Ashton Kutcher.

It's about is detecting gas based on signatures of the density and neutron logs.

When you plot density log and neutron log on a limestone compatible scale on the same log track, you will see significant separation between neutron and density log in a gas bearing zone, especially when you are in a limestone reservoir. (A limestone compatible scale is when you plot density log on a scale from 1.95 gm/cc to 2.95 gm/cc and neutron log on a scale of 0.45 to -0.15 porosity unit).

When you plot both logs together on the same plot, you will see "butterfly" like feature. Yes, "The Butterfly Effect" is the gas crossover on the neutron-density logs. It looks like a butterfly. Hence, the name "The Butterfly Effect".

The density log will tend to deflect to the left showing lower density since gas has lower density than oil or water.

The neutron will deflect to the right showing lower porosity. This happens since in gas, the neutron porosity, which reads the H+ concentration is lower than in liquid, giving lower porosity.

In the log below, we can clearly see the neutron and density log curve crossover each other. This indicates the butterfly effect where the gas bearing zone is.

(image: Oilfield Review, Spring 2011)

So, whenever you see the gas crossover (density left, neutron right), it's the indication of gas bearing formation. You might also see the effect in light hydrocarbon bearing reservoirs.

But beware, you might not see this effect at all if:

  1. You have significant mud invasion,
  2. You are in a non-limestone reservoir,
  3. You plotted the density-neutron logs on different scales, or
  4. Some other borehole and formation anomalies like badhole etc.

That's it!