Do you know that you can visually estimate porosity just by looking at your density and neutron logs?
The trick is very simple, but not a lot of people know about it.
It’s particularly useful in a relatively clean carbonate or sandstone and liquid (oil or gas) bearing zone.
So, here is the trick.
1.Plot density and neutron curve on the same track.
2.Use compatible plot scale. This means that both the density and neutron curves read the same porosity scale.
a.For limestone use 1.95g/cc to 2.95g/cc for density and 0.45 to -0.15 for neutron.
b.For sandstone use 1.90g/cc to 2.90g/cc for density and 0.45 to -0.15 for neutron. (or if your reservoir is pretty shaly, use 1.85g/cc to 2.95 g/cc for density).
3.The middle line of the track represents 15% porosity. If you divide the scale 0.45 to -0.15 by 10 division, each movement to right or left represent 6 p.u.change in porosity.
4.In fluid, both density and neutron log will give the same porosity reading.
5.In gas zone, draw a straight line in between the density and neutron curves and estimate the porosity value.
6.This works in a relatively clean, oil or water bearing formation.
Pretty neat right? Yes, if you are in a clean carbonate or a clean sandstone.
If you are in a shaly-sand, in a shale zone, in a gas bearing zone or in an unconventional reservoirs, the density and neutron curves behave differently.
The method doesn't work. It that case, you just have to read between the lines and rely on your local experience.