We can make an important error if the fluid is not water of density 1.0. We then use another series of Charts built for salted water of density 1.1 for example.
An error much more significant is made when in place of water we have light hydrocarbons. The density of the fluids is then lower than 1.
The presence of Gas for example with the density tool will give us a reading of porosity too high (b decreases) and with de neutron tool a porosity too weak. It results on the Cross-plot neutron-density a displacement of the point to the top and towards the left.
When the correction for gas is not made, the porosity read on the Chart is too low. Example the point B which after correction for gas, becomes point A.

Another factor of error is the presence of clay. Clays have an high answer in density and neutron . A rock with clay will point too high in neutron and in density, in the direction (in bottom on the right) of the shale dot. This shale dot varies according to the type of clays . We obtain it by transferring on the Chart the apparent porosity, density, and t observed in the vicinity of the clayish layers.

Chart: determination of the porosity and lithology
by density and neutron logs.

Download Chart 9

Cross-plot neutron-sonic
In the same way, we can make a cross-plot neutron-sonic while placing T of sonic in ordinate. This cross-plot gives good results for lithology, with this time a correspondence between the 0 neutron and T of 47.5 _S/foot for the sonic.

Cross-plot sonic-density
We can also combine sonic-density. In that case the determination of porosity is not very good. On the other hand, this cross-plot is interesting for the determination of the evaporitic formations.