Z/A effect

1. n. [Formation Evaluation]
The ratio of the electron density to the bulk density. The electron density is equal to the bulk density multiplied by 2Z/A where Z is the average atomic number and A is the average atomic weight of the formation. The density log actually responds to electron density, whereas the desired measurement is the bulk density. Although for most rocks 2Z/A is close to 1, it is not practical to adjust 2Z/A for each formation. Instead the electron density is scaled to give the correct bulk density in limestone filled with fresh water. Then it is found that in the majority of sedimentary rocks, the log reads the bulk density within 1%. The main exceptions are halite, sylvite and low-pressure gas, because the 2Z/A of chlorine and hydrogen are not close to 1.