SPWLA Glossary

In density logging. Common rock elements of usual interest in oil exploration, C, O, Si, and Ca all have a Z/A = 1/2, to a very high degree of approximation. For these elements and their molecules, the electron population is closely related to the unit mass or density of the material. In rocks containing elements in which Z/A differs from the Z/A value selected as a standard, the measured response will be different from the expected value. The amount that the measured bulk density will differ from the true bulk density of the rock material in question is related to the amount that Z/A differs from the standard. This influence on the interpretation of the density logging response is referred to as the Z/A effect. See also density log and compensated density log.