It is the value of the [[density]] of rock as it occurs in nature. In well logging, it is the density of the rock with the pore volume filled with fiuid. Natural density. The equation commonly used to compute [[porosity]] from [[well log]] derived bulk density is:

{{ :terms:p15f1.gif }}

where f is pore volume filled with a fluid of density r<sub>f</sub>, r<sub>b</sub> is the well-log-derived bulk density, and r<sub>ma</sub> is the [[density]] of the rock framework. See density log and [[ZA effect|Z/A effect]].