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:

where f is pore volume filled with a fluid of density r_f, r_b is the well-log-derived bulk density, and r_ma is the density of the rock framework. See density log and Z/A effect.