2. n. 
The density of the grains in a formation or core sample. As used in log and core analysis, the term 'grain' refers to all the solid material in the rock, since, when interpreting the measurements, no effort is made to distinguish grains from other solid material. The grain density of core samples is calculated from the measured dry weight divided by the grain volume. In logs, grain density is calculated from the density log, using an estimate of porosity and knowledge of the fluid content.