Density logging is a well logging tool that can provide a continuous record of a formation's bulk density along the length of a borehole.
In geology, bulk density is a function of the density of the minerals forming a rock (i.e. matrix) and the fluid enclosed in the pore spaces.
A type of active nuclear tool, a radioactive source and detector are lowered down the borehole and the source emits medium-energy gamma rays into the formation.
The number of scattered gamma rays that reach the detector, placed at a set distance from the emitter, is related to the formation's electron density,[1] which itself is related to the formation's bulk density (
The tool itself initially consisted of a radioactive source and a single detector, but this configuration is susceptible to the effects of the drilling fluid.
) can be inferred by the formula Common values of matrix density
(in g/cm3) are: This method is the most reliable porosity indicator for sandstones and limestones because their density is well known.
[1] On the other hand, the density of clay minerals such as mudstone is highly variable, depending on depositional environment, overburden pressure, type of clay mineral and many other factors.
It can vary from 2.1 (montmorillonite) to 2.76 (chlorite) so this tool is not as useful for determining their porosity.
of 1 g/cm3 is appropriate where the water is fresh but highly saline water has a slightly higher density and lower values should be used for hydrocarbon reservoirs, depending on the hydrocarbon density and residual saturation.
In some applications hydrocarbons are indicated by the presence of abnormally high log porosities.