Density Logs

By Luke Stoeckel with edits by Tom Sturman

Density logs are produced by a sonde which uses a radioactive source and a detector to emit gamma rays into the formation and detect their attenuation. Within the formation the gamma rays collide with the electrons present in the rock matrix and pore fluid, and with each collision, the gamma loses energy by a phenomenon known as Compton Scattering.
The scattered gamma rays eventually reach the detector (set at a fixed distance from the source) and are counted giving an indication of the density of the formation. It is the relationship that exists between the rock matrix density and the pore fluid density which determines the overall bulk density within the formation (Figure 2). The fluids that are present in the pore space, which can be water of hydrocarbons, often have a lower density compared to the density of the rock matrix and therefore rocks with higher amounts of fluid filled porosities yield lower bulk densities.


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Figure 2. Bulk density includes pore fluid and matrix (Holford, 2010)
Density logs can be used for quantitative and qualitative uses. Some qualitative uses are identifying lithology, shale age by compaction levels and unconformities, shale composition, mineral identification, evaporate identification, overpressure identification, fracture recognition and source rock evaluation. Quantitatively, the density log is used for porosity calculation and indirectly hydrocarbon density. The determination of porosity can be found using a relationship similar to the sonic density and whyllie equation (EQUATION BELOW);

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There are some characteristic lithology ranges and averages that occur in density logs (Figure 3), similar to the sonic calculations if an incorrect parameter is used the resultant density porosity is an incorrect representation.


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Figure 3. General given values for density of matrix and fluids present in a formation.
Due to overlapping density values between different lithologies, density alone is not diagnostic of a single lithology, however there is a general rule that:
ρCarbonate>ρSandstone>ρShale
For example, low densities in gas filled sands result in an incorrectly high porosity if an incorrect water density (ρfl) is used.