Gamma-Gamma Log: Compton Scattering, Bulk Density Measurement, and Porosity Determination in WCSB Reservoirs

A gamma-gamma log is a borehole measurement that uses an artificial source of gamma rays and a detector of gamma rays to determine the bulk density of the formation. The name is the older term for what the industry today simply calls the density log, and the two are synonymous; "gamma-gamma" describes the physics, gamma rays in from the source and gamma rays out to the detector, while "density log" describes what the measurement is used for. The tool carries a sealed chemical radioactive source, historically caesium-137 emitting gamma rays at 662 keV, mounted in a skid that is pressed against the borehole wall, with one or two scintillation detectors set at fixed spacings further up the skid. Gamma rays emitted into the formation collide with the electrons of the rock and pore fluids and lose energy through Compton scattering, a process in which a photon ricochets off an electron and continues with reduced energy and a changed direction. The denser the formation, that is, the more electrons per unit volume, the more scattering and absorption occurs and the fewer gamma rays survive to reach the detector. The count rate at the detector is therefore inversely related to electron density, and because the ratio of electrons to mass is nearly constant for most rock-forming elements, electron density is in turn very nearly proportional to true bulk density. The tool is calibrated against blocks of known density, conventionally to limestone equivalent, and reports bulk density in g/cm3 (equivalently in the old units of g/cc, and roughly 1 g/cm3 equals 62.4 lb/ft3). Bulk density is one of the most useful logs in the suite because it converts directly into porosity. Given the grain or matrix density of the rock and the density of the fluid filling the pores, the density porosity follows from a simple mass-balance equation: porosity equals the matrix density minus the bulk density, divided by the matrix density minus the fluid density. For a clean sandstone with a quartz matrix density of 2.65 g/cm3 and fresh water in the pores at 1.0 g/cm3, a measured bulk density of 2.32 g/cm3 implies about 20 percent porosity. Modern density tools also record the photoelectric factor, a low-energy absorption measurement that is a sensitive indicator of mineralogy and lithology, sharply distinguishing quartz, calcite, and dolomite. A spring-loaded caliper arm presses the skid against the wall and simultaneously measures borehole rugosity, allowing a correction for mud cake and washouts since any gap between the skid and the rock fills with low-density mud and biases the reading. In the Western Canadian Sedimentary Basin the density log is run on nearly every openhole logging job and is the primary porosity tool in the Cardium, Viking, and Montney, and crucially in carbonates such as the Leduc, Nisku, and Slave Point, where it is combined with the neutron log to identify gas zones and resolve complex lithology. The first commercial density tool was marketed by the Lane-Wells Company in the mid-1950s, and the measurement has been a cornerstone of formation evaluation ever since.

From Count Rate to Calibrated Bulk Density

The raw measurement is photon count rate, but turning that into a reliable bulk density requires correction. Dual-detector tools use a near and a far detector at different spacings; the near detector sees mostly the mud cake and shallow zone while the far detector reads deeper into the rock. By comparing the two, the processing applies a spine-and-ribs correction that removes the effect of low-density mud cake and minor standoff, producing a corrected bulk density and a density correction curve (delta-rho) that quality-control analysts watch closely. A delta-rho larger than about 0.10 to 0.15 g/cm3 signals a rough or washed-out hole where the density reading is suspect, prompting the petrophysicist to downweight that interval and lean on neutron or sonic porosity instead.

Density Porosity in WCSB Lithologies

Density porosity is only as good as the assumed matrix density, so lithology matters. Using a quartz matrix of 2.65 g/cm3 in a Viking sandstone is correct, but applying that same value to a Nisku dolomite with a true matrix density near 2.87 g/cm3 would badly underestimate porosity. WCSB petrophysicists therefore pair the density log with the photoelectric factor and neutron log to confirm lithology before computing porosity. In the Montney, a hybrid siltstone with variable dolomite and quartz content, the matrix density itself varies bed to bed, so analysts often build a variable-matrix model from the Pe and density-neutron combination rather than assuming a single grain density across the interval.

Related Terms

The gamma-gamma or density log shares its first word with the natural gamma ray log, but the two are different measurements: the gamma ray log reads natural radioactivity passively while the density log actively emits gamma rays from a source. Its primary output feeds the calculation of porosity, the pore-volume fraction that controls how much fluid a reservoir can hold. The density log is almost always interpreted alongside the neutron log, whose crossover with density is the standard gas indicator and lithology discriminator in formation evaluation.

Real-World WCSB Scenario: Gas Crossover in a Nisku Carbonate

A petrophysicist evaluating a vertical well near Drayton Valley, Alberta, logged a Nisku carbonate interval around 2,450 m with a density-neutron tool string. Over a 6 m zone the density log read a low bulk density of about 2.55 g/cm3 while the neutron porosity dropped sharply, producing a large crossover. Using a dolomite matrix of 2.87 g/cm3 confirmed by a Pe near 3.0, the apparent density porosity computed to roughly 11 percent, but the strong density-neutron crossover indicated gas rather than liquid filling those pores.

The operator perforated the crossover interval and the well flowed gas at a commercial rate, confirming the density log interpretation. Correcting the fluid density downward to account for gas in the flushed zone refined the porosity to about 13 percent, raising the gas-in-place estimate. The roughly 18,000 CAD logging program paid for itself many times over by identifying a pay zone that a porosity tool alone, without the gas-sensitive density-neutron combination, might have undervalued.