core gamma log, Oil & Gas Glossary

What Is a Core Gamma Log?

Core gamma log (also called a core gamma ray scan or core GR scan) is a continuous gamma ray measurement made on a retrieved conventional core in the laboratory by passing the core through a scintillation detector, producing a synthetic gamma ray curve that matches the depth interval cored and is used to depth-shift and orient the core relative to the wireline gamma ray log. The scan accounts for core recovery gaps, core expansion after retrieval, and depth discrepancies between driller's depth and logger's depth, enabling accurate assignment of core porosity and permeability measurements to their correct subsurface positions.

Key Takeaways

Core gamma scanning is performed at 1-cm intervals on whole core before cutting, and again on half-core after slabbing, providing millimeter-scale gamma ray resolution versus the 6-inch vertical resolution of most wireline tools.
Depth discrepancies between driller's depth and logger's depth commonly range from 10 to 30 feet in deep wells, meaning unshifted core data can be assigned to entirely the wrong stratigraphic interval.
The core gamma scan and the wireline gamma ray log display the same lithological signature because both measure natural radioactivity from potassium-40, uranium, and thorium in the formation.
A depth shift correction derived from correlating distinctive gamma ray peaks between the core scan and the wireline log is applied to all core measurements including porosity, permeability, fluid saturation, and grain density.
Without an accurate depth shift, log-core calibration errors corrupt petrophysical models, leading to systematic errors in net pay calculations and reserve estimates that can exceed 15 percent in heterogeneous reservoirs.

How Core Gamma Scanning Works

Core gamma scanning is conducted at the core laboratory immediately after the core arrives from the wellsite, before any destructive sampling or cutting. The whole core sections, still in their plastic liner or aluminum tubes, are passed at a controlled speed through a sodium iodide (NaI) or bismuth germanate (BGO) scintillation detector. The detector counts gamma photons emitted by naturally radioactive minerals in the core, producing a count rate versus depth profile at sampling intervals as fine as 1 cm. Modern core gamma scanners apply spectral decomposition to separate the potassium, uranium, and thorium contributions, generating a core equivalent of the wireline spectral gamma ray log. The resulting curve is digitized and exported for direct comparison with the wireline gamma ray log run in the same well.

After the whole core is scanned and the geologist has selected sampling intervals, the core is cut longitudinally into a working half and an archive half. The working half is scanned again to confirm that cutting has not altered the gamma signature, and to provide a reference for associating specific sample plugs with their depth positions. At this stage, confocal or hyperspectral imaging is often run simultaneously to build a color photomosaic matched to the gamma depth scale. The two gamma logs from whole-core and half-core scanning are nearly identical in peak position, with minor differences in amplitude resulting from the change in core geometry relative to the detector.

The depth shift procedure involves selecting five to ten distinctive gamma ray peaks or troughs that are unambiguous in both the core scan and the wireline log. Peaks associated with radioactive shale laminations, bentonite beds, or uranium-enriched carbonate horizons make the best tie points because they are narrow, sharp, and reproducible. The vertical offset between each core scan peak and its wireline counterpart is measured, and a piecewise depth shift function is constructed. This function is applied uniformly to all core measurements, effectively repositioning every plug sample and every mineralogy observation to the depth scale established by the wireline logging operation. When multiple core runs are available from the same well, each is shifted independently because depth discrepancies accumulate non-linearly with depth.

Fast Facts: Core Gamma Log

Measurement principle: Scintillation detection of natural gamma radiation from potassium-40, uranium, and thorium in the core
Scanning resolution: 1 cm standard; some laboratories offer 0.5 cm for thin-bedded reservoirs
Wireline GR resolution: Approximately 6 inches (15 cm) for standard tools; 2 inches (5 cm) for high-resolution tools
Typical depth discrepancy: 10 to 30 feet in wells deeper than 10,000 feet; up to 50 feet in ultra-deep or highly deviated wells
Detector types: Sodium iodide (NaI) for count rate only; bismuth germanate (BGO) or cesium iodide (CsI) for spectral decomposition
Scan speed: 2 to 5 cm per second for standard whole-core sections; slower for spectral scanning
Output formats: LAS 2.0 or LAS 3.0 digital files for direct import into petrophysical software
Standard reference: API RP 40 (Recommended Practices for Core Analysis) covers core handling and laboratory measurement procedures

Petrophysicist Tip:

Always apply the core gamma depth shift before presenting log-core crossplots to a reservoir characterization team. An unshifted porosity-versus-density crossplot from a heterogeneous sandstone can show scatter of 8 to 12 porosity units that disappears almost entirely once the shift is applied. If peak-to-peak correlation between the core GR scan and the wireline GR log leaves residual scatter, suspect core expansion: gas-charged cores can expand 2 to 6 percent after depressurization, stretching the depth scale of the lower core sections relative to the top.

Core gamma log is also referred to as:

Core gamma ray scan — the most common field term, emphasizing the laboratory scanning procedure rather than the log output
Core GR scan — abbreviated form used in laboratory reports and petrophysical software workflows
Synthetic core log — occasionally used to distinguish the laboratory-measured curve from the wireline log recorded in the borehole
Whole-core gamma scan — specifies that the measurement is made before the core is slabbed, distinguishing it from the half-core scan performed after cutting

Frequently Asked Questions About Core Gamma Logs

Why does the core gamma scan differ in amplitude from the wireline gamma ray log even after depth shifting?

Amplitude differences arise because the wireline tool measures gamma radiation through the borehole fluid, mudcake, and formation over a volume of roughly 30 liters, while the core scanner measures a 4-inch diameter cylinder in air. The wireline measurement is influenced by borehole enlargement, mud type, and tool standoff. The core scanner response depends on the diameter and density of the core section relative to the detector geometry. For correlation purposes, only the shape and peak positions matter, not absolute count rates. Amplitude normalization using a known formation interval can align the two curves if quantitative spectral analysis is required.

Can a core gamma log be used to determine the orientation of the core?

Yes, but not from the standard core gamma scan alone. Core orientation is typically established by combining the core gamma scan with oriented paleomagnetic measurements or by using a gyroscopic core orientation tool run inside the core barrel during drilling. The gamma scan provides the depth reference, while orientation tools record the rotational position of the core at the time of cutting. Some laboratories use oriented photography and structural dip measurements on slabbed core in conjunction with the gamma scan to reconstruct the true orientation of sedimentary structures and fractures relative to geographic north.

What happens if no wireline gamma ray log was run in a well that has conventional core?

Without a wireline gamma ray log, the core gamma scan still provides value as a detailed lithological strip log, but the depth shift calibration must rely on other reference logs such as density, neutron, or resistivity if these were recorded. In some cases, a memory-mode logging-while-drilling (LWD) gamma ray curve recorded during drilling can serve as the correlation reference. As a last resort, the geologist uses the core depth as recorded at the wellsite and accepts that absolute depth uncertainty may reach several tens of feet. The core scan still defines relative stratigraphy with centimeter-scale precision even when the absolute depth is uncertain.

Why Core Gamma Logs Matter in Oil and Gas

The core gamma log is the essential link between the physical core sample and the wellbore log suite. Every petrophysical calibration workflow, whether building a porosity transform, a permeability predictor, or a fluid saturation model, depends on correctly pairing core measurements with log measurements at the same subsurface depth. A 20-foot depth error in a 10-foot pay sand places core data entirely outside the zone of interest, rendering the calibration meaningless. The core gamma scan eliminates this error at minimal cost relative to the total expense of coring a well, which typically exceeds $500,000 per cored interval in deep offshore wells. Operators who skip the scan risk building reservoir models on misaligned data, a systematic error that does not become apparent until production performance diverges from simulation forecasts.

Core Gamma Log: Laboratory Gamma Ray Scanning for Core-to-Log Depth Correlation