The Gamma log is used to record the naturally occurring radiation found in the surrounding borehole rocks from three primary isotopes: Potassium-40 (K), Thorium (Th), and Uranium (U) . Clays have the highest concentration of these radioactive isotopes; hence the Gamma log is also known as the clay log, or shale log. Other applications for the Gamma log include: estimating shale/clay content, identifying certain mineral deposits such as coal, potash, and uranium, stratigraphic correlations with other logs/seismic data and boundary estimation, and monitoring movement of injected radioactive materials.
Radioactivity and Rocks
The Gamma log is used to record the naturally occurring radiation found in the surrounding borehole rocks from three primary isotopes: Potassium-40 (K), Thorium (Th), and Uranium (U) .The radioactive isotope concentration of a rock is dependent on the rock’s minerology; this relationship allows for the identification of different lithologies and boundaries. Radioactive minerals such as micas, orthoclase feldspar, and zircon are present in all rocks but are more abundant components in igneous and metamorphic rocks. While few boreholes are drilled in igneous and metamorphic material, the byproducts produced by the weathering and alteration of these rocks makes up most clastic/detrital sedimentary rocks. As igneous and metamorphic rocks get broken down, altering into clay minerals, and transported into depositional basins, concentrations of radioactive isotopes increases in these areas. Thus, common clastic rocks such as shales and immature sandstones have a higher radioactive isotope count and can be distinguished by the Gamma log. This relationship resulted in the Gamma log also being known as the ‘shale log’, however, not all shales are radioactive (and vice versa) and one should not use the Gamma log to quantify shale content. For example, most shales contain only ~2% Potassium by weight while certain evaporites can contain anywhere from ~10-50% Potassium by weight. It’s also common for calcium carbonates to have radioactive spikes due to increased Uranium concentration in sea water and marine sediments.
The Gamma Log Tool
What is commonly known as the Gamma log, is actually differentiated into the 1) Gamma Ray log, and 2) the Spectral Gamma Ray log. The Gamma Ray log is the simpler of the two and only measures the total radioactive count rate in API units. Whereas the Spectral Gamma Ray log is able to differentiate between the different radioactive elements (K, Th, U) and outputs the abundance of each in parts per million (ppm). The Spectral Gamma Ray log is particularly useful when working in carbonate rocks; radioactivity caused by Th and K is linked to clay content while radioactivity caused by U can be associated with organic matter, diagenesis, dolomitisation, or unconformities to name a few.
Gamma Logging Tool
Whether Gamma Ray or Spectral Gamma ray, the logs are recorded using passive sensing techniques.
Simple Gamma Ray
The simple Gamma Ray tool is used to produce a Gamma Ray log; the tool is made up of a scintillation counter and a photomultiplier tube. The scintillation counter is typically a crystal of sodium iodide (NaI) which produces a flash of light when bombarded with Gamma rays. The flash of light is detected by the photomultiplier tube and converted into an electric signal that is then counted going down-hole. The count is then recorded in American Petroleum Institute (API) units, which is defined as “The gamma ray API unit is defined as 1/200 of the difference between the count rate recorded by a logging tool in the middle of the radioactive bed and that recorded in the middle of the nonradioactive bed” recorded within the calibration pit. A calibration facility for API units currently exists at the University of Houston and is the world standard for the simple Gamma Ray tool, however the validity of the calibration pit has been called into question in recent years.
Spectral Gamma Ray
The Spectral Gamma Ray tool is also made up of a scintillating counter and a photomultiplier; the crystal used for the scintillating counter used is much bigger, allowing for much better sensitivity. Instead of simply counting the number of flashes produced by the crystal, the photomultiplier also notes the intensity of the light produced. The light intensity is proportional to the energy levels of the gamma radiations of the different elements; this property is exploited to identify the specific radioactive isotopes present. The flash count is then related to the isotope abundances.
Properties such as density and size of the scintillating counter can affect the instrument sensitivity, however no counter is perfect and there is no counter that can accurately detect all gamma radiation. Other factors such as tool housing, casing, and borehole fluid can also affect the count rate and lead to slight variations between gamma logs taken in the same well.
Reading Gamma Logs
In most cases, a high radioactivity on the Gamma Ray log is typically associated with shales, however this is not always the case. When interpreting Gamma logs it’s important to always remember that what is displayed is either the count of radioactive isotopes, or the individual elemental contributions of each isotope. It’s incorrect to think of the gamma log as simply a shale log and all good interpreters should be familiar with how Potassium, Thorium, and Uranium behave to some degree. Starting with the most common, Potassium is quite active and can be readily found in silicate minerals, clays, and salts. Uranium is the least abundant and is native in felsic igneous rocks and organic matter. The parent rocks weathering easily at the surface, have resulted in significant Uranium concentrations in sea water and marine sediments. Lastly, Thorium is extremely stable and unlike Potassium and Uranium, insoluble. It commonly occurs in clays, terrestrial sediments and minerals.
Shales are rocks made up primarily of compacted clays, and to a lesser extent silt and mud. Potassium commonly makes up ~2% of most shales, however because it is so abundant and reactive, it may also occur as clays in immature sandstones, or in large quantities ~10-50% in certain evaporites. Potassium can only be moderately effective at identifying shales correctly. Thorium, however is insoluble and occurs in almost constant values in shales because of this property. A typical shale will have a Thorium range from 8-18 ppm, with Thorium contributing anywhere from 40-50% of the radioactivity. Unlike Potassium and Thorium, Uranium is not associated with clay minerals and thus occurs irregularly in shales. To summarize, a shale is typically associated with a high radioactivity made up primarily by Thorium, to a lesser extent Potassium, and a variable amount of Uranium.
Isotope Ratios and Additional Uses
Additionally, Potassium and Thorium can be used as indicators of clays in carbonate rocks, while the specific K-Th ratio is used to differentiate between common clay minerals. Uranium is used to indicate increased organic content and extreme conditions of depositions such as unconformities and sequence boundaries. The Th-U ration can be used to determine sedimentary processes and depositional environments, where oxidized continental deposits show ratios above 7 and marine deposits show ratios well below 7. The K-Th and Th-U ratios have been used to improve data resolution as they are more sensitive to horizontal changes.  It is also possible to interpret facies in detrital sediments using the Gamma log on the basis of grain size, but this is not common as there is no universal correlation between Gamma log shape and grain size distribution.
Gamma logs are widely available and have a unique value when doing correlation. The processed responsible for the accumulation of radioactive isotopes, such as climate and sea level change, are global and result in latterly continuous formations. Thus the radioactivity of rocks has minor lateral variation and can be approximated as constant horizontally  . This property provides a way to form well-ties and create fence diagrams by connecting similar peaks and tracing distinguishable horizons/lithologies across the study area. Additionally, Gamma logs can be correlated to seismic data to form seismic-to-well ties. This approach uses well-logs and well-ties to identify the same mappable horizons/units on seismic data by proving the seismic interpreter with mapped horizons/units on well-logs, lithologies, depth, and other properties. The seismic-to-well ties also serves to reduce ambiguity, ground-truth the seismic data, and provide parameters needed for forward modeling and synthetic seismogram generation. However, most Gamma logs are plagued by noise which obscures finer details, while translating Gamma Ray logs to absolute values is currently not possible. The K-Th and Th-U ratios has been used as a way to generate maps and improve data resolution as these ratios are more sensitive to horizontal changes, but this is only viable in areas with abundant data.
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