Natural radiation and GR logs
GAMMA RAY TOOLS
The natural gamma ray response curve is useful for several practical applications of log data :
Determine possible reservoir rock by quickly eliminating the depth intervals occupied by shale in either open or cased hole.
Determine the amount of shale in potential reservoir rock in either open or cased hole. 3- Correlate depth on gamma ray logs in other wells to determine reservoir structural position in either open or cased hole.
Identify radioactive deposits such as potash and uranium ore, bentonite marker beds, coal seams, and potential organic source beds.
Monitor movement of injected radioactive material.
The highest radioactivity usually (but not always) occurs in shales and clays because of their concentration of potassium, thorium, and/or uranium. Quartz crystals generally exhibit strongly bonded planes in all directions, crystallizing in pure form and prohibiting impurities from invading the crystal lattice. Micas and feldspars form a large part of the Earth's potassium and decompose rapidly to clay minerals. Clays are weakly bonded, very small in grain size, and have an open lattice that encourages inclusions of the radioactive elements during and after deposition.
Equipment
The GR sonde contains a detector to measure the gamma radiation originating in the volume of formation near the sonde. Scintillation counters are now generally used for this measurement. They are much more efficient than the Geiger-Mueller counters used in the past. Because of its higher efficiency, a scintillation counter need only be a few inches in length; therefore, good formation detail is obtained.
Calibration
The primary calibration standard for GR tools is the API test facility in Houston. A field calibration standard is used to normalize each tool to the API standard and the logs are calibrated in API units. The radioactivity in sedimentary formations generally range from a few API units in anhydrite or salt to 200 or more in shales.
Measurements
GR tools measurements have a vertical resolution of about 1 ft (30 cm), but true vertical resolution depends on logging speed.
GR instrumentation is very adaptable and can be run in combination with a large variety of other logging tools.
A major advantage of the gamma ray device is that it can be run in cased holes. Although the presence of steel casing will reduce gamma ray count rates by about 30%.
Natural Gamma Ray Spectroscopy Tools
GAMMA RAY TOOLS
Spectral analysis can identify the percentages of potassium and parts per million of thorium and uranium. Any of the three traces can serve as distinct correlative elements in certain circumstances. For example, high uranium values identify organic-rich shales that represent source beds. High potassium content is found in glauconitic sands, micaceous sands, concentrations of illite clays, algal limestones, etc. Thorium-rich marker beds such as bentonite can easily be identified with spectral gamma ray data
Measurement Principle
The NGS tool uses a sodium iodide scintillation detector contained in a pressure housing which, during logging, is held against the borehole wall by a bow spring.
The NGS log provides a recording of the amounts (concentrations) of potassium, thorium, and uranium in the formation. The thorium and uranium concentrations are presented in parts per million (ppm) and the potassium concentration in percent (%).
In addition to the concentrations of the three individual radioactive elements, a total (standard) GR curve is recorded and presented in Track 1.
The total response is determined by a linear combination of the potassium, thorium, and uranium concentrations. This standard curve is expressed in API units. If desired, a “uranium free” measurement (CGR) can also be provided. It is simply the summation of gamma rays from thorium and potassium only.
To analyze the depositional environment using Spectral Gamma Ray (SGR) and PEF logs, we look at the relationship between radioactive elements and the chemical composition of the rock. Here is how you can use these logs for depositional system analysis:
1. Potassium (K) and Thorium (Th):Clay Mineralogy & Terrestrial Input
The Th/K Ratio: This is the most powerful tool for detecting changes in depositional energy and source material.
*High Th/K ratio: Usually indicates kaolinite or aluminum-rich clays, often found in high-energy, terrestrial, or continental settings (like fluvial channels or floodplains) where weathering is intense.
*Low Th/K ratio: Suggests illite or feldspar, common in marine or deltaic settings where chemical weathering is less dominant.
- Thorium (Th): Since Thorium is extremely stable and does not dissolve in water, it stays with the sediment. High Thorium concentrations usually point to a continental/proximal source (closer to the land).
2. Uranium (U): Redox Conditions & Marine Influence
- Oxic vs. Anoxic Environments: Uranium is soluble in oxygen-rich water but precipitates in low-oxygen (anoxic) conditions.
*High Uranium (with high GR): Indicates an anoxic marine environment (like a deep marine black shale or a stagnant lagoon). Organic-rich "hot shales" are classic markers of deep-sea transgression.
*Low Uranium: Generally suggests well-oxygenated, shallow-water or high-energy environments where organic matter couldn't accumulate.
3. PEF (Photoelectric Factor): Matrix and Purity
PEF helps confirm the lithology that hosts the depositional markers.
PEF ~1.8 (Sandstone): If you see this in a sequence with fluctuating Th/K ratios, you might be looking at a deltaic or fluvial system.
PEF ~5.0 (Limestone): Combined with high Uranium, this suggests a marine carbonate shelf.
PEF ~3.1 (Dolomite): Might indicate a restricted marine environment or tidal flat where evaporation and mineral replacement occurred.
4. Integrating the Data for "Facies" Identification
- Fluvial/Deltaic: Low Uranium, High Thorium, fluctuating Potassium, and PEF reflecting sand/shale sequences.
- Deep Marine Shales: Very high total Gamma Ray, dominated by a sharp increase in Uranium, with stable Th/K ratios.
- Carbonate Reefs/Platforms: Very low Gamma Ray (Low Th and K), but potentially high Uranium if the reef was associated with organic matter, and a PEF of 5.0.
By plotting Th/K vs. Th/U*, you can create a "Cross-plot" that specifically identifies the clay type and the oxidation state of the environment, which are the "fingerprints" of the depositional system.