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.

Natural radiation and GR logs

GAMMA RAY TOOLS

Calibration

Measurements

Natural Gamma Ray Spectroscopy Tools

GAMMA RAY TOOLS

Measurement Principle

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