Coal Geology

Gamma Ray Geophysical Logging in Coal Reserve Evaluation.

Gamma ray logging is one of the most commonly used geophysical logging techniques used in coal industry. It is used by professors, university students and those doing online degrees. Gamma ray logging is a method of using natural gamma radiation to characterize the rock or sediment in a borehole. K-40 isotope, associated with clay minerals, is the main source of gamma radiation in rocks. So, the maximum gamma radiation is typically associated with clay rich siltstones and mud-stones.

[ReviewAZON asin="B003YJFV0S" display="inlinepost"]In general, clean coal or a clean sandstone unit has very low gamma radiation as they have very low content of clay. Sometimes the partings in the coal many have clay impurity producing high gamma counts. Similarly, a “Sandy Shale” or “Sandstone with Shale Streaks” will have more gamma counts than a clean massive hard sandstone unit. In some of the marine clay units, apart from K-40, higher level of other radioactive minerals may present such as uranium or thorium.  In particular, shales usually emit more gamma rays than other common sedimentary rocks, such as sandstone, dolomite, or limestone potassium is a common component in their clay content, and because the cation exchange capacity of clay causes them to adsorb uranium and thorium.

Gamma radiation is also extremely useful in correlating coal seams and other rock units. For example, if you have two coal benches with variable parting in it and you are trying to tag two different benches separately, gamma radiation may give you a great support if two of the coal benches have two different gamma ray signatures.

In coal geology, gamma ray is also very useful in estimating “ash content” if you don’t have an laboratory analysis. There may be regions within a thick coal seam displaying high gamma content. Those zones are usually referred as “bones”, “shale” , “carbonaceous shale” , “boney coal” etc. depending on the nature of the gamma count.

Gamma ray is however can not be used alone to distinguish a coal seam. As we stated earlier, that if you have a clean coal just beneath clean sandstone, you can not define the boundary between the coal and sandstone just by using gamma counts as they will have similar gamma signatures. It is very common to use a density logs in such cases as coal will produce a very low density curve while sandstone will produce a high density curve.

Gamma Logs are usually calibrated using the American Petroleum Institute (API) Standards. However Gamma ray has relatively poor resolution in vertical direction. The gamma ray log, like other types of well logging, is done by lowering an instrument down the hole and recording gamma radiation at each depth.

Advantages of using gamma ray method:

1. Logging can be done through casing.
2. Performance not hindered by loss of fluid, air, or water.

However, the nature of the gamma logs are affected by the diameter of the borehole and the properties of the fluid filling the borehole and corrections may be required to use the gamma logs for calculating the thickness of a coal seam or other stratigraphic units.

References and resources:

1. Handbook of Practical Coal Geology by Larry Thomas
2. Gamma Ray Logging
3. Hydrologic interpretation of natural gamma logs

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