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. 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.

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

Coal - left for a reason!

I have been working on a reserve study for last few weeks. The coal seam is of great quality, clean, and with about black and white separation. However the geology is extremely difficult and the seam is barely over 2 feet! The seam also splits with vanishings lower bench in places and to make miming worse, it has horrible “stack rock” (interlayerd sandstone and shale). It is so difficult to even make an isopatch map in such situations as you must be consistent in whatever you are mapping. If you are mapping the one bench, you can not map the full seam in other places and so on. So, I had to divide the entire reserve in various blocks and designate “full seam mining” and “main bench mining only”. Now I can contour! Well, I started defining the two different mining conditions based on the thickness of parting and finally end up choosing % recovery line as my boundary delineation. While talking this extreme complex nature of the coal seam, my supervisor pointed to the fact “whatever is left today, left for a reason”!

Coal mining started more than a century back. Way back then coal was everywhere in coal producing countries and regions. That time people would not require a geologist to mine coal. So, the easiest to get coals are all gone by strip mining. Then more than more coals are gone which are shallow and thick and had good roof conditions. So, after a century we are left with coals that were not easy to mine in old time. With coal price being really high these days mining companies are willing to spend more and more money to reach very difficult coal seams. Many times, people are also going back to the old mines to get pillars that were left during old mining job. So, expect more and more challenging work in coal world in future. We have been mining coal for more than a century and we will be mining more many long years with every year reaching a more difficult coal seam (well, if the price keeps rising!)

Please check the video. May take few seconds to load.