Gamma ray logging


by Tom Sturman, with edits by Luke Stoeckel

The gamma ray log is a continuous measure of a formation’s radioactivity; it measures the naturally occurring radiation flowing from the formation using radioactive decay from uranium, thorium and potassium as the source. Gamma ray logging is a spontaneous wireline log and can be separated into two subcategories; the simple gamma ray log (GR) and the spectral gamma ray log (SGR) (Figure 8, below) below. The simple gamma ray shows the combined radiation of all three elements, whilst the spectral gamma ray distinguishes between each and shows them as separate logs on the same graph; thorium and uranium in parts per million (ppm) and potassium in parts per hundred (%).
gamma_ray_log_plot,_simple_and_spectral.JPG
[Figure 8: Some typical responses of the Gamma Ray and Spectral Gamma Ray. F=feldspar, M=mica, *=glauconite]
[RIDER, Malcolm; “The geological interpretation of well logs” Second edition, revised, 2002]


Geologically speaking the gamma ray log is used to calculate the volume of shale present (Equation 1, below), based upon the distribution of the three radioactive elements previously mentioned. All rocks are radioactive to some degree; igneous and metamorphic more so than sedimentary. Amongst the sediments, shales have the strongest radioactive response, however having a high gamma ray response alone is not enough to label a formation as shale, nor can we label all shale formations as having a high radioactive response. The shale volume calculation mentioned tends to give the maximum possible shale volume, and as such it should not be used as a definitive shale volume value. If we take the maximum average gamma ray log values as being 100% shale and the average lowest values to be 100% sand (0% shale), then a general scale of the shale percentage can be constructed, these two lines are usually marked onto the log, (Figure 9, below).

shale_vol_calc_using_gamma_ray.JPG
Equation 1:
Where;
V(sh)= shale volume (%),
GR(log)= GR value from log,
GR(max)=GR value from log at shale line,
GR(min)= GR value from log at sand line

gamma_ray_plot_with_sand_line_and_shale_line.JPG

[Figure 9: Sand line and shale line defined on simple gamma ray log]
[RIDER, Malcolm; “The geological interpretation of well logs” Second edition, revised, 2002]


Qualitatively speaking, the gamma ray log may be used to infer facies assemblages, lithology (to some degree), as well as depth correlation between wells. However, only its application to facies will be discussed here. The foundation of the facies interpretation is the connection between the shale content of a formation, and its grainsize. Whilst it is the shale content within a formation that the gamma ray log signifies, it displays these results in terms of grain size. For example fine-grained sands may be displayed as very shaly, a medium-grained sand as having medium shalyness, and a coarse grained sand as having low shale content. The changes in gamma ray values correspond to changes in the grain size of the formation (Figure 10, below). However it is important to note that this connection between shale content and grain size is not straight forward. In practical use; if the gamma ray log has a particular shape (e.g. Figure 10, below) then it can be interpreted as having the associated grain size changes. However, should the shape be non-existent, then this is not evidence of no grain size change. It should, at the very least, be considered as the tool not being able to identify any changes to grain size.

gamma_ray_plot_with_grain_size.JPG

[Figure 10: Changes in gamma ray values reflecting the changes in sandstone grain size]
[RIDER, Malcolm; “The geological interpretation of well logs” Second edition, revised, 2002]

As with all logging tools, the gamma ray tool has both advantages and disadvantages. In terms of the simple gamma ray tool, the main source of incorrect results comes from large scale (relative the borehole size) caves in the borehole. In these regions, an increased volume of drilling mud accumulates, thereby increasing the amount of Compton scattering, thusly reducing the gamma ray log values. To combat this potential inaccuracy, companies run calliper logs simultaneous with the gamma ray log and correlate the results. They also utilise charts published by mud logging companies which correct for borehole size, with mud property consideration. The use of a radioactive mud additive, KCL, is detected by the Potassium receptor in the tool. It results in an increase to the absolute gamma ray value. Often this is referred to as a baseline shift, because the mud volume is relatively constant throughout the hole, and as such there will be a constant increase in background radiation. Should the KCL enhanced mud enter the formation, the tools will read the formation as having a falsely higher radioactive content. In terms of the spectral gamma ray tool, the cave in effect of borehole damage is counted for by running the tool along the borehole wall. As with the simple gamma ray tool however, this is not the only downfall, there are also additives to mud which interfere with the results; barite and KCL are such additives. Their effects will vary depending on the design of the tool, as well as the algorithms used to counter them.