Spectral gamma-ray log

The spectral gamma-ray log measures natural radiation in rocks. Unlike the ordinary gamma-ray tool, which simply counts all gamma radiation photons, the spectral gamma-ray tool analyses the energy spectrum of the radiation it encounters. So, it can differentiate between photons of different energies. It's like seeing in colour instead of black and white.

Being able to see different energies, or 'colours', means we can differentiate between the radioactive decay of different elements. Elements decay by radiating energy, and the 'colour' of that energy is characteristic of that element (actually, of each isotope). So, we can tell by looking at the energy of a photon if we are seeing a potassium atom (40K) or a uranium atom (238U) decay.

In fact, all sorts of radioisotopes occur naturally in the earth. By far the most abundant are potassium 40K, thorium 232Th and uranium 238U. Of these, potassium is the most abundant in sedimentary rocks, but thorium and uranium are present in small quantities.

Potassium 40K decays to argon and calcium with some γ-emission at 1.46 MeV. However, most of the decay in the 232Th and 238U decay series occurs by α- and β- particle decay, which are not measured by the gamma-ray tool. The tool in fact measures γ-radiation from the decay of thallium 208Tl in the 232Th series, and from bismuth 214Bi in the 238U series. The spectral gamma-ray tool must be calibrated to known samples to give concentrations of 232Th and 238U from its readings. In addition to the spectral measurements, a total count is also acquired.

The concentrations of the three elements are estimated from the spectral measure- ments. The concentration of potassium is usually measured in percent (%) or per mil (‰), but sometimes in kilograms per tonne, which is equivalent to per mil. The other two elements are measured in parts per million (ppm).

Read more about the spectral gamma log at Natural Resources Canada’s website, http://gsc.nrcan.gc.ca/borehole/gamma_e.php.

Here is the gamma-ray spectrum from a single sample from 509.36 m at ODP Site 1201. The final spectrum (heavy black line) is shown after removing the Spectral gamma-ray logs background spectrum (gray region) and applying a three-point mean boxcar filter. The thin black line shows the raw spectrum. Dotted lines mark the interval boundaries defined by Peter Blum (an ODP scientist at Texas A&M). Prominent energy peaks relating to certain elements are identified at the top of the figure. The inset shows the spectrum for energies >1500 KeV at a 5× expanded scale:

Quality
The main issues affecting the quality of the logs are too calibration and drilling mud composition (Schlumberger 2003). As mentioned above, the tool relies on calibration. The measurement device consists of an NaI crystal and a photomultiplier. Both of these components are very sensitive to temperature, so calibration is especially important when the temperature of the tool is changing often (for example, in winter).

Drilling mud containing KCl (to improve borehole stability) increases the apparent potassium content of the formation, while barite acts as a gamma-ray absorber and reduces the count rates, especially in the low energies (potassium).

One of the key quality control indicators is negative readings on the uranium log (Schlumberger representative, pers. comm.). A few negative values are normal, but many zero-crossings may indicate that the tool was improperly calibrated. It is important to quality-control all of the logs, for bad readings and pick-up effects, before doing any quantitative work with them.

Interpretation
Most interpretations of spectral-gamma ray logs focus on the relationships between the three elemental concentrations. In particular, Th/K and Th/U are often used for petrophysical interpretation and log correlation. In calculating these ratios, Schlumberger use the following cut-offs: if uranium < 0.5 then uranium = 0.5; if potassium < 0.004 then potassium = 0.001 (Schlumberger 2003).

The following paragraphs summarize some examples from the literature.

According to the ODP’s Guide to Logging, high K values may be caused by the presence of potassium feldspars or micas. Glauconite usually produces a very distinctive, almost diagnostic spike in the K log. High Th values may be associated with the presence of heavy minerals, particularly in channel sand deposits overlying an erosional unconformity. Increased Th values may also be associated with an increased input of terrigenous clays. Increases in U are frequently associated with the presence of organic matter. For example, particularly high U concentrations (>~5 ppm) and low Th/U ratios (<~2) occur in black shale deposits. In the Ocean Drilling Program, a correlation can often be observed between the U log and the total organic carbon values measured in the core.

The ODP guide goes on to say that gamma-ray data may also be used to help interpret the environment of deposition. Unconformities can result in the accumulation of phosphatic nodules, which may be evident in the spectral gamma log as an anomalous spike in U. Increased U values, and in particular low Th/U ratios, may also be associated with marine condensed sequences.

An interesting literature study by Doveton & Prensky (1992) points out that Th/K is an effective differentiator of clay minerals, especially in conjunction with the photoelectric cross-section tool (this measures the photoelectric effect, in turn indicative of atomic number, and thus iron content). It also mentions that Th/K discriminates well between potassium-feldspar and clay minerals (such as illite), since both have high potassium content, but only the clay is rich in thorium as well.

The same source highlights the use of Th/U as a redox indicator. Thorium is insoluble, but uranium is soluble in its oxidized states (its lower valency state is insoluble). Therefore, low Th/U values (especially less than 2) should indicate reducing conditions (such as at flooding surfaces), and high values (more than 7) indicate uranium depletion and probable leaching (indicating, say, subaerial exposure or groundwater flushing).

Hampson et al (2005) turn instead to Th/K ratios in their study of fluvial stratigraphy. They found high concentrations of Th (> 3 ppm) on sequence boundaries in incised valleys and fluvial channels. They also found high Th/K (> 17), indicating leached potassium, in interfluvial palaeosols. They warn that similarly elevated Th/K ratios could be found in palaeosols under coal seams (where potassium is leached) or in Th-enriched mineral lags in shallow-marine sediments.

Flooding surfaces may also have their own signatures in spectral gamma-ray logs. Ruffell et al (2004) found low Th/K and Th/U at flooding surfaces in their study of a variety of rock types. They warn of the danger of confusing changes in sediment source, or fault-related mineralization, with more genetic geochemical and sedimentological effects.

In their study of marine carbonates, Ehrenberg & Svånå (2001) found that peaks in Th and K (and therefore presumably high Th/U) were associated with major transgressive surfaces. Peaks in U only, and thus peaks with low Th/U, were characteristic of minor flooding surfaces.

Demonstrating the variability of spectral gamma-ray responses, Van Bukem et al (1992) found that a peak in the Th/K ratio marked a condensed sequence (probable maximum flooding surface). Conversely, a sharp fall in the Th/K ratio followed by high U values coincided with a major transgression. In general though, they noted that the distribution of Th, K and U was generally highly correlated with the total radioactivity in the mudstones they studied.

Macfarlane et al (1988) give some interesting examples of ways to plot spectral gamma-ray data. For example, the plot below shows the Th/K ratio curve with a feldspar > mica > illite > smectite > kaolinite scale, and the Th/U plot with a fixed- U to leached-U scale.

Calculating Vshale
As for ordinary gamma-ray curves, it is possible to calculate Vsh from spectral gamma-ray data. The equation is analogous to that used for gamma-ray, but uses the CGR curve:

x = (CGRzone – CGRclean) / (CGRshale – CGRclean)

In many circumstances, x can be used as Vsh. Alternatively, one of the following corrections can be optionally applied:

Vsh =(0.5x)/(1.5–x) Vsh = 1.7 – √(3.38 – (x + 0.7)2)