Applications of Gamma Ray Logs

7/27/2019 Applications of Gamma Ray Logs

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Applications of gamma ray logs

Depth correlations and core-log integration

Total gamma-ray log curves, which are acquired with every toolstring combination, arenormally used to depth match all of the logs obtained in any one hole. The HSGR logfrom the Triple Combo is used as the base curve, and the SGR logs from all the other

toolstrings are interactively matched to it. The depth shift applied to each SGR curve is

propagated to all other logs acquired by that toolstring.

Gamma ray data can also be used for core-log integration, by correlating the naturalgamma results from the whole core multisensor track (WC-MST) with the HSGR and

SGR curves. Furthermore, because the gamma ray log responds principally to

fluctuations in the formation's mineralogy, rather than physical properties such aslithification, it is particularly useful for making regional, inter-hole comparisons between

major lithostratigraphic units (Figure 1).

Figure 1: Regional correlation of major lithostratigraphic units, using total gamma raydata from Leg 189.

Identification of lithology, facies and depositional environment

Naturally radioactive elements tend to have a far greater concentration in shales than in

other sedimentary lithologies, and therefore the total gamma-ray log and, in particular,

the corrected gamma-ray log (HCGR and CGR) and the Th log are frequently used to

derive a "shale volume" (see Ellis 1987 and Rider 1996). In addition, the shape of the


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gamma log curve may be used to reconstruct downhole fluctuations in grain size, and

infer changes in sedimentary facies: the standard approach is to interpret bell shaped

gamma curves as a fining-upwards sequence and funnel shaped gamma curves as acoarsening-upward sequence (Serra & Sulpice 1975). However, these methods are only

likely to be of use in simple sandstone/shale formations, and are subject to error when a

significant proportion of the gamma ray radioactivity originates from the sand sizeddetrital fraction of the rock (see Heslop 1974 and Rider 1990).

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 inparticular low Th/U ratios, may also be associated with marine condensed sequences

(Myers & Wignall 1987). Doveton (1991) used Th/U ratios to estimate paleo-redox

conditions at the time of deposition, which he used to identify generally transgressive andregressive intervals.

Mineralogy / Geochemistry

The concentrations of the three main radioactive elements in the formation can often beused to give an indication of the mineralogy and/or geochemistry. For example, 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 beassociated with an increased input of terrigenous clays (Hassan et al. 1976) (Figure 2).

Figure 2: Spectral gamma-ray data from Hole 1124C, showing high Th values in a

mudstone unit between 420-430 mbsf.

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 (


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.

Figure 3: Spectral gamma-ray data from Hole 1172D, showing high U values in an

organic-bearing claystone unit between ~622-640 mbsf.

In sandstones, high K values may be caused by the presence of potassium feldspars ormicas (Humphreys & Lott 1990, Hurst 1990). Glauconite usually produces a very

distinctive, almost diagnostic spike in the K log (Figure 4).

Figure 4: Spectral gamma-ray data from Hole 1171D, showing high K values due to the

presence of glauconite.

In ocean floor volcanics, K can become significantly enriched in secondary alteration

minerals, which are typically found where the formation is more permeable and intense

fluid-rock interactions can occur (Breweret al. 1992). An example of this can be seen in

ODP Hole 896A, where the lowest K values occur in relatively impermeable massiveflows, whereas higher and more variable K concentrations can be correlated with the

more permeable pillow lavas and breccias (Brewer et al, 1998).

More quantitative attempts have been made to derive a mineralogy from the spectralgamma-ray log, which generally involve cross-plotting Th against K (Quirein 1982),

PEFL against K (Schlumberger 1991), or PEFL against Th/K (Schlumberger 1991).

However, the validity of these methods is questionable (Hurst 1990), and it is unlikely

that they are applicable in a wide variety of sedimentary environments.

Cyclostratigraphic analysis

Spectral gamma-ray data can also be used for cyclostratigraphic analysis of theformation, to help identify the frequency of paleoceanographic and/or climatic change

(Figure 5). Data acquired by the recently developed Lamont Multisensor Gamma ray
http://www.ldgo.columbia.edu/BRG/ODP/LOGGING/MANUAL/Pat/fig3.htmlhttp://www.ldgo.columbia.edu/BRG/ODP/LOGGING/MANUAL/Pat/fig4.htmlhttp://www.ldgo.columbia.edu/BRG/ODP/LOGGING/MANUAL/Pat/fig3.html


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Tool will be particularly valuable for time series analysis, due to its very high resolution

(~8 cm).

Figure 5: Spectral gamma-ray data (A) and preliminary spectral analysis (B and C) from

1170D. The power spectrum show the results of spectral analysis over the entire logged

section (B) and the interval where the Th and K data show the most pronounced cyclicity

(C).
http://www.ldgo.columbia.edu/BRG/ODP/LOGGING/MANUAL/Pat/fig5.html

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Applications of Gamma Ray Logs