Downhole logging tools

Gamma-ray logs, which are acquired with every tool string, are normally used to depth match all of the logs obtained in any one hole. The gamma ray from the Triple Combo is normally used as the base curve, and the gamma ray logs from all the other tool strings are manually or automatically matched to it. The depth shift applied to each gamma ray curve is propagated to all other logs acquired by that tool string.

Gamma ray data can also be used for core-log integration, by correlating the natural gamma results from the Whole Core Multisensor Track (WC-MST) with the log curves. Furthermore, because the gamma ray log responds principally to fluctuations in the formation mineralogy, rather than physical properties such as lithification, 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 gamma ray data 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 the Th log are frequently used to derive a "shale volume" (Ellis, 1987 and Rider, 1996). In addition, the shape of the 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 a coarsening-upward sequence (Serra and Sulpice, 1975). These methods, however, 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 sized detrital 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 in particular low Th/U ratios, may also be associated with marine condensed sequences (Myers and 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 and regressive intervals.

The concentrations of the three main radioactive elements in the formation can often be used 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 be associated with an increased input of terrigenous clays (Hassan et al., 1976) (Figure 2).



Figure 2: Spectral gamma-ray data from Hole 1124C, ODP Leg 181, 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 (<~2) occur in black shale deposits (Adams and Weaver, 1958). In ODProgram, a correlation can often be observed between the U log and the total organic carbon values measured in the core (Figure 3).

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Figure 3: Spectral gamma-ray data from Hole 1172D, ODP Leg 189, 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 or micas (Humphreys and Lott, 1990, and 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, ODP Leg 189, 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 (Brewer et al. 1992). An example of this can be seen in ODP Hole 896A, ODP Leg 148where the lowest K values occur in relatively impermeable massive flows, 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 spectral gamma-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.