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- TIIE GA~1\IA RAY AYD SPECTRAh CA\I CIA RAY' LOCS - <br />1. NON-RADIOACTIVE ELEMENTS <br />50 <br />c <br />metlian grain size -+ 0.037 0.063 0.125 0.25 0.50 7.0 mm <br />SILT V. FINE FINE ME CSE <br />median grain size -+ <br />Figure 7.16 Radioactive elements in de«ital rocks. Typical sandstone composition shown against groin size for the Reindeer <br />Formation, Mackenzie Delta, Beaufort Sea. Radioactive elements vary with grain size. It is a deltaic sand of Lower Tertiary age. <br />(Redrawn from Nentwich and }'ore, 1982). <br />0.037 0.063 0.725 0.25 0 50 [.0 <br />SILT V. FINE FINE ME CSE <br />Table 7.15 Potassium content of some common tletrital materials (from Serta, 1979; <br />Edmundson er al„ 1979; Dresser Atlas, 1983; Schlumberger. 1985). <br />9o potassium Average Gamma ray value <br />Mineral species by weight 90 (APf) <br />n <br />Glauconite• <br />3.2-5.8 <br />4.5 <br />75'-90` <br /> Muscovite 7.9 - 9.8 9.8 140' - 270 <br /> Botite 6.2 - 10,1 8.7 90` - 275 <br /> Microline 10.9 - 16 Iti 220 - 280' <br />~ <br />v Orthoclase I I.8 - I4 14 220 - 280' <br />a <br />`Detrital or authigenic <br />'For Sin hole, 1.2g/em' mud. 3'fin Nal scintillator <br /> <br />t <br />7.8 pualitative use of the <br />simple gamma ra~~ log <br />Lithology <br />As a first indicator of lithology, the gamma ray log is <br />extremely useful as it suesests where shale may be <br />expected (Figure 7.1). Moreover, as shown above, the <br />higher the gamma ray value, the higher the percentage of <br />shale (Figure 7.15). But the log is only a firs[ indicator, <br />The radioactivity of some typical lithologies other than <br />shalt is now considered. This shows [hat ary lithology <br />indicated by the simple gamma ray log must be con- <br />firmed by other logs. <br />Radioactivity ojsandstones and <br />otherarenaceous rocks <br />Quartz, the principal component of the coarse-grained <br />detrital rocks, shows no radioactivity. Sandstones conse- <br />quentlyusually show low gamma ray values (Figure 7.1). <br />However, associated detrital minerals are radioactive. The <br />most common of these are feldspars. micas, heavy <br />minerals and lithic fragments (Figure 7.16). The first two <br />groups contain potassium (Table 7.15), the third thorium <br />(Table 7.12) and the last contains shale. These all cause <br />sandstones with high [o moderate gamma ray values. <br />79 <br />2. RADIOACTIVE ELEMENTS <br />