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- Tllt GEU LOGICAL I\TtK YKt1A1 IU:\ OF BELL LOGJ - There are many examples of radio~e sandstones that may be quoted. The fine-grained mica sands of the North Sea (Nyberg eral., 1978) aze a typical, well-known example (Figure 7:17). Some marine sands contain glauconite and, if the concenvations are sufficiently high, render the sands radioactive (Figure 7.18). in fact radioactive sandstones are Far more common than real- ized. Arkoses are radioactive by definition (Table 7.16). Thorium, as previously described, is present in heavy- mineral suites. Placer silts (concentrations of heavy minerals) are frequently radioactive, producing a spiky aspect to the gamma ray log (Figure 7.19). However, this is the only case, and in general detrital grain mdioactivity is caused by potassium (Table 7.16). For sandstone reservoir studies, identifying clay as opposed to non-clay radioactive elements is important. 25 E r n 0 0 50 Neglecting radioactive stone intervals as being shales means missing essential reservoir. The fact [ha[ only potassium should be causing detrital mineral radioactivity in sandstones (e.g. Table 7.15) is used in the interpretation of the spectral gamma ray log to separate shale mdioactivity from detrital grain radioactivity (see befoty, 'Quantitative uses of the specVal gamma ray'). Radioactivity in carbanares Carbonates in their pure state are not radioactive and this aids their identification (Figure 7.1). Nonetheless, in certain facies, carbonates contain organic matter and this is frequently mdioactive due to uranium. This is certainly the case in the example given (Figure 7.20) and it is pro- posed (Hassan, 1973) that pure carbonate radioactivity is due only to uranium. Shaly carbonates will show the presence of potassium and thorium. (7 y O o J rT 0 n 0 H > W n a ~- quartz 65% le ltlspar 4% clay [5% medium sand (3S0µ) quartz 50% mica 20% clay 20% pyrite 5% v. line sand (1O0µ) quartz 56% leldspar 6% clay 76% garnet [0% coarse sand (1O00µ) N W m Figure 7.17 Radioactive sand, the 'mica sands' of the North Sea Jurassic. They ate fine-grained shallow marine sandstones with perhaps 209c clay but IS - 3090 mica, mainly muscovite, which causes the radioactiviry. Table 7.16 Radioactivity in sandstones. Species !\lineral Radioactive element Mica sand Muscovite/biotite 'eK Glauconitic sand Glauconite '0K Arkose Potassic feldspar i0K Placer silt Heavy minerals Th 80 Radioactivuy in evaporiles The most common evaporites, such as sa][ and anhydrite, give extremely and abnormally low values on the gamma ray log. However, the high radioactivity in some evapor- ates caused by potassium content has already been mentioned (Table 7.9). The log example shows a typical aspect of this ecaporite radioactivity. Frequently there are extreme contrasts between the potassium and non- potassium-bearing zones (Figure 7.21). Volumetrically, potassium rich evaporates are rare. : ~ :, 0 50 100 150