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P tlIIJlO II ~I ,II` <br />dl l~ III II I n I i ' <br />..7il IIWC , W __ __- _'-girl r <br />- THE GA\1 YIA RAY AhD SPECTRAL GANNA RAY LOGS - <br />u <br /> <br />2 (~ <br />O <br />W CALI (in.l <br />I6 <br />6 <br />~ O ---- <br />- <br />____ <br />O = <br />~+ !- <br />N .~ GR IAPII <br /> 0 100 <br />~~ <br />~ <br />m <br />~- '. <br />_ <br />_- <br />~n ~ . <br /> ~ organc aM1ab y <br />coal <br />!~ II <br />~U I~ <br />I~ <br />V <br />~~ I' <br />V <br />_v_ <br />- <br />I <br />= , <br />1 organic aM1ala - <br />:ool r <br />U U <br />U ...::.~:~'.::: I <br />0 <br />Sm <br />~10m <br />15m E <br />s <br />o° <br />~20m <br />Figure 7.23 Gamma ray characteristics of coal (very low <br />values) and organic rich shale (eery high values) in a deltaic <br />sequence. <br />necessary element in the li[holoeic vocabulary. Both <br />uranium and thorium originate in the acid to intermediate <br />igneous rocks, but their distribution is very iregular since <br />they are associated with secondary minerals such as <br />apatite. Potassium is present, especially in the acid <br />igneous rocks, principally in the alkali (potassic) <br />feldspars. The net result is that basic igneous rocks have <br />low radioactivity, while the internediate and acid types <br />show progressively higher values (Keys, 1979; Sanyal et <br />al., 1980) (Table 7.17). <br />The example shows a typical basalt which may be con- <br />fusedwith sand (Figure 7.24). <br />Unconjormities <br />Unusually high gamma ray values often occur as narow, <br />isolated peaks. Considering the geochemistry of the <br />radioactive minerals, these peaks are generally associated <br />with uranium concentrations. As discussed (see 'Uranium' <br />above) uranium concentrations indicate extreme conditions <br />of deposition. Experience has shown that these conditions <br />frequently occur around unconformities where a long <br />passage of time is represented by little deposition. The <br />minerals associated may be uranium-enriched phosphates <br />or uranium-enriched organic matter (see also Chapter I5, <br />and Figure 14.19). <br />Facies arsd grairs si;,e <br />An interesting and fairly comprehensive scheme for facies <br />identification in detrital sediments (sand-shale) has been <br />developed using gamma ray log shapes. The basis For the <br />SILL <br />composition bask, <br />i.e. DYroaene antl <br />plagioclase <br />Figure 7.24 Low gamma ray values through a basic sill. <br />It may be confused with a sandstone interval. <br />Table 7.17 Radioactive elements in igneous and volcanic <br />rocks (from Serta, er ai., 1980; fieures approximate). <br />Rock type Th (ppm) U (ppm) Kr09D Typical <br />radioactivity <br />Acid <br />intrusive I-25 I-8 4. 11-2.00 <br /> High <br />Acid <br />extrusive 9-25 ~ ?-7 2.00-b.00 <br />Basic <br />inwsive 0.5-5 0.3-2 0.90-2.?0 <br /> Low <br />Basic <br />exwsive 0.5-10 0.2-i 1.40.2.50 <br />- Very low <br />Ultrabasic - 0.0001- 1.6 <br /> 0.03 <br />scheme is the relationship between grain size and shale <br />content. Il is shale content that the gamma ray log indicates, <br />but. it is interpreted in terms of goon size. For example, a <br />coarse-grained sand will have a very low shale content, a <br />medium-grained sand some shale, and afine-grained sand <br />may be very shalt'. The changes in grain size will he <br />followed by changes in gamma ray value (Figure 7 ~5). <br />83 <br />