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<br />- TIIE G.~)1.\IA Ral~ ADD SPECTR.~h G9~I )11 R:11' LOGS - <br />Table 7.13 Thorium abundance in clay minerals. <br />(From Hassan er al.. 1976; Dresser Atlas. 1983). <br />Thorium ppm <br />Mineral (approximate average) <br />Bauxite 8-132 (42) Afore continental <br />Kaolinitz 18-26 <br />Illite-muscovite 6-22 <br />Smectite 10-24 ~ <br />Glauconitz 2-8 Afore marine <br />the clay grain sized fraction, thorium shows an affinity <br />for tertes[rial clay minerals. For example, it shows highzr <br />concentrations in kaolinites (of tertesttial origin) than <br />in glauconites (of marine origin) (Hassan et al., 1976; <br />Figure 7.1, Table 7.13). In the coarse grained szdiments, <br />thorium minerals may be found as sit[-sized heavy min- <br />eral concentrations or placer deposits (see 'sandstone <br />radioactivity' below). <br />Despite its lack of solubility, thorium is however, <br />widely and relatively evenly distributed in sedimznts. So <br />much so that in shales it is used as a base level from <br />which the relative abundance of the other radioactive <br />elements, especially uranium, is measured (Section 7.10). <br />7.6 Radioactivity of shales and clays <br />[n pe[roleum borehole logging the commonest natural <br />radioactivity (by volume) is found in shales (clays). A <br />high gamma ray value frequently means shale. A typical <br />shale analysed by a spectral gamma ray tool shows that <br />each of the three elements, U, Th, and K, is contributing <br />(Figure 7.13) and an analysis of shales in general shows <br />GAMMA RAY dPl O SPEC rRAL GAFKIA RAY <br /> <br />0 50 100 - nonum loom uranmmloon) oolassmm i <br /> o. a ix ~sio x. a a o~ 2 a <br /> s~aie <br />snare .aloe comoos~oon <br /> 10 PPm in. <br />GR= ]5 API <br /> t oom u. <br /> 23 K <br /> <br /> <br />o- <br />25 - <br />50 - <br />]9 <br />Figam 7.13 A typical shale interval analysed by a spectral <br />gamma ray tool, The log shows thz individual contributions o! <br />thorium, potassium and uranium to the overall radioactivity. <br />Table 7.14 Average radioactive mineral content and contribu- <br />tion to total shale radioactivity (this is only onz set of figures <br />among severatl. <br />'Average 'Range `contribution <br />content to total <br />radioaclivin~ 9a <br />Uranium 4 ppm ?ppm - 6 ppm 299c <br />Thorium I?ppm 8 ppm - 18 ppm ~124c <br />Potassium 2.090 2.09c - 3.59c 299c <br />'A1ycrs, K. pzrs. comm. <br />*using the average 6gurrs (column 2) <br />the relative con[tibution of each element to the overall <br />radioactivity (Table 7.14). <br />Sut the gamma ray log should not be used as a 'black <br />bo.x' shale indicator either qualitatively or quantitatively, <br />as is commonly the case. The behaviour of the individual <br />radioactive elements in clay minerals and clays in general <br />is so diffzrent, as the preceding geochemical descriptions <br />indicate, that there is a nezd for more detailed under- <br />standing. <br />Potassium is involved in the chemical make up of clay <br />mineral structure and, despite the varia[ions of this in <br />specific clay mineral species (Table 7.8), has a fairly con- <br />sistentcontent inmost shales, of around 29r - 3.591. This <br />is the cast since shales are generally a mix of several of <br />the clay mineral types. Potassium therefore is a moder- <br />ately good 'shale indicator'. However, potassium occurs <br />in detrital minerals such as feldspars as well as in clay <br />minerals, so [hat in sand-shale mixtures. potassium may <br />occur in both the shalzs and the sands and cannot alone be <br />used as a shale indicator and dzscriptor (see Section 7.9). <br />Uranium dis[ribution is very irregular as has bzen shown, <br />because its affinity is to secondary components and not <br />thz main the rock forming minerals. Thus. in the average <br />shale it may contribute only 109c - 309c of thz total <br />radioactivity (Table 7.14) but in certain cases this can <br />increase dramatically (e.g. Table 7.10, Figure 7.31). Since <br />its distribution is not related to clay volume, uranium is a <br />poor 'shale indicator'. For this reason, on spectral gamma <br />ray logs. a curve is plotted without the uraniurtt content <br />(the CGR) to give a better clay volume estimace (Section <br />7.4, Figure 7.6). <br />The behaviour of thorium in shales is not fully under- <br />stood. Experience shows tha[ despite iu varying content <br />in clay mineral species (Tablz 7.13), it has a constam <br />value in almost all naturally occurring shalzs. The average <br />value is about 12.0 ppm (ranee 8_18 ppm) for a typical <br />shale, contributing bztween 4090 - 50'ir of the overall <br />shale radioactivity (Table 7.14 and ref.). Considering <br />therefore the constant average value and thz high pzrcent- <br />age contrihution to thz overall radioactivity, thorium is a <br />very good 'shale indicator'. In mixtures of sand and shale, <br />thorium will occur onl}' in the shale fraction (except in <br />rare occurrences). <br />~~ <br />