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-..ia_......~.~~.~-~..._-.._-_.. - .~..~_._`__-_.rl • • - TIIE GEO LUGIC:\L I\TF.RPRF.TdTlO\ OF RBLL LOGS - for this reason that the simple gamma ray log has been called the 'shale log', although modem thinking shows that it is quite insufficient to equate gamma ray emission with shale occurrence. Not all shales are radioactive, and all that is radioactive is not necessarily shale (Figure 7.1) -see Section 7.6. Principal uses The gamma ray log is still principally used quantitatively Table 7.1(a) Principal uses of the gamma ray log. to derive shale volume. Qualitatively, in i[s simple (ortn, it can be used to cortelate, to suggest facies and sequences and, of course, to identify lithology (shalyness). The spec- tral gamma ray can be used additionally to derive a quantitative radioactive mineral volume and a more accurate shale volume. Qualitatively it can indicate dom- inantclay mineral types, give indications of depositional environment, indicate frac[ures and help to localize source rocks (Table 7.In,b). Discipline Used for KnoNing Quantitative Petrophysics Shale volume (Vsh) gamma ray (mac) gamma my (rein) Qualitative Geology Shale (shaliness) gamma ray (max) gamma ray (min) Litholoey typical radioactivity values Dlineral identification Mineral radioactivity Sedimentology Facies Clay/grain size relationship Sequence Stratigraphy Parasequence 8 condensed Clay/grain size & organic sequence identification matter/radioactivity relationships Stratigraphy correlation - Unconformity identification - Table 7.1(b) Principal uses of the spec«al gamma ray log. Distipline Used for Knowing Quantitative Pe«ophysics Shalt volume (VN) Th (max).Th (min) for pure shale Radioactive mineral Vin (Th), K (max), volume K (min) for shale Semi-quantitative Geology Dominant clay .and qualitative material Detrital clay mineral suite Sedimentology& Condensed ;ectiun Sequence recognition from S«atigraphy excess uranium Th. K, U content of individual clay minerals Radioactive content of individual clay minerals Normal U and Th content or Th/U ratio of shales Climatic changes? Th/K ratio changes in shale Geochemistry Marine source rack identification Rexrvoir geology Fracture detection Uranium contribution to radioactivity Uranium content of organic matter ~ 6S