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l <br />I <br />~, <br /> <br />• I/~d nl E"0 ~ ~ ~Jb r 5 C ~, 6 ~• III I II I II I II (III III <br />THE GAMMA RAY AND SPECTRAL <br />GAMMA RAY LOG <br />7.1 Generalities <br />The log <br />The gamma ray log is a record of a formation's adioactiv'iry. <br />The radiation emanates from naturally-occumng uranium, <br />thorium and potassium (see below). The simple gamma ray <br />log gives the radioactivity of the three elements combined, <br />' SHALE <br />compact :•.~. ~.. <br />SANDSTONE ___ ~~:~: ~:~: ~:•:':~:•:' <br />porous :: , , , , , <br />LIMESTONE <br />ARKOSE :':~:'~'~~:':': <br />':~~::: <br />F <br /> <br />MICACEOUS . <br />..... <br />~ ~ "' <br />M~.•.•M.•.•.~ <br />SANDSTONE :•:•M:~:~:M• <br />SILT/FINE SAND ....::'~•:~:~ <br />KAOLINITE <br />Pure <br />MONT- clay <br />MORILLONITE <br />miner <br />ILLITE <br /> <br />ORGANIC RICH/ <br />BLACK SHALE <br />COAL <br />FINING~UP Shaly <br />SANDSTONE <br />Clean :- <br />SHALE <br />ve <br />while the spectral gamma ray log shows the amount of each <br />individual element contributing to this radioactivity. <br />The geological significance of radioactivity lies in the <br />distribu[ion of these three elements. Most rocks are <br />radioac[ive to some degree, igneous and metamorphic <br />rocks more so [han sediments. However, amongst the <br />sedimenLS, shales have by far the stronger[ radiation. It is <br />GAMMA RAY LOG SPECTRAL GAMMA RAY <br />(natural radioactivity) U ppm <br />Scale: API units Th <br />a0 Bo iID i60 100 <br />0 <br />glauconitic <br />Heavy mineral <br />bantls <br /> <br />SHALE BASE <br />t LINE <br />Figure ).1 The gamma ray log and spectral gamma ray log: soma typical responses. The gamma ray log shows natural radioactivity. <br />The spectral gamma ray log gives the abundances o(the naturally radioactive elements, thorium, Th and uranium, U in parts per <br />million (ppm) and potassium. K in rk. F-feldspar. Al =mica, • = glauconite. <br />67 <br />-c <br />~: <br />N. <br />i~ <br />-J- <br />~k'_ <br />u: <br />i ~`. <br />i y. <br />~+ <br />,. <br />I) <br />~. <br />1J <br />~p . <br />n <br />i <br />