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- THE GEOLOGICAL <br />•~ <br />u <br />O <br />J <br />I) <br />INTERPRETATION OF WELL LOGS - <br />-~ ~ <br />J 0 50 GAMMA T00 RAY API I50 20 <br /> <br />DOLOMITE (argillaceous) <br /> <br /> <br /> <br />AY - <br />(saliferous) <br />CL <br />ANHYDRITE <br /> K K <br />POTASSIUM SALTS <br />(some halite) <br />K K <br />K K <br />K K ~_ <br /> <br /> <br />~-- <br />HALITE (salt) <br />.+. <br />ANHYDRITE <br />with tlolomite bands <br />0 <br />25 <br />c <br />N <br />N <br />C <br />V <br />L <br />50 <br />Figure 7.21 Potassium salts giving very high peaks of radioactivity in an evaporite sequence. (The lithology comes from an <br />interpretation of combined logs and cuttings). Permian, North Sea. <br />Coa! and organic rich shale <br />(source rock) radioactivity <br />The relationship between organic matter and uranium <br />enrichment is the basis for being able to identify organic <br />rich shales (source-rocks) using the gamma ray lag and is <br />discussed under Section 7.10 (Organic matter and source <br />rocks: uranium content). In practice, although high <br />gamma ray values often correspond to organic matter <br />rich, source-rock intervals (Figure 7:22), such intervals <br />do not always have a high gamma ray value (i.e. <br />Schmoker, 1981 and Section 7.10 below). <br />Coals have low gamma ray values (Figures 7.1, 7.23). <br />The contrast in [his response between pure coals and <br />organic shales is remarkable, especially when, in typical <br />% ORGANIC CARBON <br />0 Z ~ 6 B t0 <br />GAMMA RAY API <br />50 500 <br />cyclic deltaic sequences, a low gamma ray coal is imme- <br />diately overlain by a high gamma ray, organic-rich shale <br />(Figure 7.23). I[ appears that uranium, which is adsorbed <br />by organic maner to cause the high gamma ray values in <br />the organic rich shales, is not adsorbed by organic matter <br />in~ terrestrial swamps where no clay is present. Clay <br />seems to be a catalyst to the adsorption. Thus, pure coals <br />have the typical low gamma ray response while shaly <br />coals have a gamma ray value which depends on the shale <br />(or ash) content (Kays] and Christoffel, 1989). <br />Igneous and volcanic rock radioactivity <br />igneous rocks are no[ volumetrically important in pevo- <br />leum wells, but occur sufficiently frequently to be a <br />I50 300 <br />MATTER TYPE <br />E <br />saProoe~ 50% <br />saproPel 90% <br />sao~o0e~ 20-504 <br />cm~w~es <br />saorooe150-BOY <br />Figure 7.22 High organic carbon values and the total gamma ray giving good correlation, in this case due to uranium associated <br />with organic matter. <br />82 <br /> <br />