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-THE GE~ <br />GAMMA RAY API <br />t) t>Q <br />m <br />C <br />0 <br />C y <br />A <br />~ L <br />C <br />A <br />N <br />(D <br />v <br />A <br />U <br />0 <br />5 <br />20m <br />35m <br />ICAL IiVTERPRETATION OF R"ELL LOC: <br />a <br />c <br />as <br />N <br />E m <br />m ~ '" <br />v `m <br />I L C d 0 <br />m m :` E U <br />$IZC <br />x <br />n0 <br />Er <br />(7 <br />¢I <br /> 0 <br /> <br /> U <br /> O <br /> 0 <br />. ~ <br />. ~ <br />. ~ <br />. O <br />• • • • > <br />• • • • • ~ <br />• m <br /> <br />I <br />. . ~ <br />(clay volume) <br />50 100% <br />¢V ! I ~ <br />sand silt clay <br />coarse tine <br />LITHOLOGY/GRAIN SIZE <br />A, LOGS B. GRAPHIC RELATIONSHIP (schematic) <br />Figure 7.25 Facies from the gamma ray log. (A) The changes in sandstone grain size aze reflected in changes in the gamma ray <br />value. This allows a facies to be suggested. (B) Graphic representation of the variation of grain size with gamma ray value. Here <br />it is expressed as a sVaight line but the relationship is very variable. It should parallel the clay volume change. <br />This method of indicating facies with the gamma ray <br />log, however, is not straightforward. The relationship <br />between grain size and shale content is very variable, as is <br />the relationship between shale volume and gamma ray <br />value (see 'Shale volume'). Empirically, if [he gamma ray <br />log shows a typical shape it can be taken as indicating <br />grain-size changes. A lack of shape is not evidence for <br />lack of grain-size choose since it cannot be interpreted <br />(Rider, 1990). <br />The facies scheme derived from the gamma ray log is <br />fully described in Chapter 14. <br />Correlation <br />The gamma ray log is one of the most frequently-used <br />logs for correlation. l[ has 'character'. is repeatable, is not <br />affected b}• depth, it gir•es some indication of litholog}' <br />and is simple (Figure 7:?6). Moreover, it is almost always <br />run and the sensitivity scales are alwa} s relatively similar. <br />Generally, because it is used for correlation, it is repro- <br />duced on the well completion log, the document used to <br />reassemble the essential drilling and geological data at <br />the end of a well (see Chapter 1 I ). <br />Besides its availability, the gamma ray log has inherent <br />34 <br />advantages for cortelation, especially when this concerns <br />shales. The gamma ray value of shale formations is often <br />variable, depending on the various amounts of clay, <br />minerals, carbonate and organic matter present. <br />Horizontally, at the same stratigraphic level, these various <br />elements tend to show only slight variability in the <br />complex mix, i.e., the depositional environment which <br />convols the mix is laterally persistent.The complexity <br />does not persist through time, as most abrupt changes are <br />vertical. There are changes, amongst others, in source and <br />age. Thus, the gamma ray log value in shales remains <br />constant laterally but changes vertically. These are ideal <br />characteristics for correlation. <br />Recently, correlation with the gamma ray log has taken <br />on a new significance. It is suggested that gamma ray <br />peaks in shale sequences represent condensed sections, <br />(maximum Flooding surfaces in sequence stratigraphic <br />parlance) which are effective time lines and should be <br />correlated. This subject is considered in detail later in the <br />book (Chapter IS) and in more detail in terms of the <br />gamma ray below (Section 7.10). <br />In sandstones, gamma ray log shapes are often used to <br />correlate. However, the shape is a facies characteristic <br /> <br />