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Th• Gamma Nay log/10 ~ TABLE 70-. CONVERSION FROM OLD UNITS TO API UNITS FOR SCHLUMBERGER GAMMA RAY lOGS API Equipment Old Unit Units Per Old Unif GNT-F or G Gamma Ray 1 Itgm Ra-eq/ton 16.5 GNT-J, K Gamma Ray, GLD-K 1 ltgm Ra-eq/ton 11.7 BOREHOLE CORRECTION CURVES The Gamma Ray deflection is a function nor only of the radioactivity and density of the Formations, bur also of hole conditions (hole diameter, mud weight, casing . , .) since the macerials interposed between the councer and the formations absorb gamma rays. As shown by Charts Por-7 and Por-8 the corrections tan be important in large holes and also in cased holes. lv[any logs were formerly calibrated such that a 358" sonde eccentric in a IO" ttncased borehole filled with 10 Ib/ gal non-radioactive mud would read directly the aue radio- aaiviry of the formations. For different borehole conditions, corrections have [o be made in order to determine the read- ings which would correspond to "standard" borehole condi- tions, hence the true radioactiviry.'t' Charts Por-7 and Por-8 are also applicable with good accuracy, to logs calibrated in this way. APPLICATIONS OF THE GAMMA RAY LOG 1. The Gamma Ray Log is particularly useful For defin- ing shale beds when the SP curve is rounded (in eery resistive formations) or flat (Rmr -^= R„~), or when the SP curve can- not be recorded (non-conductive muds -empty holes - cased holes). 2. The Gamma Ray Log reflects the proportion of shale and, in some regions, can be used quantitatively as an indi- caror of shale content (See Chapcer I-16. ) 3. The Gamma Ray Log is used for the detection and evaluation of radioactive minerals, such as potash or uranium ore. The radioactivity reading corrected For borehole effect (Chart Por-7 or Por-8) is practically proportional to the ]i 0 Content, approximately 15 API units for fir of f::0. This proportionalin~ is due to the Fur that potassium 40 emits mono-energetic gamma rays of L46 file\'. The Gamma Ray may also be used [o detect and ro evaluate uranium deposits, but in this case there is no simple proportionality between Gamma Ray deflections and the "richness" of the deposits. 4. The Gamma Ray Log can also be used Eor the delinea- tion of nun-radioactive minerals including coal beds.'' " S. The Gamma•Log is used for correlations in cased holes. The simultaneous recording of the Gamma Ray and of a Casing Collar Locator makes it possible to position perfor• acing guns very accurately. As compared with the corre- sponding open-hole log, the deflections on the cased-hole log are somewhat attenuated due to absorption of the gamma rays in the steel casing and cement. 6. The Gamma Ray Log is sometimes used in connec- tion with radioactive tracer operations.`31 7. Another specialized application is sometimes made possible by a phenomenon peculiar to old wells which have produced for long periods. The radiation level of zones be- lieeed to have experienced large-scale passage of formation n~arers has been observed to increase significantly, giving useful workover information. BIBLIOGRAPHY 1. Blanchard, A. and Dea~an, J. T.: "The Calibration of Gamma Ray Logs", Pet. Eag., Aug., 1953. ?. American Petroleum Institure: "Recommended Practice for S[andard Calibruion and Form for Nuclear Logs", API FP3i, 8ept., 1959. 3. Tixier, hl. P. and Alger, R. P.: "Log Evaluation of Non- Metallic Mineral Deposits", SPIt`'LA SynrPoriun4 1967. -1. Tittman, J.: "Radiation Logging: Physical Principles", in University of Kansas Petroleum Engineering Confer- ence, April 2 and 3, 1956. 5. "Schlumberger Production Log ]nrerprerarion'", Schlum- berger Limited, 1970. S9