Natural radioactivity of reservoir rocks of oil and gas in East Baghdad field is due to not only their content of clay material, but also the existence of uranium located in the skeleton of rocks. In connection with this, for the shale content determinatio n of reservoir rocks, it is necessary to exclude the uranium contribution from the overall intensity of the gamma radiation. The paper presents the results of a research, using the data of spectral (standard) gamma ray logs -SGR, in which more accurate dete rmination of the shaliness (clay content) in the productive zones of the Zubair section of East Baghdad field (In Iraq) is proposed. With the help of a computer program, a formula that allows us to remove the effect of uranium, which affects the readings o f gamma ray logging, is obtained. Keywords: Uranium, clay content, gamma ray, CGR, SGR *Author for Correspondence E-mail: sudad.hameed@koyauniversity.org INTRODUCTION As indicators of the clay content and shale in the formation rocks, unsealed by a well, many types of logging can be used, in particular , the spontaneous potential (SP), density, neutron, and gamma ray logging. Gamma ray tools record naturally occurring gamma rays in the formations adjacent to the wellbore. This nuclear measurement indicates the radioacti ve content of the formation rocks. Effective in any environment, gamma ray tools are the standard devices that are used for the correlation of logs in cased and open holes. One of the most effective gamma ray log tools, based on the measurements of the natural radioactivity of the formation rocks, was proposed in 1980. Its readings are analysed from the point of view of determining the nature and quantity of radioactive eleme nts [1]. The tool can be considered as an improved indicator of clay content of the formation rocks. This logging device detects the presence of uranium U, thorium Th and potassium K in rocks, and subjected to determine the volumes of these radioactive min erals. After calculation of the content of uranium, thorium and potassi um by the method of weighted least squares the total gamma intensity of the radiation can be represented as a linear combination of the intensities of radiation of each of the se elemen ts. Since , the presence of clay affects the results of the determination of petrophysical parameters of formation rocks, such as saturation and porosity, it is necessary to introduce relevant corrections to the clay content [2]. Different bulk clay volumes for many intervals of Zubair section was determined using two different gamma ray logs data (spectral gamma ray log, SGR and corrected gamma ray, CGR). The CGR log is the log at which the effect of uranium is excluded from the overall gamma ray int ensity of the formation rocks. CALCULATION OF SHALE VOLUME (CLAY CONTENT) The interpretation data of gamma ray logs could be used as a single indicator of clay content of the formation rocks based on the following basic relationship [3]: 𝑉𝑠ℎ=𝐼𝛾−𝐼𝛾(min ) 𝐼𝛾(max )−𝐼𝛾(min ) (1) Where; 𝐼𝛾(min ), Minimum intensity of the radioactivity that corresponds to the clean formation rocks. Calculation of Clay in the Hydrocarbon Formation Rocks Al-Obaidi Sudad H. JoPET (2017) 1-3 © STM Journals 201 7. All Rights Reserved Page 2 𝐼𝛾(max ), Maximum intensity of the radioactivity that corresponds to the formation of net clays. 𝐼𝛾 , The intensity of the natural radioactivity of the formation rocks studied. In practice, there are two types of logging curves of gamma ray (GR) logs. The first corresponds to a standard curve of GR, where GR indications are represented as a linear combination of the radioactivity of potassium, thorium and uranium, in accordance with the following equation: 𝐼𝛾=𝐴𝐼𝑇𝐻+𝐵𝐼𝑈+𝐶𝐼𝐾 (2) Where , A, B and C are coefficients obtained from the calibration of the logging tool. This calibration takes into account the intensity of thorium radioactivity, I Th, the intensity of uranium radioactivity, I U and the intensity of potassium radioactivity, I K. The second type is the corrected gamma ray log CGR, where the proportion of uranium is excluded from the total gamma activities of the formation rocks . The CGR log represents a linear combination of gamma ray activities, only thorium and potassium, of the formation rocks: 𝐼𝛾𝑐𝑜𝑟𝑟 .=𝐴𝐼𝑇𝐻+𝐶𝐼𝐾 (3) Since , the existence of uranium is associated most with radioactive materials of the formation rocks other than those found in clay [1], such as organic sediments, then the data of CGR log can significantly reduce the distorting effect of slit and organics on th e assessment of the shale in the reservoir rocks. Therefore, CGR log is considered to be a good indicator of clay content. To determine the shale volume of the formation rocks using the data of the CGR log, Eq. (1) is used. Due to the fact that CGR log is not always available in well logging complex due to its high cost, the data of SGR log is undertaken for sand bodies in wells where CGR log is recorded (EB -55, EB -56, EB -77, EB -79), at the same sand bodies depths of wells (EB -18 and EB -15) where the CGR i s not recorded, are used in a regression program together with data of the CGR log data. This regression program has been developed to give a linear relationship to determine the values of 𝐼𝛾 corrected, using the data of standard GR log, as shown in Fi gure 1. Fig. 1: Dependence of I( γcorr)=f(Iγ) for Sand- stone I ntervals of Zubair Formation. In this Figure 1, the data of standard gamma ray log and corrected gamma ray log are plotted to obtain a single fundamental equation, which can be further applied to each interval of Zubair section; 𝐼𝛾𝑐𝑜𝑟𝑟 .=0.850 𝐼𝛾−1. 841 (4) Since the clay content coefficient that is calcu- lated according to any separately used well logging method , often turns out to be overes- timated, in practice, the clay content of the formation rocks is recommended to be calcu- lated by several methods of well logging [4]. Thus, for an amendment to the clay content calculated according to the data of well log- Journal of Petroleum Engineering & Technology Volume 7, Issue 1 ISSN: 2231 -1785 (Online), ISSN: 2321 -5178 (Print) JoPET (2017) 1-3 © STM Journals 201 7. All Rights Reserved Page 3 ging, which consequently affects the values of the petrophysical parameters of the formation rocks, the lowest estimated values of clay con- tent are used [5]. To determine the effectiveness of the proposed method ( Eq. (4)), the shale volumes of formation rocks calcul ated by the proposed method are compared with the values obtained , using other methods such as a cross - plot method of Neutron log -Density log; standard (spectral) gamma ray log (SGR) (Table 1). The comparison of the clay content volumes obtained using the proposed method with those values obtained by other methods, clearly shows that the proposed method gives the lowest values. Table 1: Comparison of Vsh Calculated by Proposed Method and Vsh Calculated by Cross -Plot (Neutron and Density Logs ) and SGR Log Methods . Depth (m) 𝑽𝒔𝒉,% Proposed Method 𝑽𝒔𝒉,% Spectral GR Log Method 𝑽𝒔𝒉,% Cross -Plot Method 3019 3022 3028 3045 3110 3115 3136 3155 3163 3169 3177 3181 3189 3197 3217 3258 3270 3289 3305 18 8 13.6 18 12 6 6 12 24 6 18 6 6 0 6 0 6 6 18 23.3 11.5 18.1 23.3 16.2 9.2 9.2 16.2 30.4 9.2 23.3 9.2 9.2 2.1 9.2 2.1 9.2 9.2 23.3 19 10 14 18.8 14 7 7.5 13 25.5 7.3 18 7.1 6 5 6.5 0 6.8 7.5 20 CONCLUSIONS As a result, it is f ound that the proposed method, in which the excluding of uranium effect from the overall intensity of the gamma radiation, gives a better estimation of clay content of formation rocks than the original method which takes into account the presence of the uranium component. As a consequence , this will lead to a more accurate determination for the petrophysical parameters of the formation rocks, such as saturation and porosity, and consequently to more accurate hydrocarbons reserve estimation.