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BASIC WELL LOGGING ANALYSIS – THE GAMMA RAY (GR) LOG 1 Hsieh, Bieng-Zih Fall 2009 THE GAMMA RAY (GR) LOG Gamma ray logs measure natural radioactivity in formations and because of this measurement, they can be used for identifying lithologies and for correlating zones. 2 THE GAMMA RAY (GR) LOG As shale content increases, the gamma ray log response increases because of the concentration of radioactive material in shale. Shale-free sandstones and carbonates have low concentrations of radioactive material, and give low gamma ray readings. 3 GR LOG THE GAMMA RAY (GR) LOG (CONT.) However, clean sandstone (i.e. low shale content ) may also produce a high gamma ray response if the sandstone contains potassium feldspars (鉀長石), micas (雲母), glauconite (海綠石), or uranium-rich (高含量鈾的) waters. In zones where the geologist is aware of the presence of potassium feldspars, micas, or glauconite, a Spectralog can be run in addition to the gamma ray log. 5 THE GAMMA RAY (GR) LOG (CONT.) The Spectralog breaks the natural radioactive of a formation into the different types of radioactive material: (1) thorium (釷), (2) potassium (鉀), and (3) uranium (鈾). If a zone has a high potassium content coupled with a high gamma ray log response, the zone may not be shale. Instead, it could be a feldspathic (長石質的), glauconitic (海綠石質的), or micaceous (雲母質的) sandstone. 6 THE GAMMA RAY (GR) LOG (CONT.) Besides their use with identifying lithologies and correlating zones, gamma ray logs provide information for calculating the volume of shale in a sandstone or carbonate. 7 GR LOG CURVE The gamma ray log is recorded in track ＃1, usually with a caliper. Tracks ＃2 and ＃3 often contain either a porosity log or a resistivity log. 8 SHALE VOLUME (VSH) CALCULATION Because shale is more radioactive than sand or carbonate, gamma ray logs can be used to calculate volume of shale in porous reservoirs. The volume of shale can then be applied for analysis of shaly sands. 9 CALCULATION OF THE GAMMA RAY INDEX (IGR) Calculation of the gamma ray index is the first step needed to determine the volume of shale from a gamma ray log (the following formula from Schlumberger, 1974). IGR= GRlog GRmin GRmax GRmin Where: IGR = gamma ray index GRlog = gamma ray leading of formation GRmin = minimum gamma ray (clean sand) GRmax = maximum gamma ray (shale) 10 FIND GRMAX, GRMIN FOR SAND FORMATION (SAND-B) SAND-B GRmin GRmax 11 CHART FOR CORRECTION THE GAMMA RAY INDEX (IGR ) TO THE VOLUME OF SHALE (VSH). Curve 3 Curve 2 unconsolidated, Tertiary rocks consolidated, older rocks Curve 1 Vsh=IGR 12 PRACTICE – VSH CALCULATION FROM GR LOG Step 1 Setup shale base line Step 2 Analyze SAND intervals (formations) 13 PRACTICE – VSH CALCULATION FROM GR LOG Step 3 Calculate IGR between 13680 -13720 ft 14 PRACTICE – VSH CALCULATION FROM GR LOG Step 4 Calculate Vsh between 13680 -13720 ft Note: this formation is a kind of consolidated rocks (you should use curve 2) 15 REVIEW – GR LOG Gamma ray logs are lithology logs that measure the natural radioactivity of a formation. Because radioactivity material is concentrated in shale, shale has high gamma ray readings. Shale-free sandstones and carbonates, therefore, have low gamma ray readings. Gamma ray logs are used to: (1) identify lithologies; (2) correlate between formations; and (3) calculate volume of shale. 16 END OF THIS CHAPTER 17