Basic well Logging Analysis – The Gamma Ray (GR) Log

1. Basic well Logging Analysis – The Gamma Ray (GR) Log Hsieh, Bieng-Zih Fall 2009

2. The gamma ray (GR) log • Gamma ray logs measure natural radioactivity in formations and because of this measurement, they can be used for identifying lithologiesand for correlating zones.

3. 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.

4. GR Log

5. 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.

6. 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.

7. 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.

8. 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.

9. 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.

10. 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= • Where: IGR = gamma ray index • GRlog = gamma ray leading of formation • GRmin = minimum gamma ray (clean sand) • GRmax = maximum gamma ray (shale)

11. Find Grmax, Grmin for sand formation (SAND-B) SAND-B GRmin GRmax

12. Chart for correction the gamma ray index (IGR ) to the volume of shale (VSH). • Curve 3 • unconsolidated, Tertiary rocks • Curve 2 • consolidated, older rocks • Curve 1 • Vsh=IGR

13. Practice – Vsh calculation from GR log • Step 1 • Setup shale base line • Step 2 • Analyze SAND intervals (formations)

14. Practice – Vsh calculation from GR log • Step 3 • Calculate IGR between 13680 -13720 ft

15. 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)

16. 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.

17. End of this chapter