Pure Matrix GR, an Indicator of Rock Matrix Gamma Radioactivity and its Applications
- Hongtao Wang (Tongji University) | Rui Zhao (Anhui Provincial Bureau of Coal Geology)
- Document ID
- Society of Petrophysicists and Well-Log Analysts
- Publication Date
- August 2016
- Document Type
- Journal Paper
- 390 - 396
- 2016. Society of Petrophysicists & Well Log Analysts
- 5 in the last 30 days
- 99 since 2007
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The downhole gamma-ray (GR) log records the radioactivity of the surrounding formation, which is closely related to lithology. However, other influential factors, including porosity, density and background radioactivity, do exist and will obviously impact variations of the GR log in high-porosity formations. Therefore, a new concept, pure matrix GR (Nm), is proposed to independently indicate the radioactivity of the matrix. To obtain Nm, a theoretical relation between the GR log and these factors is suggested in our research. Then an approximate method to calculate Nm is put forward based on the assumptions that background radioactivity is constant and fluid radioactivity is negligible. Finally, the effectiveness of this method is demonstrated by numerical simulation and application in IODP Leg 349, Site U1431.
The downhole gamma-ray (GR) log measures the total gamma radiation originating from potassium-40 (40K) and isotopes of the uranium (U) and thorium (Th) series in formations (Ellis and Singer, 2007). Because shales generally have the highest level of radiation in sedimentary strata, the GR log is used to identify lithology qualitatively and to estimate the shale volume (Vsh) of the formation quantitatively. Other formations, which are rich in sylvite, feldspars and micas etc., also have high levels of radiation and can be recognized by the GR log, or with the help of spectral GR logs (Rider, 1996; Ellis and Singer, 2007). Among other applications, the GR log is also used as an effective indicator of palaeoenvironment and paleoclimate (Goldberg, 1997; Deconinck et al., 2003; Schnyder et al., 2006; Tian et al., 2008, Nejad et al, 2010; Paulissen and Luthi, 2011).
These direct applications of the GR log are based on the fact that it is mainly sensitive to the rock matrix rather than the fluid.
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