Log analysis for unconventionals
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Shale volume estimation
The estimation of the shale volume in the zone of interest can be performed in the available wells using the normalized version of the Gamma-Ray log (GRn), and using both neutron and density porosity logs. Following the concept that the increase in radioactivity of the organic-rich shales is related to their organic matter content, the GR and spectral GR responses need to be corrected for uranium before estimating clay content. This element forms compounds that sorbs to clays and organic material in both cases where their depositional environment is anoxic marine or oxidizing lacustrine (Ahmed and Meehan, 2016). In case spectral GR data are not available for a particular well (Well A), its correction for uranium should be conducted by means of a linear empirical relationship constructed using both, the corrected spectral GR of another well in the area of interest (Well B) plotted against its original log (Figure XX). The following equation is used for the GR correction for uranium:
Where, GRc=corrected GR, GR=total GR, and U=uranium in ppm.
After correcting the GR for uranium to remove the effect of the organic matter, the normalization of the GR log is conducted as a method of reducing mud weight and hole size effects (Crain et al., 2014). The normalization process follows the assumption that all pure shales in an area have the same GR values, and that all clean sands have the same GR log reading using the following equation:
Where, GRn = normalized corrected GR in API units, GRmin = GR clean sand value to normalize to, GRmax = GR shale value to normalize to, GR = total GR, GRlow = GR clean sand value in the well/zone, GRhigh = GR shale value in the well/zone.
To calculate Vshale from GRn, the following methodology was applied: