Volume 28, Issue 12
  • ISSN: 0263-5046
  • E-ISSN: 1365-2397


The presence of mixed carbonate and siliciclastic rocks complicates the choice of appropriate petrophysical tools for shale volume estimation. In this case study, conventional uranium-corrected gamma-ray and spectral gamma-ray logs were initially used for shale volume estimation. The results, however, were not supported by the nuclear magnetic resonance (NMR) log. Significant discrepancies were found in the reservoir where a considerable shale volume was estimated from the gammaray logs. Scanning electron microscope (SEM) analysis verified that potassium is present in clay and as potassium feldspar. Knowing the proportion of minerals from SEM analysis allows calibration of the spectral gamma-ray log for the potassium content of the clay at selected depths. Calibrated spectral gamma-ray values at the selected depths and the uniformity of the spectral gamma-ray logs are used to correlate the spectral gamma-ray logs over the intervals between the selected depths. The corrected spectral gamma-ray logs were used to recalculate shale volume, and resulted in a significantly reduced estimate. Validation of the result with the NMR log confirms the SEM and gamma-ray calibrated shale volume estimates, and suggests that the NMR log is a valuable tool for shale volume evaluation.


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  • Article Type: Research Article
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