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Comparison of Quantitative Mineral Analysis By X-Ray Diffraction and Fourier Transform Infrared Spectroscopy
Accurate description of mineralogy bears on oilfield operations and can be used to calibrate downhole logs and develop petrophysical models for oilfield exploration and development. X-ray diffraction (XRD) is the industry-standard for quantitative mineral analysis, but the accuracy of the method varies substantially due to differences in sample preparation, sample measurement, and data-analysis techniques. This paper reviews the alternative transmission Fourier transform infrared (FTIR) spectroscopy technique for quantitative analysis of sedimentary formations including recent analytical improvements, and examines the differences between XRD and FTIR mineralogy in several case studies. The FTIR procedure has been modified to address two challenges, namely cloudy pellets and the presence of adsorbed water in FTIR spectra. Quantitative mineral analysis by FTIR is enabled using a generalized least-squares inversion against a library of mineral standards with unique IR spectra. Mineralogy results from FTIR are compared with those obtained using XRD techniques commonly practiced by commercial XRD laboratories on over one hundred sedimentary samples spanning carbonate, sandstone and shale lithologies. Accuracy of the mineralogy is evaluated using whole-rock chemistry. Bulk major-element (Si, Ca, Al, K, Na, Mg, Fe, and S) concentrations are reconstructed from the mineralogy by assigning chemical compositions to each mineral, and are then compared against measured elemental concentrations obtained on identical splits of the samples using independent and validated chemical-analysis techniques. Conformance between the reconstructed and measured elemental concentrations is assessed by quantifying the difference between the two. For the sets of samples examined here, FTIR results are consistently better than those from XRD using this conformance criterion. The data indicate that FTIR can provide quantitative and accurate mineral analysis of sedimentary formations, which can be used, for example, to calibrate and validate quantitative petrophysical interpretations in oilfield operations.
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