Dual-Range FT-IR Mineralogy and the Analysis of Sedimentary Formations

Abstract

Fourier transform infrared spectroscopy (FT-IR) has been successfully used as an alternative to X-ray diffraction in the analysis of sedimentary minerals. Many common minerals exhibit unique absorbance spectra in the mid-IR range, which extends from 400 cm -1 to 4000 cm -1 . Several authors have shown that the mineral concentrations of an artificial mixture can be quantitatively derived from the mixture spectrum and the spectra of the mineral standards using the mid-IR. There are some limitations to the use of FT-IR that can result in errors in such an analysis, particularly for real sedimentary rock samples as opposed to laboratory mixtures. A new procedure was developed that combines mid-IR and far-IR into a single spectrum covering the range 300 cm -1 to 5200 cm -1 . The inclusion of the far-IR enables a better analysis of carbonate minerals since the metal-oxygen bond energies are included. Other improvements include compensation for variable light scattering by potassium bromide pellets, wavelet-based weighting of spectra, and use of end-member feldspar standards. Analysis of over 1000 oilfield samples shows that for many minerals, available library mineral standards have appreciably different FT-IR spectra than are found in oilfield samples. As a result, the mineral standard set has been expanded and now includes 49 mineral species of 29 distinct sedimentary minerals including 5 calcites, 4 dolomites, 5 kaolinites and 4 illites. The sample’s FT-IR spectrum is solved as a linear combination of the standard spectra, so the individual mineral species’ concentrations are available. The accuracy of the analyses is confirmed by comparing chemical concentrations measured on a split of the samples with concentrations reconstructed from the mineral concentrations. The new level of quantitative analysis has already resulted in a quantitative lithology analysis for total clay, carbonate, anhydrite and sand that is implemented for open- and cased-hole geochemical log analysis.