Quantitative analysis of kerogen content and mineralogy in shale cuttings by Diffuse Reflectance Infrared Fourier Transform Spectroscopy

摘要

Diffuse reflectance infrared Fourier transform spectroscopy(DRIFTS) has been used for qualitative investigation of hydrocarbons,source rocks,core and cuttings.However,in the past,quantitative analysis has been unreliable.In the DRIFTS technique,an infrared laser shines onto the material and a detector measures the reflected light intensity as a function of frequency.The reflected light frequency spectrum is related to characteristic vibrational modes of elemental bonds in the material.In some cases these modes have been linked to compounds or minerals. This paper demonstrates that quantitative analysis of kerogen,calcite,illite,kaolinite, quartz,dolomite and smectite is possible for cuttings obtained from a well drilled with oil-based mud in a formation in Texas.The cuttings mineral concentrations solved from DRIFTS spectra were compared with mineralogy determined by the highly accurate transmission dual-range Fourier transform infrared(DRFT-IR) technique developed in our laboratory and the concentrations are similar to within a few percent.The kerogen content from DRIFTS was compared to the organic carbon content determined by a LECO elemental analyzer and the agreement is better than ± 1 wt % kerogen. Our procedure removed the oil-based mud from the cuttings sample and then reduced the particle size.The samples were analyzed with a Bruker Alpha-R DRIFTS unit over the range of 375 – 4000 cm~(-1) .After converting the reflectance data to Kubelka-Munk units, the spectra were analyzed by assuming that the Beer-Lambert law holds at every wavelength.In this analysis,the mineralogy was determined from over 2500 simultaneous least squares solutions of the unknown absorbance as a linear combination of the mineral standard spectra.Mineral concentrations were evaluated with artificial mixtures of minerals and by comparison with the DRFT-IR and LECO measurements on well cuttings.