AAPG Annual Convention and Exhibition

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Geochemical Screening Using Infrared Spectroscopy and Multivariate Calibration: Mudrock Mineralogy, Major Element Chemistry, and Organic Content


Basic characterization of mudrocks from unconventional petroleum plays involves assessment of inorganic (major minerals or oxides) and organic properties. Traditionally, analytical tools like X-ray fluorescence (XRF) spectroscopy, inductively-coupled plasma mass spectrometry and atomic emission spectroscopy (ICP-MS/AES), and X-ray diffraction (XRD) have been applied to inorganic characterization; while combustion (total organic carbon, TOC) and pyrolysis (Rock-Eval) methods have been employed to quantify organic properties like hydrocarbon-generating potential. One method that can provide information on both inorganic and organic properties is Fourier transform infrared (FTIR) spectroscopy. Many different modes of FTIR spectroscopy are available, including transmittance, diffuse reflectance, and attenuated total reflectance (ATR), each with their own specific advantages and disadvantages. The focus of this study is to determine the utility of using ATR-FTIR for characterizing geochemical properties of shales and mudrocks. ATR-FTIR can provide qualitative information on the organic matter content and mineralogy of powdered rock samples via a rapid, non-destructive measurement that can be used to guide sample selection for additional analyses. By combining ATR-FTIR with multivariate calibration methods, like partial least-squares regression (PLSR), semi-quantitative results on major element chemistry, bulk mineralogy, and organic geochemistry can be acquired. Utilizing the combination of ATR-FTIR and PLSR allows for extensive characterization of large numbers of outcrop, core, or cuttings samples in less time than any of the traditional methods listed previously using instrumentation that is less expensive and easier to use. A case study utilizing ATR-FTIR and PLSR of the Eocene Green River Formation in the Piceance and Uinta Basins will be presented, including an extensive calibration set of data collected on core and cuttings samples. Models developed using the Piceance-Uinta calibration set were applied to other samples from the Piceance and Greater Green River Basins as well as from other unconventional shale formations. The results of these tests illustrate the utility and limitations of estimating inorganic and organic properties of mudrocks using a combination of infrared spectral data and chemometric modeling.