Kerogen thermal maturity and content of organic-rich mudrocks determined using stochastic linear regression models applied to diffuse reflectance IR Fourier transform spectroscopy (DRIFTS)

Abstract

A method is presented to estimate thermal maturity and concentration of sedimentary organic matter (kerogen) in organic-rich mudrocks using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The maturity estimate is based on stochastic regression model built on a diverse set of mudrocks containing type II kerogen and spanning a range of age, lithology, kerogen content, and maturity. The model accounts for variations in intensities of both aliphatic and aromatic CH absorption bands. It has been validated on over forty mudrock samples with vitrinite reflectance values from ∼0.3 to 3%Ro. The maturity estimate is accurate on average to better than 0.2%Ro up to vitrinite reflectance values of at least 3%Ro, on par with uncertainties associated with vitrinite reflectance measurements.Estimating the kerogen content of mudrocks using infrared (IR) spectroscopy is challenging because the absolute intensities of aliphatic and aromatic CH absorption bands in infrared spectra are influenced by both kerogen concentration and composition, and composition changes as a function of maturity and type. A scaling factor for type II kerogen standard spectra has been derived as a function of maturity, such that the maturity estimate from the model is used to apply the scaling necessary to estimate kerogen content. The resulting kerogen concentrations agree with reference concentrations within about 1.5wt% for marine mudrocks. Thus, this approach provides a rapid and integrated analysis of kerogen maturity and content, in addition to established mineral analysis.