Evaluating Molecular Evolution of Kerogen by Raman Spectroscopy: Correlation with Optical Microscopy and Rock-Eval Pyrolysis

Abstract

Vitrinite maturity and programmed pyrolysis are conventional methods to evaluate organic matter (OM) regarding its thermal maturity. Moreover, vitrinite reflectance analysis can be difficult if prepared samples have no primary vitrinite or dispersed widely. Raman spectroscopy is a nondestructive method that has been used in the last decade for maturity evaluation of organic matter by detecting structural transformations, however, it might suffer from fluorescence background in low mature samples. In this study, four samples of different maturities from both shale formations of Bakken (the upper and lower members) Formation were collected and analyzed with Rock-Eval (RE) and Raman spectroscopy. In the next step, portions of the same samples were then used for the isolation of kerogen and analyzed by Raman spectroscopy. Results showed that Raman spectroscopy, by detecting structural information of OM, could reflect thermal maturity parameters that were derived from programmed pyrolysis. Moreover, isolating kerogen will reduce the background noise (fluorescence) in the samples dramatically and yield a better spectrum. The study showed that thermal properties of OM could be precisely reflected in Raman signals.

Potential Fossilized Sulfide-Oxidizing Bacteria in the Upper Miocene Sulfur-Bearing Limestones From the Lorca Basin (SE Spain): Paleoenvironmental Implications

“…All the spectra display a relevant fluorescence and, excluding the spectra from the spar-calcite cement (Figure 5A, spectrum b), all the other spectra show two additional broad bands (∼1350–1370 cm -1 and ∼1580–1600 cm -1 ; Figure 5A, spectrum c) that are indicative of poorly organized carbonaceous material, possibly finely scattered because not visible under the microscope. The ∼1580–1600 cm -1 band (i.e., the ordered band, also called “graphite-like” G band) is generated by the sp2 bonds typical of the crystalline carbon, whereas the ∼1350–1370 cm -1 band corresponds to the disordered, i.e., amorphous, D band and it is related to poor symmetry in the crystalline structure (Kelemen and Fang, 2001; Khatibi et al, 2018). The coarse-grained sparry cement filling the prismatic and lenticular pseudomorphs after gypsum (Figure 4A, 5B) shows Raman peaks at 1085, 281, 154, and 711 cm -1 , which are typical of “pure” calcite (Figure 5A, spectrum a).…”

Structure Evolution of Organic Compounds in Shale Plays by Spectroscopy (1H & 13C−NMR, XPS, and FTIR) Analysis

“…It's important to note that selected kerogen samples were isolated using HCl and HF, and details of sample preparation and organic matter extraction procedure can be found in Abarghani et al [23], and Khatibi et al [24]. The relative proportion of different carbon types in the samples was quantified through curve fitting of the 13 C−NMR spectrum, which was conducted with the ratios of Gaussian to Lorentzian distribution; the full width at half−maximum (fwhm).…”

Molecular Weight Distribution of Kerogen with MALDI-TOF-MS

“…Selected kerogen samples were isolated using HCl and HF, and liquid state hydrocarbon was extracted before MALDI-TOF-MS experiment. Further details about these samples and conventional geochemical analysis, organic matter extraction and the Bakken Formation can be found in Khatibi et al [18], and Abarghani et al [19].…”