--> --> Correlative Study With Raman Microscopy and Optical Petrography for Investigation of Organic Matter Within Mudstones

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Correlative Study With Raman Microscopy and Optical Petrography for Investigation of Organic Matter Within Mudstones


Accurate quantification and differentiation of organic matter in sedimentary rocks is integral to obtain a complete understanding of a potential source or unconventional reservoir. Inaccurate estimates of production potential can be caused by misidentification of solid bitumen and overestimation of kerogen volumes. Raman microscopy has emerged as a tool with high potential for non-destructive, objective evaluation of carbon species, including organic matter in rocks. High spatial resolution (1-µm) and the ability to identify components based on molecular fingerprint makes Raman microscopy well suited to analysis of organic matter. Here, we present the results of classical optical petrography and Raman microscopy analyses of a region of interest (ROI) in a thin section of Paleozoic siliceous mudstone. Preliminary analysis of Raman spectral map data identified quartz, pyrite and marcasite, as well as two apparent organic carbon phases: one concentrated in wavy lenses aligned with bedding, and the other dispersed within radiolarian fossils and the surrounding matrix. Optical petrography including bright-field and fluorescence imaging and bitumen reflectance measurements, concluded that the sample is in the mid-to-early oil window and that the two carbon phases are actually a single type of amorphous organic matter with varying fluorescence and concentration. Optical petrography also identified solid bitumen as low-to non-fluorescent accumulations occurring only in the transport network of the organic rich laminations, though identification within the ROI was difficult due to poor surface polish of organics. Outside of the ROI, bitumen was also observed in large spherical accumulations. In light of these results, a more rigorous study of the spectral data was undertaken, which included an analysis of principal components in the carbon spectrum between 1000 and 1800 cm-1 and an examination of the fluorescence background in raw spectra. Two carbon species were identified in the data by principal components analysis. However, in this case the second component correlated with the solid bitumen, as identified by optical petrography, and occurred in lower- to non-fluorescent regions within the lenses of amorphous organic matter. This case study illustrates how optical petrography can be used to guide interpretation of Raman microscopy data. The Raman spectra can be used to build a reference library for advanced characterization of organic matter in source rocks.