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Application of Raman Spectroscopy in Understanding Organic Matter Heterogeneity and Maturation Rate

Abstract

Solid organic matter (OM) in sedimentary rocks produces petroleum and solid bitumen along with CO2 and water when it undergoes thermal maturation. The solid OM is a geomacromolecule, usually representing a mixture of various organisms with distinct biogenic origins and can have high heterogeneity in composition. Programmed pyrolysis is a method to reveal bulk geochemical characteristics of the dominant OM while detailed organic petrography is required to reveal information about the biogenic origin of contributing macerals. Despite advantages of programmed pyrolysis, it cannot provide information about the heterogeneity of chemical compositions present in the individual OM types. Therefore, employing other analytical techniques is necessary. In this study, we compared geochemical characteristics and Raman spectra of two sets of naturally and artificially matured Bakken source rock samples. A continuous Raman spectral map on solid bitumen particles was created from the artificially matured hydrous pyrolysis (72 hrs. and 280-360 oc) residues to show the systematic chemical at the microscale. Parameters derived from spectroscopic including band separation were plotted for both sets against thermal maturity to compare maturation rate/path for these two separate groups. The outcome showed that artificial maturation through hydrous pyrolysis does not follow the same trend as naturally-matured samples although both have similar solid bitumen reflectance values (%SBRo). Furthermore, Raman spectroscopy of solid bitumen from artificially matured samples indicated the heterogeneity of OM decreases as maturity increases. This represents an alteration in chemical structure towards more uniform compounds at higher maturity. This study may signify the potential of Raman spectroscopy as an alternative to the conventional (pseudo) Van Krevelen diagram, by revealing the underlying chemical changes of organic matter. Finally, observation by Raman spectroscopy of chemical alteration of OM during artificial maturation may assist in the proposal of improved pyrolysis protocols to better resemble natural geologic processes.