Sulfur speciation in kerogen and bitumen from gas and oil shales

• Published in: Organic geochemistry Vol. 68; pp. 5 - 12
• Main Authors: Pomerantz, Andrew E., Bake, Kyle D., Craddock, Paul R., Kurzenhauser, Kurt W., Kodalen, Brian G., Mitra-Kirtley, Sudipa, Bolin, Trudy B.
• Format: Journal Article
• Language: English
• Published: Oxford Elsevier Ltd 01.03.2014; Elsevier
• Subjects: Earth sciences; Earth, ocean, space; Exact sciences and technology; Hydrocarbons; Marine and continental quaternary; Sedimentary rocks; Surficial geology; shale; hydrocarbons; oxidation; recovery; speciation; sedimentary rocks; thermal evolution; bitumens; kerogen; organic materials; chemical composition; flow; clastic rocks; oil shale; sulfides
• ISSN: 0146-6380; 1873-5290
• DOI: 10.1016/j.orggeochem.2013.12.011

•We find marked differences in sulfur speciation of kerogen and bitumen from shales.•Kerogen is dominated by reduced sulfur, bitumen more abundant in oxidized sulfur.•Propose that bitumen can be generated by oxidation during kerogen cracking.•Composition of bitumen suggests it could act as a naturally occurring surfactant.•Developed a sample preparation allowing quantitative sulfur speciation in kerogen. The chemical and physical structure of immobile organic matter partially controls both the thermal evolution of organic rich shales and hydrocarbon production from these unconventional fossil fuel resources. This organic matter is typically classified into two fractions: kerogen, which is defined as insoluble in organic solvent and bitumen, which is defined as soluble. Kerogen and bitumen are complex materials that are not yet completely characterized and often considered to be compositionally similar except for molecular weight. Here we present a novel method for measuring sulfur speciation in kerogen and we report measured sulfur speciations of kerogen and bitumen from three shales. We observe a general trend of dissimilarity between kerogen and bitumen, with kerogen being dominated by non-polar sulfur forms (such as elemental, sulfide and thiophene) while bitumen is more abundant in polar sulfur forms (sulfoxide). We propose that this difference in sulfur speciation results from a mechanism involving oxidation of non-polar sulfur forms in kerogen during bitumen generation. Additionally, the measured chemical composition of bitumen suggests that it could act as a naturally occurring surfactant, impacting fluid flow and therefore the feasibility of economic hydrocarbon recovery from shales.