Sulfur speciation in kerogen and bitumen from gas and oil shales

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.