Sulfur Species in Source Rock Bitumen before and after Hydrous Pyrolysis Determined by X-ray Absorption Near-Edge Structure

Abstract

The sulfur speciation of source rock bitumen (chloroform-extractable organic matter in sedimentary rocks) was examined using sulfur K-edge X-ray absorption near-edge structure (XANES) spectroscopy for a suite of 11 source rocks from around the world. Sulfur speciation was determined for both the native bitumen in thermally immature rocks and the bitumen produced by thermal maturation of kerogen via hydrous pyrolysis (360 °C for 72 h) and retained within the rock matrix. In this study, the immature bitumens had higher sulfur concentrations than those extracted from samples after hydrous pyrolysis. In addition, dramatic and systematic evolution of the bitumen sulfur moiety distributions following artificial thermal maturation was observed consistently for all samples. Specifically, sulfoxide sulfur (sulfur double bonded to oxygen) is abundant in all immature bitumen samples but decreases substantially following hydrous pyrolysis. The loss in sulfoxide sulfur is associated with a relative increase in the fraction of thiophene sulfur (sulfur bonded to aromatic carbon) to the extent that thiophene is the dominant sulfur form in all post-pyrolysis bitumen samples. This suggests that sulfur moiety distributions might be used for estimating thermal maturity in source rocks based on the character of the extractable organic matter.

Universal Curves Describing the Chemical and Physical Evolution of Type II Kerogen during Thermal Maturation

“…Sulfur K-edge X-ray absorption near-edge structure (XANES) spectroscopy widely serves as a technique to quantitatively probe the speciation of sulfur in carbonaceous materials, including kerogen, petroleum, and petroleum derivatives. ,,,,,− The technique characterizes electronic transitions between 1s orbitals and vacant orbitals with 3p character, of which the energy is dependent upon the oxidation state of sulfur (transition energy increases with the oxidation state) and, hence, sulfur bonding. Measurements are performed at synchrotron light sources, owing to the high energies (∼2450–2500 eV) and energy resolution (∼1 eV) required for the identification and quantification of different sulfur species.…”

Thermal Transformation of Sulfur Species and Hydrogen Sulfide Formation in High-Pyrite-Containing Lacustrine Type-II Shale during Semiopen Pyrolysis

“…Previous studies have investigated its genesis and significance to the depositional environment and organic matter enrichment. , However, little attention has been paid to the thermal reactivity of pyritic sulfur; the organic sulfur form in the Chang 7 shale has not yet been characterized. X-ray photoelectron spectroscopy (XPS) and sulfur X-ray absorption near-edge structure (S-XANES) have been used to quantify the distribution of sulfur speciation and understand its evolution with thermal maturity in both natural conditions and pyrolysis systems. ,,− As S-XANES requires a synchrotron radiation source, XPS is usually the preferred method.…”

Sulfur Differentiation in Organic-Rich Shales and Carbonates via Open-System Programmed Pyrolysis and Oxidation: Insights into Fluid Souring and H2S Production in the Bakken Shale, United States

“…4,[10][11][12]14,17,18 Moreover, to this day, little is still known about the amount and forms of sulfur in most kerogens. 19,20 Even total organic sulfur in kerogens [TOS = S1-S + (S2 − S4)S] is not routinely determined, mainly because of common contamination by pyrite that makes the analysis and interpretations derived from it challenging. 7,10 The main reason for this lies in the technical challenges to separate organic from inorganic sulfur moieties prior to analysis.…”

“…This way, the measured total sulfur in the kerogen is corrected for the estimated pyritic sulfur to calculate the organic sulfur content. 8,14,20 However, all of these wet-chemistry-based techniques are cumbersome and often not totally satisfactory. 10 Alternative methods exist to overcome the pyrite contamination problem occurring with wet chemistry.…”

“…are still measured by traditional wet chemistry separation with combustion-based elemental analysis. 14,20 Although XANES and other sophisticated analytical tools can provide partial answers to the problem, sulfur can also be studied by customizing oxidation and pyrolysis ovens. Pioneering experiments in this area were performed by Madec and Espitalie, 28 where organic sulfur was differentiated from pyritic sulfur (i.e., present in sulfides like pyrite) through programmed pyrolysis in a customized Rock-Eval apparatus.…”