Gwchimica d Cosmochimica Acla zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Vol. 55, pp. 801-814 C o p yrig ht 0 1991 Pe rg a m o n Pre ss pk. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Printed in U.S.A. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA 0016-7037/91/$3.00 + .OO Sulfur speciation in heavy petroleums: Information from X-ray absorption near-edge structure GEOFFREY S. WALDO,’ ROBERT M. K. CARISON,~J. MICHAELMOLDOWAN,’ KENNETH E. PETERS,’ and JAMESE. PENNER-HAHN”* ‘Department of Chemistry, University of Michigan, Ann Arbor, MI 48 109-1055, USA ‘Chevron Oil Field Research Company, PO Box 1627, Richmond, CA 94802-0627, USA zyxwvutsrqponmlkjihgfedc (Received May 3 I, 1990; accepted in revised form December 10, 1990) Abstract-The chemical speciation of sulfur in heavy petroleums, petroleum source rock extracts, and source rock pyrolysis products was studied using X-ray absorption near-edge structure (XANES) spec- troscopy. The good energy resolution (ca. 0.5 eV) at the sulfur K edge and the strong dependence of XANES on the sulfur environment combine to give excellent sensitivity to changes in the electronic and structural environment of the sulfur. This has permitted identification and approximate quantitation of different classes of sulfur-containing compounds (e.g., sulfur, sulfides (including disulfides and polysulfides as a group), thiophenes, sulfoxides, sulfones, sulfinic acids, sulfonic acids, and sulfate) in a series of petroleums and petroleum source rocks. Our results indicate that the sulfur speciation of geological samples can be correlated with differences in source depositional environment, thermal maturity, and aromaticity. We report organosulfur compositions for the asphaltene, maltene, and liquid chromatographic fractions of two sulfur-rich oils. In addition, we find that the organosulfur species in some, but not all, oils are subject to oxidation upon storage and thus may also be susceptible to oxidation in shallow reservoirs exposed to oxic waters. This work illustrates the utility of XANES as a direct spectroscopic probe for the quantitative determination of sulfur species in geological samples. INTRODUCDON NEXT TO CARBON AND hydrogen, sulfur is the most abundant chemical element in petroleum, occuring at concentrations of over 10 wt% in some heavy oils (HUNT, 1979; TISSOTand WELTE, 1984; ORR and SINNINGHE DAMSTB, 1990). In- creased use of sulfur-rich petroleums and increased appre- ciation of the environmental impact of sulfur combustion products have led to a growing interest in understanding the chemical nature of the sulfur in oil and oil precursors (SIN- NINGHE DAMSTB et al., 1988a, and refs. therein). The present work describes the use of sulmr X-ray absorption spectroscopy for determining the speciation of sulfur compounds in pe- troleums. This contributes to petroleum geochemistry, has potential utility in predicting the geological distribution of oils with high or low sulfur contents, and also has important implications for enhanced oil recovery and refining operations involving sulfur-rich petroleums. A graph of frequency vs. sulfur content for a large collection of crude oils shows a bimodal distribution (TISSOT and WELTE, 1984). This distribution is used to classify “high- sulfur” (greater than 1 .O wt% sulfur) and “low-sulfur” (less than 1.0 wt% sulfur) oils. The total sulfur concentration of many oils correlates with various other properties, including specific gravity, viscosity, and aromatic hydrocarbon content (GRANSCH and POSTHUMA,1974; ORR, 1986). However, be- yond such phenomenological correlations, the role of sulfur in the geochemical transformations which give rise to petro- leum is a current geochemical concern. There is a need for better characterization of the abundance and distribution of * Author to whom correspondence should be addressed. sulfur species in, for example, kerogens, asphaltenes, and high- molecular weight fractions. Several factors are generally believed to affect the concen- tration of sulfur in oil. Among these are source characteristics (marine vs. terrestrial), presence or absence of dissolved sul- fate, depositional environment (carbonate vs. elastic se- quences) (TISSOT, 1981; VALITOV, 1983; HUGHES, 1984; TANNENBAUM and AIZENSHTAT, 1985) thermal maturity (Ho et al., 1974; ORR, 1986; SHOU and MYHR, 1988), ther- mochemical sulfate reduction (ORR, 1974; KROUSE et al., 1988; SASSEN, 1988), and the extent of biodegradation (MA- GOONand ISAACS,1983). Furthermore, the precise mode of sulfur incorporation in source organic matter, i.e., via H2S attack at susceptible centers (CASAGRANDE et al., 1979; MANGO, 1983; SINNINGHE DAMSTI? et al., 1989b,c), or mechanisms involving elemental sulfur or polysulfides (DOUGLASand MAIR, 1965; MARTIN and HODGSON, 1973; DE Roo and HODGSON, 1978; CASAGRANDE and NG, 1979; AIZENSHTAT et al., 1983; PRZEWOCKI et al., 1984; WHITE et al., 1988; KOHNEN et al., 1989) remains an active area of inquiry. It is clear that both H2S and polysulfide can react with functionalities such as double bonds during very early diagenesis (SINNINGHE DAMST~et al., 1989~; KOHNENet al., 1990a,b). The finding that sulfur compounds in the asphalt- enes and various chromatographic fractions of oils have uni- form 634S isotopic compositions (THODE and MONSTER, 1970) suggests that the sulfur in each of the fractions comes from sulfur species with identical sulfur isotopic composi- tions. Chemical characterization is essential both for general un- derstanding of organosulfur transformations associated with petroleum formation and alteration, and for the development of organosulfur biological markers (SINNINGHEDAMSTI~ et 801