59th Annual Report on Research 2014

The goal of the research was to test if sulfur radical chemistry plays critical role in initial cracking of organic hydrocarbons during petroleum formation, and if such sulfur radical chemistry can leave unique isotopic signature as anomalous ³³S abundance that can be detected by high-precision multiple sulfur isotope analysis. The research was designed to follow up recent discoveries by Oduro et al. (2011) and Watanabe et al. (2009) of the generation of anomalous S-33 signals from thermochemical reactions involving organic sulfur compounds, and to test the requirements for generation of such signals. In addition, we carried out experiments to test the reactivity of thiophenes to investigate the preservation of sulfur isotope signals in thiophene sulfur.

Harry Oduro (former postdoctoral fellow) examined sulfur isotope variations in different sulfur-bearing phases of petroleum source rocks by using sequential chemical extraction technique. He tested two petroleum source rocks, the Monterey Shale and the Ghareb Limestone formations, and successfully extracted various sulfur-bearing fractions, including acid volatile sulfur (FeS and other metal monosulfide), pyrite sulfur (FeS₂), elemental sulfur (S₈), organic sulfur (bitumen and kerogen sulfur), and sulfate for high-precision sulfur isotope analysis using the SF₆ technique. They show δ³⁴S variation as large as 35 ‰; two formations yield different isotope systematics among different phases, reflecting different sources of sulfur. There are measurable ³³S signals but their small signals do not demonstrate the production and preservation of anomalous ³³S signals in these petroleum source rocks studied.

For the second set of experiments, common sulfur-bearing amino acids (cysteine, methionine, taurine, and glutathionine) and diphyenyl disulfide were thermally decomposed under vacuum. The products yield mass-dependent isotope fractionation, as opposed to previous reports of mass-independent fractionations by thermochemical sulfate reductions by certain amino acids. Thermal decomposition of trithiane, however, produced ³³S anomaly up to +1.3 ‰. The origin of the anomalous fractionation is currently under investigation. This study demonstrates that the generation of anomalous ³³S signals is highly specific to certain compounds or reactions. Harry's results were presented in Goldschmidt Conference 2013 and Organic Geochemistry Gordon Conference in 2014.

The third part of the project was to follow up the recent report by Armani et al (2013) on the different sulfur isotope signals between benzothiophene (BT) and dibenzothiophene (DBT) in petroleum. In order to test if the different isotope signals reflect different reactivity between BT and DBT, Andrew Whitehill (Graduate student) and Eoghan Reeves (Postdoc) carried out experiments to test isotope mixing at high-temperature and high-pressure conditions. In this experiment, ³⁴S enriched (at 200‰) elemental sulfur was mixed with equal weight of BT or DBT. They were sealed in gold capsules in triplicates and heated for 40 hours at 350°C and 350 bar. GC-MS analysis of the dichloromethane extracts from BT experiments showed the formation of benzothieno-benzothiophene, and products from DBT include thianthrene. After removing elemental sulfur, residual organic sulfur fractions were analyzed for multiple-sulfur isotope ratios. The products yield clear signal from ³⁴S enriched sulfur from ³⁴S labeled elemental sulfur, demonstrating the transfer of sulfur from elemental sulfur to organic sulfur fractions including BT, DBT, benzothieno-benzothiopnene and thianthrene. This experiment confirms that BT and DBT are reactive under experimental conditions. Surprisingly, BT products yield lower degree of sulfur mixing between elemental sulfur and organic-S fractions (22 to 28 %) compared to DBT that yield 39 to 62 % sulfur mixing. Next step will be to isolate BT and DBT from experimental products for isotope ratio analysis to test the incorporation of sulfur into BT or DBT. The experiments demonstrate that ³⁴S-enriched sulfur can be used for experimental studies to understand the fate of sulfur during thermal alteration of petroleum.

References: Oduro et al., 2011, Proc. Nat. Aca. Sci. 43, 17635; Watanabe, 2009, Science, 324, 5925; Amrani et al., 2013, Geochimi Cosmochimi Acta, 84, 152