Formation of h2s and sulfur-rich bitumen from a reservoired heavy oil in the presence of elemental sulfur

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Organic Geochemistry 41 (2010) 951–958

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Organic Geochemistry
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Formation of H2S and sulfur-rich bitumen from a reservoired heavy oil in the presence of elemental sulfur
Isabelle Kowalewski a,*, Philippe Schaeffer b, Pierre Adam b, Daniel Dessort c, Anne Fafet a, Bernard Carpentier a
a b cGeology–Geochemistry Division, IFP, BP311, 92506 Rueil-Malmaison, France Laboratoire de Biogéochimie Moléculaire, Université de Strasbourg, UMR 7177, ECPM, 25 rue Becquerel, 67200 Strasbourg, France Total, CSTJF, Avenue Larribau, 64000 Pau, France

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The hypothesis of a low temperature natural ‘‘vulcanization” process as a potential mechanism for theformation of insoluble bitumen and H2S in reservoirs which have not undergone biodegradation, high thermal stress, or thermochemical sulfate reduction (TSR) has been investigated using laboratory simulation experiments. The experiments, involving elemental sulfur (32S and 34S), a low maturity oil and water at 200 °C in sealed gold tubes, clearly showed that elemental sulfur is able to induce theformation of H2S, polar compounds (resins and asphaltenes) and insoluble bitumen most likely by way of sulfur cross-linking processes. Specific organic sulfur compounds (thiophenes, dithiophenes and thienothiophenes) formed by the reaction of sulfur and hydrocarbons may be potential markers of such a process, but remain to be detected in natural samples. Ó 2010 Elsevier Ltd. All rights reserved.Article history: Received 30 September 2009 Received in revised form 22 April 2010 Accepted 3 May 2010 Available online 7 May 2010

1. Introduction Bitumen and H2S have been found worldwide in many carbonate petroleum reservoirs (Sassen, 1988; Carpentier et al., 2007; Cai et al., 2003). Bitumen is referred to as black viscous, semi-solid or solid (solid bitumen) organic matter in a reservoir(Curiale, 1986; Pittion, 1983; Lomando, 1992). It can form permeability barriers in reservoirs (tar mats) that impact on the field development plan and calculation of the amount of oil in place. For these reasons, it is important to know where tar mats occur in the field (both laterally and vertically) and what controls their distribution. The presence of H2S, a corrosive and toxic gas, has adverse effectson production and safety. In geological settings, the processes responsible for the formation of bitumen and H2S include anaerobic biodegradation by sulfate reducing bacteria (SRB) below 100 °C (Machel, 2001), thermal decomposition of oils (Mort, 2004; Huc et al., 2000; Kowalewski et al., 2009) and thermochemical sulfate reduction (TSR) above 110 °C (Worden and Smalley, 1996; Cai et al., 2001;Hanin, 2002), as well as oil mixing and oil–gas mixing at the reservoir temperature (Carpentier et al., 1998). Bitumen and H2S also can be formed by way of artificial processes occurring during water injection for thermally enhanced oil recovery (Thimm, 2001; Kowalewski et al., 2008).
* Corresponding author. Tel.: +33 1 47526459; fax: +33 1 47527019. E-mail address: (I.Kowalewski). 0146-6380/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.orggeochem.2010.05.002

More recently, the presence of insoluble bitumen, which cannot be explained by the above processes, was observed in low temperature reservoirs (Gao et al., 2001; Walters et al., 2006). A new reservoir alteration process involving ether cross-linking reactions was thusproposed to explain the formation of H2S and solid bitumen at temperatures Sresins > Saromatics. From experiments T5 and T6, it can be deduced that water did not influence the extent of sulfur incorporation and the stable sulfur isotopic composition of the fractions. All of the added elemental sulfur was consumed during the laboratory simulation experiments. About 40 wt.% of the elemental sulfur was...
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