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Compositional and stable carbon isotopic fractionation during non-autocatalytic thermochemical sulfate reduction by gaseous hydrocarbons

Geochimica et Cosmochimica Acta

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, , , and
DOI: 10.1016/j.gca.2014.05.004

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Abstract

The possibility of autocatalysis during thermochemical sulfate reduction (TSR) by gaseous hydrocarbons was investigated by examination of previously reported laboratory and field data. This reaction was found to be a kinetically controlled non-autocatalytic process, and the apparent lack of autocatalysis is thought to be due to the absence of the required intermediate species. Kinetic parameters for chemical and carbon isotopic fractionations of gaseous hydrocarbons affected by TSR were calculated and found to be consistent with experimentally derived values for TSR involving long-chain hydrocarbons. Model predictions based on these kinetic values indicate that TSR by gaseous hydrocarbon requires high-temperature conditions. The oxidation of C2–5 hydrocarbons by sulfate reduction is accompanied by carbon isotopic fractionation with the residual C2–5 hydrocarbons becoming more enriched in 13C. Kinetic parameters were calculated for the stable carbon isotopic fractionation of gaseous hydrocarbons that have experienced TSR. Model predictions based on these kinetics indicate that it may be difficult to distinguish the effects of TSR from those of thermal maturation at lower levels of hydrocarbon oxidation; however, unusually heavy δ13C2+ values (>−10‰) can be diagnostic of high levels of conversion (>50%). Stoichiometric and stable carbon isotopic data show that methane is stable under the investigated reaction conditions and is likely a product of TSR by other gaseous hydrocarbons rather than a significant reactant. These results indicate that the overall TSR reaction mechanism for oxidation of organic substrates containing long-chain hydrocarbons involves three distinct phases as follows: (1) an initial slow and non-autocatalytic stage characterized by the reduction of reactive sulfate by long-chain saturated hydrocarbons; (2) a second autocatalytic reaction phase dominated by reactions involving reduced sulfur species and partially oxidized hydrocarbons; (3) and a final, or late-stage, TSR reaction in which hydrocarbon oxidation continues at a slower rate via the non-autocatalytic reduction of sulfate by gaseous hydrocarbons.

Additional Publication Details

Publication type:
Article
Publication Subtype:
Journal Article
Title:
Compositional and stable carbon isotopic fractionation during non-autocatalytic thermochemical sulfate reduction by gaseous hydrocarbons
Series title:
Geochimica et Cosmochimica Acta
DOI:
10.1016/j.gca.2014.05.004
Volume
139
Year Published:
2014
Language:
English
Publisher:
Elsevier
Contributing office(s):
Central Energy Resources Science Center
Description:
15 p.
Larger Work Type:
Article
Larger Work Subtype:
Journal Article
Larger Work Title:
Geochimica et Cosmochimica Acta
First page:
472
Last page:
486
Number of Pages:
15