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Chemistry

The Fuel to Hydrocarbon Formation

fuel

© J. Chatin/CNRS Photothèque

Subterranean rock showing traces of hydrocarbons.


It was previously believed that the early steps of hydrocarbon formation were triggered by lack of oxygen, or resulted from the work of micro-organisms that turn organic lipid residue into stable sedimentary compounds. But researchers have recently challenged this latter hypothesis by revealing that hydrogen sulfide (H2S) is the main agent in the initial stage of hydrocarbon development.1 This means that the initial process is purely chemical, not biological. The hydrocarbon that emerges from this transformation matures, over time, into petroleum.

These findings are the result of a partnership between Dr. Pierre Albrecht's unit at the University Louis Pasteur of Strasbourg (CNRS) and the Federal Institute of Technology in Zurich. To explain the role of H2S in creating stable hydrocarbons, the team carried out molecular studies on recently deposited sediments from Lake Cadagno in Switzerland. Below 11 meters, the lake has an anoxic2 layer that contains H2S and resembles the conditions in which hydrocarbons are formed in marine environments, making it an ideal location for this type of research.

Experiments in the laboratory setting corroborated evidence that H2S causes unstable organic lipids to become hydrocarbons through a series of chemical reactions. The H2S molecule first attaches to the organic lipid's unsaturated (or hydrogen poor) carbon chain to create a thiol.3 Then the sulfur is eliminated, leaving the hydrogen component of H2S attached to the carbon, which results in a stable saturated hydrocarbon. “This allows us to better understand the fundamental environmental processes that lead to petroleum formation. It is something that up to now was not well elucidated,” says Albrecht. This same process of hydrogenation probably also occurs in underwater “black smokers” (sulfide chimneys), whose hot water sources contain significant portions of H2S. Three billion years ago, early life forms could have originated through chemical interactions produced in the sulfide chimneys. “Many people think that life could have started in such places through chemical reactions between simple carbon compounds, in which H2S might have acted as a reducing agent,” explains Albrecht, whose team is currently exploring this hypothesis.

 

Melisande Middleton

 

 

 

 

Notes :

1. Hebting et al., “Biomarker evidence for a major preservation pathway of sedimentary organic carbon,” Science. 312 (5780): 1627-31. 2006.
2. Anoxic: without oxygen.
3. Thiol: organic compound resulting from attaching a sulfhydryl group (-SH) to a carbon.

Contacts :

Pierre Albrecht
Université Louis Pasteur, Strasbourg.
albrecht@chimie.u-strasbg.fr


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