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Paris, June 19, 2006

The first stages of petroleum formation

Contrary to what is often thought, the first step in forming natural gases and petroleum is a purely chemical process, and not a biological one. This discovery by the “Natural Substances / Molecular Chemistry” unit ("Substances naturelles/ Chimie moléculaire" CNRS, University of Strasburg I, France) challenges the theory that microorganisms are responsible for initiating the transformation of organic lipid residues into stable sedimentary compounds. The results are published in Science of June 16 2006.

Proteins, nucleic acids and sugars rarely last very long after organisms have died.  It is mainly hydrocarbons that are found in rocks that are hundreds of millions of years old, and these are sometimes in the concentrated form of petroleum, gas or coal. These compounds come from the conversion of biological lipids, with their carbon double bonds, into saturated compounds that are more resistant to decomposition. Up to now, knowledge about the conditions in which these changes took place was only fairly superficial. However, two main hypotheses were generally favored. One was that preservation is enhanced by the absence of oxygen, and the other was that microorganisms are responsible for the transformations.

 

The team of chemists from CNRS “Natural Substances / Molecular Chemistry” unit, and Louis Pasteur University Strasbourg, directed by Pierre Albrecht, has recently challenged this second hypothesis. The researchers worked with a geologist from the Federal Institute of Technology in Zurich, to examine recent sediments from the bottom of Lake Cadagno in Switzerland. The particular conditions of this lake make it an ideal model of the situation that existed in many marine basins during the Earth's history. Over time source rocks were formed in which the deposited organic material matured into petroleum. Lake Cadagno has a two-layer stratification: On the top is an oxic (oxygen containing) layer, and below 11 m, an anoxic layer (without oxygen). The lower layer is anaerobic because sulfate-rich underwater springs feed the lake. The sulfate is converted into hydrogen sulfide (H2S) by bacteria in the sediment and in the lower layer of the lake. In between the oxic and the anoxic layer, highly specialized red bacteria use the hydrogen sulfide for a particular type of photosynthesis, which stops the sulfide rising into the upper layer of water, where it would be harm the fauna.

 

The researchers have now discovered that the conversion of certain organic compounds, which are typical of bacteria and algae, into compounds that can be preserved long-term takes place in the anoxic layer, in the presence of hydrogen sulfide, very soon after the organisms have died. These compounds stabilized by hydrogenation are already found in the upper layers of recently deposited sediment. The lack of selectivity of hydrogenation reactions, and the wide variety of chemical forms of degradation products are a feature of a purely chemical process and not a more precise, selective biological product. The team was able to confirm this hypothesis by laboratory studies carried out between 50 and 90°C, which recreate the conversions that take place in the natural medium. The chemical reaction takes place in two steps: First H2S is added to a double bond of the carbon chain to form thiol [1], then this group is reduced which eliminates the sulfur atom and leaves a hydrogen atom. So it is obviously the hydrogen sulfide that is acting as a hydrogenating agent.

 

Therefore is would seem that, contrary to what was generally believed, the first step in the process of forming petroleum and other fossil energy sources is a purely chemical one. These hydrogenation reactions which start the preservation process of large quantities of subterranean carbon may also have played an important part in relation to so-called “Black Smokers” [2] where submarine springs of hot water rich in H2S are active. The chemical reactions taking place around these “Black Smokers”, similar to those which have been discovered in Lake Cadagno, may have given rise to the first forms of life over three billion years ago. The research team is currently studying this hypothesis.

Notes:

[1] An organic compound with a sulfhydryl group (-SH) attached to a carbon atom).

[2] Jets of water at 300-400°C coming from cracks in the ocean ridges and containing black metallic particles in suspension. This phenomenon is caused by the magma under the ridges heating the seawater on the ocean bed.

Contact information:

Researcher
Pierre Albrecht
TEL. +33 3.90.24.26.34
albrecht@chimie.u-strasbg.fr

Press
Isabelle Bauthian
TEL. +33 1 44 96 46 06
isabelle.bauthian@cnrs-dir.fr


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