First steps of petroleum formation deciphered
A purely chemical process in the early phase of sediment deposition plays a crucial role in the stabilization of existing but dead organic compounds and their subsequent conversion to petroleum. The thesis that it is primarily microorganisms that convert the unsaturated carbon chains into stable forms must therefore be revised.
A research team from the Swiss Federal Institute of Technology in Zurich and the Louis Pasteur University in Strasbourg headed by Pierre Albrecht examined water samples and sediments from Lake Cadagnose in the canton of Ticino for their study. The mountain lake has some special features, as Stefano Bernasconi from the Geological Institute of ETH Zurich explains. The body of water, which is around twenty meters deep, has an extremely stable stratification: the water at the top contains oxygen, while the conditions below are anaerobic. The two water packets are separated by a sharp layer about a meter thick, which is home to highly specialized, reddish-colored bacteria.
Anaerobic is the bottom layer because underwater springs bring sulphate water into the lake. The sulphate is converted to hydrogen sulphide by the bacteria in the sediment and in the lower water layer. This creates the conditions for better preservation of dead organic material. The reddish bacteria in the boundary layer use the hydrogen sulfide for a special form of photosynthesis and thus prevent it from escaping into the upper water layer - similar conditions were found in many sea basins in geological times, in which crude oil was formed.
The researchers have now found that the conversion of certain organic compounds typical of bacteria and algae in the presence of hydrogen sulfide appears to begin shortly after the organisms have died. Partially saturated carbon chains can already be found in the uppermost layers of sediment that have only recently been deposited. It is also striking that the double bonds are broken at any point along the carbon chains. According to Bernasconi, this indicates that the conversion is not caused by microorganisms, because they would usually start their metabolism at certain points.
The chemical reaction takes place in two steps: First, a simple sulphur-hydrogen group binds to the carbon chain. In a second step, this group is then reduced; the sulfur atom is dissolved out so that only one hydrogen atom remains.
The group was able to confirm their thesis with laboratory tests. The scientists had allowed artificial solutions to react with the corresponding organic compounds at 50 to 90 degrees Celsius for a few weeks: Exactly the same conversion took place in the laboratory as was observed in the sediments of the lake. © ETH Life