Lack of oxygen promotes petroleum
A long-standing theory of petroleum formation -- that it is formed from organic matter starved of oxygen -- has received new support.
An AustralianUS research team used a technique called Nuclear Magnetic Resonance (NMR), usually applied in medicine, to analyse marine sediments from all over the world. Their results appear in this week's Science.
"This is the first extensive application of solid-state NMR to ocean sediment," said researcher Mr Jeff Baldock, from CSIRO Land and Water.
"It has proved to be very successful."
Mr Baldock and colleagues from the University of Washington, Seattle, examined samples of 15 different marine sediments collected from all over the world.
This use of Nuclear Magnetic Resonance is unusual. The technique is used in medicine as a diagnostic tool, and in polymer science and biochemistry.
"There are only four or five places around the world using NMR to look at natural substances," said Mr Baldock, whose work looks at the decomposition of organic matter.
Petroleum is formed from non-protein alkyl carbon, which is in organic matter that has not fully decomposed. NMR was used to measure levels of non-protein alkyl carbon in samples from different parts of the sea floor.
They found that sediments with less exposure to oxygen had higher levels of non-protein alkyl carbon, and vice versa. This suggests that rocks originating from coastal sediments are more likely to yield petroleum.
"Coastlines have organic matter and sediment material flowing from rivers so organic matter is buried quickly," said Mr Baldock.
"But out in the deep ocean, sediment is formed from plankton material raining down and it is much slower."
Deep sea organic matter is exposed to oxygen for a longer period and has time to fully decompose before being buried, so is less likely to form petroleum.
The research will allow geologists to identify rocks that were originally from the continental shelf which are source rocks for petroleum.
"These are more likely to have petroleum in them than the rocks that originated from sediments in the middle of the ocean," Mr Baldock explained.
Another outcome will be an increased use of NMR as a tool for analysing organic materials, he said.
"Chemists like to work in sterile environments, they don't like messing up their NMRs with dirt."
"But once they see the potential of this tool, a lot of places will start running organic analyses."