Re: Nature and MIT - April is a Dangerous month for Science
Looking at this thread from a few months back (which seems to have veered markedly off-topic), it may be worth making a few comments.
Disputin's definition of a fossil fuel as being something that can be used as a fuel which is dug up out of the ground is etymologically correct, but virtually nobody uses that definition in practice. Under that definition I'd say uranium ore could reasonably be classified as a fossil fuel. It obviously requires a lot of work to turn uranium ore into nuclear fuel, but a lot of processing effort is also required to turn crude oil into petrol or other practical fuels, and even natural gas requires some processing effort, like hydrogen sulphide, carbon dioxide and water vapour being removed for practical use.
The carbon 13 depletion argument is pretty strong for petroleum being of biogenic origin, as the C13 ‰ depletion range for petroleum ties up very well with photosynthesis, but a much greater C13 depletion than produced by photosynthesis can be observed with natural gas, which also varies with depth below the ground surface, suggesting that physical diffusion through the ground is causing a lot of its C13 depletion. So it is more difficult to pin down whether natural gas is abiogenic or biogenic.
I came across an interesting alternative theory for the origin of fossil fuels a few years ago called the "anhydride theory" by C Warren Hunt. A free to view copy of the theory is available on the scribd.com site:
As I understand Hunt's theory, he thinks natural gas is mainly abiogenic. Petroleum is biogenic, but produced by microbes which convert methane coming from the Earth's interior to higher hydrocarbons at depth rather than being biological material that found its way down from the surface. (Presumably this microbial action might produce a similar C13 depletion to photosynthesis). So natural gas and petroleum would not be fossil fuels in the sense of them being fossilised material that originally started at the surface. Hunt's theory (if valid) might provide an explanation for why petroleum can be biogenic but also occur at what some might see as implausibly large depths below the ground surface or seabed.