The key claim for the theory of biogenic petroleum origin is that low concentrations of the C13 isotope are proof of biogenic origin because photosynthesis preferentially selects C12 over C13.
See Richard Heinberg's The Abiotic Oil Controversy.
Geologists trace the source of the carbon in hydrocarbons through analysis of its isotopic balance. Natural carbon is nearly all isotope 12, with 1.11 percent being isotope 13. Organic material, however, usually contains less C-13, because photosynthesis in plants preferentially selects C-12 over C-13. Oil and natural gas typically show a C-12 to C-13 ratio similar to that of the biological materials from which they are ASSUMED [Emphasis added] to have originated. The C-12 to C-13 ratio is a generally observed property of petroleum and is predicted by the biotic theory; it is not merely an occasional aberration.Also see Ugo Bardi & Dale Allen Pfeiffer's No Free Lunch.
The isotopic ratios of a substance (particularly the ratio of carbon and hydrogen isotopes) provide us with a profile of the substance, a sort of isotopic fingerprint which indicates how the substance was generated. Most naturally occurring carbon is isotope C-12, with a small percentage of C-13 (1.11%) and a trace of radioactive isotope C-14. Organic matter, however, has a lower ratio of C-13 because photosynthesis preferentially concentrates C-12. Hydrocarbon reserves reflect their organic origin in their C-12/C-13 ratio.This all comes from the biogenic geologist Barbara Sherwood Lollar's paper: Abiogenic formation of alkanes in the Earth's crust as a minor source for global hydrocarbon reservoirs.
Here, using carbon and hydrogen isotope analyses of abiogenic methane and higher hydrocarbons in crystalline rocks of the Canadian shield, we show a clear distinction between abiogenic and thermogenic hydrocarbons.Given the fact that geologists know nothing of the universe we live in and given the fact that these individuals are factually challenged I was highly suspicious of this claim. What I discovered is that a lack of C13 isotope proves nothing: Hydrocarbons in Hydrothermal Vent Fluids.
We did, however, observe anomalous amounts of m/z 31 (13C12CH6aq) (relative to that predicted from natural [biogenic] carbon abundances)How are biogenic theorists going to explain this so-called anomaly?
Is methane the first direct sign of extra-terrestrial life?
The C-12 to C-13 ratio of methane alone is not always proof of life. For example, the "Lost City" hydrothermal vent field in the Atlantic Ocean did not show a clear isotope signature, says James Kasting, professor of earth and mineral science at Penn State University.Carbon stars which are totally deficient in C13 have been observed: The Abundance Ratio of C12 and C13 in Carbon Stars.
"The methane is not that strongly fractionated, but they still think it might be biological," says Kasting. "At Lost City, you can't figure out if it's biological or not by the isotopes. How are we going to figure that out on Mars?"
Two hydrogen deficient carbon stars HD13716 and HD182040 show no trace of C13N features.Needless to say, there is no photosynthesis occuring on those stars. And the authors conclude with the following:
The above results revealed that C12/C13 ratio in carbon stars shows a wide range of variety.John Valley, Geology professor at the University of Wisconsin also admits lack of C13 proves nothing.
High C-12 to C-13 ratios, he says, "when present in sufficient quantity, are very strong evidence of organic activity, although I don't use the word 'proof.'"So there you have it: lack of C13 proves, you guessed it, absolutely nothing.
Abiogenic hydrocarbons with low C13 content have also been observed in Japan so the "scientists" conclude they must be a mix of biogenic and abiogenic hydrocarbons.
The isotope composition of methane and the relationship between methane/ethane ratio and δ13C-value of methane suggest that these light hydrocarbon gases are mixtures of thermogenic and abiogenic components. The abiogenic hydrocarbon may be attributed to magmatic hydrocarbon gases equilibrated with carbon dioxide at fO2 defined by the fayalite-magnetite-quartz buffer (FMQ)The reality is that in 2007 ICP-MS showed that all oil has inorganic geochemistry: The Inorganic Geochemistry of Oil.
Of course the final nail in the coffin would have to be this gem from Giardini and Melton (1991): EVIDENCE THAT STABLE CARBON ISOTOPES ARE NOT A RELIABLE CRITERION FOR DISTINGUISHING BIOGENIC FROM NON-BIOGENIC PETROLEUM
The isotopic abundance of presumably-pristine primordial carbon has been determined by analyzing carbon dioxide entrapped in a 8.65 carat natural diamond of African origin. The results were 12C = 98.9275% and13C = 1.0725%, which giveδ13C = -35.2‰/00. This value is well within the range used to assign a biogenic origin to carbon-containing compounds, i.e., more negative than -18.0‰/00. Similar negative values have been reported for some natural diamonds and carbon-bearing meteorites. It is concluded, therefore, that stable carbon isotopes can be an unreliable criterion for assigning a biogenic origin to petroleum.