The late Jack Kenny argued that carbon isotopes could not be used as a determinant of whether a substance had a biological origin or provenance and one of the lines of evidence used was the measured isotopic ratio of carbon 12/13 in a primeval diamond reported by Giardini et al 1982.
In that report it was shown that the carbon isotope ratio of the diamond, compared to the Pee Dee Belemnite standard was -35.2%. Pee Dee Belemnite standard ratios more negative than -18% are indicative of a biological origin. On this basis and from the dating of the particular diamond which indicated an age crystallisation of 3.1 x 10^9 years, “well before any record of biological life on Earth”, it was concluded that carbon isotope (C12/13) ratios can’t be used to determine a biogenic provenance.
Not so, because while the carbon isotope ratio is the result of measurement and calculation, the statement that there was no biological life on the Earth is not a fact but a belief, an imaginary construct.
Rather the empirical procedure is to conclude that this particular diamond had a biogenic origin or provenance and that the belief of life not existing at the time is actually wrong; clearly life was present 3.1 x 10^9 years ago as indicated by the data.
So it is quite likely that petroleum is biogenic AND derived from the mantle.
(reference: Giardini, A. A., Melton, C.E., Mitchell, R.S., (1982), J Pet. Geo., 5, 2, 173-190)