A study has been made of the sulphur isotope ratios for petroleum from many different oil fields in the United States and Canada. The S³⁴/S³² ratio for petroleum varies by about 4.5 per cent as compared with a total spread of about 9 per cent for terrestrial sulphur. The single large oil pools appear to be uniform in isotopic content. Also, oils from widely separated locations in Alberta, found in the same reservoir rock, have similar sulphur isotope ratios. On the other hand, the actual sulphur content of these latter oils may vary by a factor of 10. This result indicates that there is little or no fractionation of the sulphur isotopes in the maturation of the oil in which sulphur is lost. The fact that the H₂S and its associated petroleu have similar sulphur isotope content is further evidence for this view.

The sulphur isotope ratio for petroleum would seem, therefore, to give information concerning the isotopic content of the source sulphur at the time of petroleum formation. This in turn would reveal something about the environment in which the petroleum was formed. The very high S³⁴ content of the petroleum in the recent sediments of the Uinta basin in Utah confirms the non-marine origin of these deposits. The decrease in S³⁴ content with age and depth of petroleum deposit in the basin suggests a gradual S³⁴ enrichment of the sulphate over a period of time in the enclosed lake, probably due to bacterial action.