Early investigations of the sulphides and sulphates in sedimentary rocks showed wide variations in the abundance of the sulphur isotopes. More recent investigations of petroleum and related materials show that these also vary markedly in their sulphur isotope content. It is now known that the biological sulphur cycle and in particular the bacterial reduction of sulphate is largely responsible for the fractionation of the sulphur isotopes in nature.

In the interpretation of the sulphur isotope distribution data in terms of natural processes and earh history, it is essential that we know the base levels of isotope ratio from which fractionation began as well as the extent of fractionation which can occur in any process. Since petroleum is usually formed in a marine or marine-like environment, it is important that we know the sulphur isotope ratio of the sulphate in the oceans and how this ratio has been changing throughout geological time. An extensive study of evaporites in various sedimentary basins has been carried out by Thode and Monster (1962). It is clear from their results that the sulphur isotope ratio of the evaporites reflects the environment and sulphur isotope ratio in the basin at the time of deposition. On the assum tion that the lowest S³⁴ enrichment found in the evaporites of a given geological period from various sedimentary beds will give the closest approach to the value of the ocean sulphates of the period Thode and Monster have determined the S³⁴ content of the ancient seas. The results clearly show that the sulphur isotope ratios of the oceans have changed with time in a complex but cyclic fashion.

The sulphur isotope ratios of petroleum samples also reflect these changes. In particular, these ratios appear to reflect the isotope level and environment of the basin during the time of petroleum formation. However, the petroleum ratios are displaced from those of the contemporaneous evaporites by about 15^pmil which is the fractionation to be expected in the bacterial reduction of sulphate. These results suggest that it is the reduced sulphur which becomes incorporated into the petroleum and that ocean sulphate, or sulphate of a large inland sea, is the source of petroleum sulphur. It is not surprising, therefore, that the pattern of isotope distribution in petroleum and related materials in non-marine sediments is completely different from that of those of marine origin. These res lts will be discussed from the point of view of petroleum exploration.

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