It has now been established that considerable sulphur isotope fractionation occurs in the biological sulphur cycle and that the bacteria] reduction of sulphate, which leads to the enrichment of S³⁴ in sulphate and its depletion in sulphide, is largely responsible for the wide fluctuations in isotope ratio which occur in marine sediments.

In this regard, present-day ocean water sulphate is remarkably uniform in sulphur isotope content, both in depth and in geographical location at a value of ^dgr = S³⁴ = +20 (20 parts per mil enriched in S³⁴ with respect to sulphur in meteorites) and provides a base level in isotopic ratio from which fractionation can be reckoned. However, in dealing with ancient sediments and petroleum, we need to know the S³⁴ content of the ancient oceans or seas.

Recently (Thode and Monster, 1963) a study of the sulphur isotope distribution in the marine evaporites of some ten sedimentary basins of several continents was carried out. From this study it has been possible to estimate the sulphur isotope ratio for the various ancient oceans and to establish the pattern of change throughout geological time.

The pattern of change for petroleum sulphur appears to be parallel with that for the evaporites and ancient seas. However, the petroleum sulphur is, in general, depleted in S³⁴ by about 15^pmil with respect to the contemporaneous gypsum anhydrite deposits. This displacement of ~15°^pmil in the S³⁴ content, which is about the isotope fractionation expected in the bacterial reduction of sulphate, is strong evidence that sea water sulphate is the original source of petroleum sulphur and that it is first reduced by bacterial action in the shallow muds before being incorporated into the petroleum. The lack of any sulphur isotope fractionation in the plant metabolism of sulphate would seem to rule out plant sulphur as a major source of petroleum sulphur.

Since the ^dgrS³⁴ values for petroleum pools in a given horizon, e.g., Devonian (D-2), are fairly uniform over a large sedimentary basin and since these values vary from one horizon to another depending on the S³⁴ content of the contemporaneous seas, sulphur isotope studies should be useful in solving migration problems.

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