It has now been established that considerable sulfur-isotope fractionation occurs in the biological sulfur cycle and that the bacterial reduction of sulfate, which leads to the enrichment of S³⁴ in sulfate and its depletion in sulfide, is largely responsible for the wide fluctuations in isotope ratio which occur in marine sediments.

In this regard, present-day ocean-water sulfate is remarkably uniform in sulfur-isotope content, both in depth and in geographical location at a value of = S³⁴ = +20 (20 parts per mil enriched S³⁴ with respect to sulfur in meteorites), and provides a base level in isotope 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 sulfur-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 sulfur-isotope ratio for the various ancient oceans and to establish the pattern of change throughout geological time.

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

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