Homogeneity of Sulfur and Carbon Isotope Ratios S34/S32 and C13/C12 in Petroleum

Abstract

The sulfur isotope ratios, S34/S32, of six different fractions obtained from each of three Saskatchewan crude oils and a McMurray bitumen showed very little deviation (^scsim 1 permil) from the ratios obtained for total sulfur in these samples. In the oils, the fractions consisted of a high- and a low-boiling fraction with a subdivision of the high-boiling fraction into asphaltenes, saturates, aromatics, and polar compounds. The bitumen was separated into asphaltenes, saturates plus aromatics, dark oils, and three classes of resins. Organically bound sulfur in recent marine sediments exhibits a range of sulfur isotope ratios of as much as 50 permil. This sedimentary sulfur is likely to be at least in part of the precursor of sulfur in petroleum. As the isotopic heterogeneity of the precursor is lost in the oil in the reservoir rock, homogenization of sulfur isotope ratios takes place at one or more stages in the chain of processes leading from organic sedimentary material to a mature petroleum. The two major processes postulated as being responsible for homogenization are mixing through migration and sulfur isotope exchange reactions. Variations in C13 content of the oil fractions also were found to be very small. No fraction differed by more than 0.8 permil in its C13/C12 ratio from that of the total sample from which it was derived.