Sulfur Isotope Abundances and Genetic Relations of Oil Accumulations in Middle East Basin

Abstract

A sulfur isotope study was made of sulfur in crude oils from various northern Iraq oil fields and of sulfate sulfur of gypsum and anhydrite deposits in the Arabian Gulf-Iraq-Iran basin. The sulfur isotope ratios (S34/S32) of 42 oil samples from Tertiary and Cretaceous reservoirs were found to be very uniform, the average ^dgrS34 value being --5.4 ^pmil, relative to Canyon Diablo troilite. By contrast, three oils from reservoir rocks of Triassic age showed a distinctly different S34/S32 ratio, the average ^dgrS34 being +2.4 ^pmil. The isotopic composition of oils from different depths--one above the other--is particularly uniform. In the Kirkuk field, oils from four levels--Tertiary and Upper, middle, and Lower Cretaceous--are identical within the reproducibility of the determinations (^dgrS34 = --5.5 ^pmil). In the Bai Hassan field, ^dgrS34 = --7.25 ^pmil for oils from the same four levels. These findings give strong support to Dunnington's view that the major oil accumulations in Cretaceous and Tertiary traps in northern Iraq originated from a common source. The sulfur isotopic composition of contemporaneous marine sulfates can be considered representative of the level from which isotope fractionation started. As a result of fractionation, reduced sulfur present as organic compounds in the oils is depleted in S34 relative to the sulfate from which it was formed. On the basis of ^dgrS34 values for Jurassic, Cretaceous, and Tertiary evaporites of the Middle East, a Jurassic origin for the oils is most favored, and a Tertiary origin is least likely.