Abstract
The sulfur content and sulfur isotopic composition of crude oils have been determined for 82 samples from the oil-field region of northeastern Japan.The sulfur content of crude oils lies in the range of 0.01 to 0.56wt% with a mean value of 0.28wt% and the sulfur content of condensates is usually less than that of oils. The δ34S value of crude oils ranges from -4.6 to +13.3‰ relative to CDT. The δ34S values in the Akita district show a bimodal pattern and those in the Niigata district are divided into three groups. Oils from the Iwafune-oki oil field have the heaviest δ34S value, while crude oils from the West structure of the Aga-oki oil and gas field have the lightest.Crude oils from each field are characterized by a narrow range of δ34S values, although both sulfur content and δ34S value of crude oil are affected by secondary alteration processes such as water washing and biodegradation. The δ34S value of each field shows both regional and local variations which may be attributed to the heterogeneity of sulfur in source rock. These facts suggest that the δ34S value of crude oil is useful for oil-to-oil correlations.
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