Abstract
The preliminary study was carried out on the carbon isotopic ratios of about 30 samples of crude oil obtained from some oil fields in Akita and Niigata prefectures, northern Japan.
The reservoir strata of these oil fields are mainly composed of sandstone and mudstone except a few cases of volcanic tuff and of dolomite, and these rocks are ranging from Miocene to Pliocene in age.
Massspectrometric measurement of carbon isotopic ratios is performed by the method proposed by McKinney and Craig with a little modification.
Carbon isotopic ratios show rather uniform distribution through the samples and the δ-value in P.D.B. scale have an average of-23.7‰ with the range from-24.7 to-22.2‰. These features are closely resembled to those of oil from North America that sammarized by Degens. (E.T. Degens, 1969).
The importance of carbon isotopic investigations on the separate fractions of distillation of crude oils is pointed out for the discussions on the origin of petroleum.
