Nitrogen isotopic geochemical characteristics of crude oils in several basins of China

Nitrogen isotopic ratios of crude oils with different physical and chemical properties in several basins of China have been measured and their geochemistry has been studied elementally combined with molecular organic geochemistry methods. The data indicate that δ15N is in the range of -6‰–20‰. They vary with the oils in different sedimentary types of basins. The oils forming in the limnic environment have lower δ15N with the value of about 1‰–5‰ whereas the oils in saline or semi-saline environment have higher δ15N, with most samples being above 10‰ and some exceeding 17‰. The average nitrogen isotopic ratio of oils from the Ordovician formation in the Tarim Basin is the lowest, with most samples having δ15N below zero, which reflects possibly the characteristic of the oils originating mainly from the organic matter formed in marine carbonate sedimentary environment. The δ15N distribution can be altered by fractionation. It increases obviously because of biodegradation and decreases during hydrocarbon migration. However, the effect of nitrogen isotopic fractionation in the short distance migration is unconspicuous.