Effects of source facies and maturity on individual carbon isotopic compositions of oil

Abstract

Carbon isotopes have been used extensively in tracing the sources of oil. However, primary source facies and secondary alteration controls on oil isotopic compositions have not been well resolved, resulting in application uncertainties. A case study was undertaken for an alkaline lacustrine oil system in a lower Permian formation in the Junggar Basin, NW China. Results indicate that increasing maturity causes the carbon isotopic composition to become heavier for only short–middle-chain compounds, whereas source facies-related carbon assimilation controls the compositions of short-, middle-, and long-chain compounds. In particular, light-carbon assimilation during organic-matter degradation makes the isotopic composition lighter, whereas heavy carbon from the water mass makes it heavier. Accordingly, oils in this study area were divided into Type U and Type N oils based on individual compound carbon isotopic compositions, reflecting the difference in source facies in a highly saline and reducing stratified water environment. The results provide a better understanding of the controls on carbon isotopes in oil in sedimentary basins, reducing the uncertainty in oil–source correlation and addressing the origin of oil.