Influences of carbon isotope fractionation effects on identification of deep crude oil sources

Abstract

Deep source rocks undergo high degrees of evolution， leading to a significant decrease in biomarker content in crude oil as maturity increases. The decrease affects the reliability of the oil source correlation with conventional biomarker parameters. To better use the δ13CPDB value for identifying deep crude oil sources， authors analyzed the δ13CPDB values of four group components in the soluble organic matter from naturally evolved source rock profiles in Dongying Sag， Jiyang Depression， Bohaiwan Basin and the products of physical experiments on hydrocarbon generation and expulsion. Considering the relative content of alkane， authors discussed the carbon isotope fractionation effects in the crude oil from deep source rock. Additionally， authors proposed the working thought and procedure for identifying the source of deep oil with the δ13CPDB values. The carbon isotope fractionation effects resulting from high thermal maturity are obvious in deep source rocks. The δ13CPDB values of the four group components of soluble organic matter from source rocks （alkane， aromatics， non-hydrocarbon， and asphaltene） exhibit a significant elevation with increasing burial depth. Specifically， the δ13CPDB values of alkane exhibit the most pronounced change， exceeding 5‰， while aromatics show a change of around 4‰. The changes in the δ13CPDB value of non-hydrocarbon and asphaltene are relatively small， ranging from 2‰ to 3‰. As burial depth increases， the alkane contents in soluble organic matter increase from around 40% to over 80%. Correspondingly， the δ13CPDB values of alkane increase from around -30‰ to over -24‰， and the alkane contents show a great correlation with the variations of its δ13CPDB values. Similar results can also be observed from the δ13CPDB values of group components from physical experiments on hydrocarbon generation with the same type of source rock samples. In these experiments， the variations in the δ13CPDB values of alkane can reach 4‰， while those in aromatics， non-hydrocarbon， and asphaltene are relatively low， ranging from 1‰ to 3‰. When utilizing carbon isotopes to identify the source of deep oil， it is essential to first check the correlation between alkane contents and their δ13CPDB values to identify if the carbon isotope fractionation effects derived from thermal maturation exists. Subsequently， the δ13CPDB values of alkane should be properly recovered before utilizing it to identify the source of the oil. According to the abovementioned methods， authors identified the source of light oil from Chepaizi Uplift in the Western region of Junggar Basin. It confirmed the presence of carbon isotope fractionation effects in the light oil derived from thermal evolution. It shows a great possibility that the light oil is sourced from the underlying high-maturity source rock of the Permian， which is of important practical significance in the evaluation and deployment of the petroleum exploration target in this area.

Keywords

• deep formation
• high-mature oil
• carbon isotope fractionation
• source of crude oil
• light oil in chepaizi area