Stable Carbon Isotopic Composition of Selected Alkylnaphthalenes and Alkylphenanthrenes from the Tarim Oilfields, NW China

The present study aimed firstly to use a set of crude oil samples and a dataset to provide new evidence for source input contribution in selected aromatic isomers for discrimination of oils from three oilfields from Tarim Basin and identify the key factor (s) controlling the isotope composition. Thus, the present research showed that the delta C-13 values of alkylnaphthalenes and alkylphenanthrenes plotted against P/DBT and Ga/C30H ratios is a reliable and convenient tool for discrimination of organic matter variations in different oilfields. More importantly, molecular ratios and different diagram plots revealed that the selected oil samples would be derived from a mixing of indigenous organic matter from the terrestrial (in Kuqa area) and marine (in the cratonic area) depositional environments prior the apparition of the Yakela Faulted Uplift. Thus, Daolaoba, Yakela, and Tahe oils were made up of organic materials from both marine and terrestrial sources. Furthermore, marine organic matter input dominates oils from the Tahe and Yakela, with a minor input from terrestrial sources. The oils from Daolaoba were assigned to be from a mixing of marine and terrestrial material inputs. The controlling factors assessment revealed that biodegradation has an insignificant effect on the set of oils; however, the source input and the thermal maturity together control the isotopic compositions of individual aromatic isomers from these three oilfields.

Automated summary

This study provides new evidence for source input contribution and identifies the controlling factors of isotope composition in oils from three oilfields in the Tarim Basin. It suggests that the delta C-13 values of certain aromatic isomers plotted against specific ratios can effectively discriminate organic matter variations in different oilfields. The study also reveals that the selected oil samples are derived from a mixing of marine and terrestrial sources, with marine organic matter dominating the oils from Tahe and Yakela.