Geochemical characteristic of different-lithofacies source rocks and its implications for ultradeep hydrocarbon exploration in the lower cambrian Yuertus formation, Tarim basin

The Tarim Basin harbors abundant deep to ultra-deep hydrocarbon resources, yet detailed oil-source correlation remains to be further investigated. As a potential key source rock, the organic geochemical characteristics of the Yuertus Formation (Є₁y) warrant additional research. This study integrates biomarker, carbon, and sulfur isotope data from Є₁y source rocks in existing wells and outcrops across the eastern, northern, and northwestern Tarim Basin to better constrain the distribution, hydrocarbon generation potential, biomarker features, and oil-source relations of Є₁y. Results reveal that Є₁y exhibits diverse lithofacies, including calcareous mudstones, siliceous shales, and shales as potential source rocks. The calcareous mudstone shows the highest total organic carbon (TOC) content (up to 29.8%), characterized by Type II kerogens and mature to over-mature thermal maturity. Stable biomarkers including triaromatic steroids (TAS), triaromatic dinoflagellate steroids (TDSI), and aryl isoprenoids (1-alkyl-2,3,6-trimethylbenzenes, ATMBs) reveal significant differences among the three lithological types of source rocks: Calcareous mudstones exhibit biomarker distribution patterns similar to those of classical Ordovician source rocks (C₂₆R + C₂₇S TAS < C₂₈S TAS, TDSI < 0.6), while simultaneously containing high abundances of ATMBs characteristic of the Є₁y. Siliceous rocks display typical Cambrian characteristics with low C₂₈ TAS, high TDSI, but absence of ATMBs. Shales present typical Cambrian features with low C₂₈ TAS, high TDSI, and low ATMBs. Oil-source correlation based on n-alkane' carbon and sulfur isotopes suggests that calcareous shales are likely one of the primary sources for currently explored oils. These findings enrich the fundamental understanding of deep and ultra-deep hydrocarbon exploration in the Tarim Basin and provide new insights into the reassessment of oil-source relations.