Thermal maturity, source characteristics, and migration directions for the Ordovician oil in the Central Tabei Uplift, Tarim Basin: Insight from biomarker geochemistry

The central of Tabei Uplift, Tarim Basin in western China, including Rewapu, Yueman, and Fuyuan (RYF) blocks, can offer biomarker geochemistry of Ordovician oils and their valuable information for deep petroleum system. Biomarker analysis was performed on the Ordovician oils from the RYF using gas chromatography-mass spectrometry to constrain thermal maturity, oil source, and possible oil migration directions. The homogenization temperature of fluid inclusions and the burial history corrected by vitrinite reflectivity were used to reflect the oil charging periods. The oil samples were characterized by the complete occurrence of n-alkanes, dominated C-20-C-21-C-23 terpanes, low abundance of homohopanes in the m/z 191 mass spectrogram, and a dominance of C-29 among C27-29 regular steranes in the m/z 217 mass spectrogram. A weak and negligible biodegradation indicated by the biomarker signatures suggested favorable preservative conditions for the oils. Biomarker maturity parameters indicated that the oils were mainly in the late "oil window." The oils were suggested to be obtained from reducing marine sedimentary environments and belong to the same crude oil family. The ratios related to the triaromatic steroids indicated that the Cambrian-Lower Ordovician and Middle-Upper Ordovician source rocks contributed to the Ordovician petroleum system, and the Middle-Upper Ordovician source rocks were likely the major contributors for the current oil samples. The filling tracers suggested that the main oil migration was from southwest to northeast along the Shunbei No. 1 fault rather than from the Manjiaer Depression in the southeast.