Abstract: In the context of ultra-deep oil, biodegradation is often regarded as a secondary process. The detection of 25-norhopane compounds in ultra-deep oil from the Shunbei oilfield presents an opportunity to explore the role of biodegradation in ultradeep petroleum accumulation. This study analyzes petroleum geochemistry and accumulation using gas chromatographymass spectrometry, carbon isotope analysis, fluid inclusion analysis, single-well burial-thermal history, and thermal evolution simulations of source rocks. Gas chromatography, mass spectrometry, and carbon isotope analyses indicate that the oil in the Shunbei oilfield belongs to the same oil family. The oil exhibits high maturity, and the source rocks were formed in a weakly oxidizing-reducing environment with marine Type II or Ⅱ / Ⅲ organic matter as the primary contributor. Fluid inclusion and burialthermal history analyses reveal two primary periods of oil charging in the study area: the Late Caledonian and Indosinian periods. The detection of 25-norhopane compounds and complete n-alkanes in the crude oil further supports the occurrence of these two distinct oil charging periods. The integration of burial-thermal history with petroleum charging time suggests that the oil is predominantly a mixture of biodegraded oil from the Late Caledonian period and fresh oil from the Indosinian period. Therefore, biodegradation plays a crucial role in understanding the hydrocarbon accumulation processes in the Shunbei oilfield.

Key words: biodegradation, fluid inclusion, petroleum accumulation, Shunbei oilfield, Tarim Basin