Study on the Relationship Between β-Carotane Abundance and Biodegradation: A Case Study of Crude Oils from the Mahu Sag, Junggar Basin

The crude oil along the periphery of the Mahu Depression in the Junggar Basin has generally undergone varying degrees of biodegradation, and a high abundance of β-carotane has been commonly detected in the crude oil. To further clarify the source of β-carotane in the crude oil and the relationship between its relative abundance and the degree of biodegradation, organic geochemical analytical techniques were employed to conduct a systematic geochemical molecular compound analysis of 47 oil samples collected from different stratigraphic levels and regions within the study area. In the Fengcheng Oilfield, the biodegradation degree of heavy crude oil across the entire study area consistently exceeded PM5. However, for all other areas outside the Fengcheng heavy crude oil zone, the biodegradation grades of the crude oil were either PM5 or below. The biodegradation grades in the slope zone ranged from PM0 to PM1, while those in the fault zone ranged from PM1 to PM5. Using parameters such as β-carotane/nCmax, Gammacerane/C30 hopane, and C24TeT/C26TT for oil source comparison analysis, it is interpreted that the crude oil in the study area is most likely derived from the contribution of halophilic green algae within the Fengcheng Formation source rock. The variation in β-carotane abundance in the crude oil is predominantly influenced by biodegradation processes: within the PM0–PM5 range, as the degree of biodegradation increases, the β-carotane-related parameters (β-carotane/nCmax, β-carotane/Pr) gradually rise and exhibit strong correlations with density and viscosity, enabling a quantitative characterization of the biodegradation degree within this range. At PM6, β-carotane begins to be degraded and is gradually depleted during this biodegradation phase. Crude oil biodegradation is more severe in fault zones; longitudinal variations indicate that the degree of biodegradation in Jurassic and Triassic oil samples increases as the burial depth decreases. In the Uerho area, some Permian-age crude oil samples with relatively high β-carotane abundance (indicating severe biodegradation) are still present—this is inferred to be attributable to the combination of the disruption caused by tectonic uplift during the early accumulation phase and subsequent burial beneath deeper strata. The relative content of β-carotane can be used to predict the physical properties of crude oil and the degree of biodegradation in different regions, providing geological theoretical insights and a basis for oilfield exploration and development.