Molecular Geochemical Characteristics and Geological Significance of the Well B6 Crude Oil of the Tarim Basin

Multiple biomarker datasets and compound-specific sulfur isotopic compositions (δ34S) of dibenzothiophenes (DBTs) were analyzed for crude oil from Well B6 on the Maigaiti Slope, Tarim Basin. The very low concentrations of DBTs (124.9 μg/g oil), diamondoids (92.7 μg/g oil), and thiadiamondoids (0.20 μg/g oil), together with the absence of 25-norhopane, indicate that the B6 oil has not undergone significant secondary alteration, including thermochemical sulfate reduction (TSR), extensive thermal cracking, or biodegradation. No clear evidence of oil mixing was observed either. Aliphatic and aromatic biomarker distributions suggest that the parent source rocks contain type I–II1 kerogen, with dominant algal and bacterial organic inputs deposited under low-salinity, weakly reducing conditions, broadly comparable to those of the Upper Ordovician Lianglitag Formation source rocks (UOLS). Oil–source correlation using compound-specific δ34S values of DBTs indicates that B6 oil is derived from UOLS (or similar undiscovered source rocks), not from Cambrian source rocks. This is consistent with biomarker evidence. As the first identified Ordovician-derived oil showing relatively light DBT δ34S values (average ~6.41‰), close to those of Ordovician kerogen (average ~5.62‰), and with minimal secondary overprinting, B6 oil has strong potential to serve as a UOLS end-member oil. This will likely open new exploration opportunities for deep hydrocarbon from previously untapped strata in the southwestern Tarim Basin.