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The origin of the marine oils in the Tarim Basin has long been a disputed topic. A total of 58 DST (drill stem test) crude oil and 8 rock samples were investigated using a comprehensive geochemical method to characterize and identify the origin of the Ordovician oils in the Tazhong Uplift, Tarim Basin, northwest China. Detailed oil-oil and oil-source rock correlations show that the majority of the oils have typical biomarker characteristics of the Middle-Upper Ordovician (O 2+3 ) source rock and the related crude oil. These characteristics include a distinct "V-shaped" relative abundance of C 27 , C 28 and C 29 regular steranes, low abundance of dinosterane, 24-norcholestanes, triaromatic dinosteroids and gammacerane. Only a few oils display typical biomarker characteristics indicating the Cambrian-Lower Ordovician (∈-O 1 ) genetic affi nity, such as linear or anti "L" shape distribution of C 27 , C 28 and C 29 regular sterane, with relatively high concentrations of dinosterane, 24-norcholestanes, triaromatic dinosteroids and gammacerane. It appears that most of the Ordovician oils in the Tazhong Uplift were derived from the O 2+3 intervals, as suggested by previous studies. However, the compound specifi c n-alkane stable carbon isotope data indicate that the Ordovician oils are mixtures from both the ∈-O 1 and O 2+3 source rocks rather than from the O 2+3 strata alone. It was calculated that the proportion of the∈-O 1 genetic affi nity oils mixed is about 10.8%-74.1%, with an increasing trend with increasing burial depth. This new oil-mixing model is critical for understanding hydrocarbon generation and accumulation mechanisms in the region, and may have important implications for further hydrocarbon exploration in the Tarim Basin.

There exists a petroleum system rich of oil and gas around Halahatang depression, where the oil and gas possess obvious local distinctions of properties in different parts. The research proved that the discovered crude oil and natural gas in the region derived mainly from O 2+3 source rock, and the differences of its properties were controlled by the oil and gas filling intensity. The comprehensive study result shows the oil and gas reservoirs of the region mainly underwent three important accumulation phases: late Caledonian-Early Hercynian epoch, late Hercynian epoch, and Yanshan-Himalayan epoch. In the first phase, the oil and gas derived mostly from Cambrian source rock, which formed the primary ancient oil reservoirs, then suffered strong degradation and remained a great quantity of pyrobitumen in the high position of Tabei uplift in the present. In the second phase, the O 2+3 source rock of Manjia'er depression started its generation of hydrocarbon, which accumulated in the high position of Tabei uplift afterwards, and then biodegradated to heavy oil in the late Hercynian epoch. In the last phase, the O 2+3 source rock of southern Halahatang depression and margin of Manjia'er depression started its peak of generating liquid hydrocarbon, which mostly accumulated in the trap formed before the Indo-China and Yanshan epoch, and in somewhere the heavy oil suffered dilutions in various degrees or serious gas invading, to lead to obvious crude oil divergence. marine facies, Mid-Upper Ordovician source rock, oil and gas source, accumulation stage, Halahatang depression, Tarim Basin

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