Genesis features of crude oil in Ordovician, Yubei area, Tarim Basin

The Ordovician oil in Yubei area has middle density,low freezing point and high waxy content.The distribution of normal alkane is complete while the baseline of saturated hydrocarbon chromatogram shows various degrees of excursion.It shows upside-down phenomenon that the δ¹³C value of nonhydrocarbon and asphaltene is smaller than that of saturated and aromatic hydrocarbon.All of these indicate that the reservoir underwent no less than two periods of oil charging.The early charging oil experienced strong biodegradation and mixed with the later higher mature charging oil.Geochemical properties are mainly attributed to the late charging oil with weak phytane predominance (Pr/Ph≤1.0),abundance of gammacerne (G/C₃₀-hopanes 0.14-0.24),and higher C₃₅-hopane/ C₃₄-hopane value (>1.0) characteristics,showing reduction-strong reduction sedimentary environment.The predominance of n-heptane (46.47%-48.57%),and C₂₃-tricyclic terpane and higher tricyclic terpane/hopane indicate that the original source materials of later oil are homonemeaes.High heptane value and tetramethylnaphthalene index suggest that the oil is in high mature stage and probably generated from the Cambrian source rock.