Characteristics and Genetic Type of the Oils in the Tazhong Uplift

The central uplift of Tarim Basin (Tazhong Uplift) has become a hot exploration target in western China. Totally 104 oil samples were sampled and analyzed by conventional geochemical as well as compound specific isotopic approaches. The oil were divided into four genetic types. Type-Ⅰ is characterized by relatively heavier values of n-alkanes carbon isotope (-29.6‰~-29.1‰) , and generally a reversed L shape of relative abundance of C27, C28 and C29 regular steranes with 4-metyl sterane and dinosteranes as well as gammacerane relatively enriched whereas diasteranes and diahopanes less developed. This type of oil shares typical features of Cambrian-low Ordivican derived oil (TD2, ∈-O1) in the Tarim basin. Type-Ⅱ has relatively lighter carbon isotope values of n-alkanes (-34‰~-35.6‰), and is poor in dinosteranes with V shape of C27, C28 and C29 regular steranes. This type of oil correlates well middle-upper Ordovician derived oil (YM2, O1) in the basin. Type-Ⅲ is distinguished from other oils by completely high concentrations of dibenzothiophene, which was primarily situated in TZ4 and TZ1-6 Blocks. Type-Ⅳ is mixed oil of type-Ⅰ and Ⅱ, and characterized by an intermediate carbon isotope value of type-Ⅰ and Ⅱ. This type oil is predominant in amount and widely distributed in the Tazhong Uplift. Oil -oil correlations showed that the oil analyzed was sourced from at least two sets of source rocks (i.e., ∈-O1 and O2+3 , respectively). The un-consistency of the biomarkers (indicating ∈-O1 genetic affinity ) and the compound specific compounds of n-alkanes (indicating more of O2+3 genetic affinity ) in some oils analyzed indicates that the oils were widely mixed with different sources. The physical and chemical properties of the oils vary with tectonic belt and horizons suggest such typical characteristics of composite basin as multi-source and multi-stage of hydrocarbons migration and accumulation of the oils as well as obvious heterogeneities of carbonatite reservoir rock. The oil-mixing model suggested in this study is significant for further oil exploration.