塔中隆起原油特征与成因类型

塔里木盆地塔中隆起油气性质多样、分布与成因复杂, 为揭示油气的特征与成因, 对塔中及外围104个原油样品进行了精细地球化学研究.依据单体烃碳同位素、特征生物标志物分析, 将塔中原油分为4种类型: (1) 寒武系成因原油, 具有较重正构烷烃单体烃碳同位素(-29.6‰~-29.1‰)、甲藻甾烷较发育及C₂₇、C₂₈、C₂₉规则甾烷呈反“L”型或线型分布等特征; (2) 中、上奥陶统成因原油, 具有较轻的正构烷烃单体烃同位素(-34‰~-35.6‰)、甲藻甾烷等不太发育与C₂₇、C₂₈、C₂₉规则甾烷呈“V”型分布等特征; (3) 富含含硫芳烃-二苯并噻吩原油, 主要分布于塔中4井区; (4) 混源油, 单体烃碳同位素特征界于Ⅰ、Ⅱ类原油之间, 是塔中最为主要的原油类型.油-油对比与油气性质分析表明, 塔中地区至少有两套主力烃源岩供烃.塔中部分原油生物标志物显示寒武系-下奥陶统成因特征, 而单体烃碳同位素却与中上奥陶统成因原油更为接近, 这种不同馏分的不一致现象系不同成因原油混源的结果, 反映单一应用生物标志物指标有其局限性.塔中油气性质具有分带、分块、分层特征, 反映叠合盆地多源、多期成藏、储层非均质性等多种特性. 

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.