准噶尔盆地玛南地区原油地球化学特征及成因

Geochemical characteristics and genesis of crude oil in Ma′nan area,Junggar Basin

准噶尔盆地玛南地区在二叠系、三叠系及侏罗系多层系均取得很好的勘探成果,不同层系原油密度、凝固点差异较大,原油密度分布在0.8054~0.9071 g/cm3之间,凝固点分布在-29.6~24.4℃之间,但相关原因尚不清楚。依据原油生物标志物及正构烷烃单体碳同位素将该地区原油分为4类:Ⅰ类原油Pr/Ph值分布在0.99~1.20之间、β-胡萝卜烷和类异戊二烯烃含量较高,Pr/n C17和Ph/n C18>0.7、Ts含量低,Ts/Tm值主体小于0.3,藿烷/甾烷<1.0,C27/C28甾烷值分布在0.52~0.70之间,C27/C29甾烷值分布在0.36~0.57之间,单体烃碳同位素值曲线呈水平分布(-31.1‰~-27.7‰),不同碳数间碳同位素值最大偏差分布在0.3‰~1.6‰之间,来源于偏还原的风城组二段烃源岩;Ⅱ类原油Pr/Ph>1.20、β-胡萝卜烷和类异戊二烯烃含量低、Pr/n C17和Ph/n C18<0.5、Ts含量高、Ts/Tm>0.3,藿烷/甾烷>1.2,C27/C28甾烷值分布在0.38~0.64之间,C27/C29甾烷值分布在0.25~0.39之间,单体烃碳同位素值曲线呈山谷分布(-36.2‰~-30.7‰),不同碳数间碳同位素值最大偏差分布在4.4‰~5.1‰之间,来源于弱氧化—弱还原的风城组三段烃源岩;Ⅲ类原油为Ⅰ类和Ⅱ类原油混源油,单体烃碳同位素值曲线呈山谷分布(-34.0‰~-28.4‰),但不同碳数间碳同位素最大偏差相对小,分布在2.6‰~3.6‰之间,主要是Ⅰ类原油不同比例混入Ⅱ类原油导致不同碳数正构烷烃间碳同位素变化幅度降低;Ⅳ类原油为生物降解原油,正构烷烃基本消失,β-胡萝卜烷和类异戊二烯烃丰度高,甾烷和萜烷分布与Ⅰ类原油相似,来源于风城组二段烃源岩。玛南地区原油物性差异主要受到成熟度、生物降解、生源和混合作用的控制。成熟度是造成原油物性变化的最主要因素,随着成熟度增大,原油密度逐渐变轻,而全油碳同位素组成逐渐变重,三环萜烷/藿烷值和Ts/Tm值逐渐增大。生物降解作用导致Ⅳ类原油中正构烷烃组分降解,原油中异构烷烃和环烷烃富集,含蜡量和凝固点降低,并伴随着密度增大。由于风城组二段烃源岩类型好于风城组三段烃源岩,且有机质中藻类贡献相对更多,导致Ⅰ类原油生成的异构烷烃含量高,其含蜡量和凝固点较Ⅱ类原油低,而混合作用造成Ⅲ类原油物性介于Ⅰ类和Ⅱ类原油之间。

The Permian,Triassic and Jurassic oil-bearing strata in Manan area of Junggar Basin have achieved good exploration results.The crude oil density and freezing point in different strata vary greatly,with the distri⁃bution of crude oil density ranging from 0.8054 g/cm3 to 0.9071 g/cm3,and the distribution of freezing point ranging from−29.6℃to 24.4℃,but the reasons have not been studied.According to biomarkers and carbon isotopes of n-alkane monomer,the crude oil in this area is divided into four types.(1)TypeⅠcrude oil are de⁃rived from source rocks of the second member of Fengcheng Formation,which formed in a reduced sedimentary environment,with Pr/Ph ratio ranging from 0.99 to 1.20,high content ofβ-cartane and isoprenes,Pr/nC17 ra⁃tio and Ph/nC18 ratio are basically higher than 0.7,low content of Ts,Ts/Tm ratio is mainly less than 0.3,hopanes/steranes ratio are less than 1.0,C27/C28 sterane ratio are between 0.52 to 0.70,C27/C29 sterane ratio are between 0.36 to 0.57.The carbon isotope curve of monomer hydrocarbon shows a horizontal distribution(−31.1‰to−27.7‰),and the maximum deviation among different carbon numbers are between 0.3‰and 1.6‰.(2)TypeⅡcrude oil are derived from source rocks of the third member of Fengcheng Formation,which formed in weakly oxidized and weakly reduced sedimentary environment,Pr/Ph ratio are basically higher than 1.20,low content ofβ-carotene and isoprene,Pr/nC17 ratio and Ph/nC18 ratio are basically less than 0.5,Ts con⁃tent was high,Ts/Tm ratio are higher than 0.3,hopanes/steranes ratio are higher than 1.2,C27/C28 sterane ra⁃tio are between 0.38 to 0.64,C27/C29 sterane ratio are between 0.25 to 0.39,and the carbon isotope curve of monomer hydrocarbon showed a valley distribution(−36.2‰to−30.7‰),the maximum deviation among dif⁃ferent carbon numbers are between 4.4‰to 5.1‰.(3)The distribution of typeⅢcrude oil is mixed source oil of typeⅠand typeⅡcrude oil,and the carbon isotope curve of monomer hydrocarbon showed a valley distribu⁃tion(−34.0‰to−28.4‰),but the maximum deviation among different carbon numbers are relatively small,ranging from 2.6‰to 3.6‰,the main reason is that different proportions of typeⅠcrude oil mixed with typeⅡcrude oil lead to the decrease of carbon isotope variation among n-alkanes.(4)TypeⅣcrude oil are derived from source rocks of the second member of Fengcheng Formation,suffering from mild biodegradation,normal alkane is basically disappeared,β-carotene and isoprene are abundant,the distribution of steranes and terpenes is similar to that of typeⅠcrude oil.The difference of crude oil physical properties in Manan area is mainly con⁃trolled by maturity,biodegradation,biogenesis and mixing.Maturity is the most important factor that causes the change of physical properties of crude oil.With the increase of maturity,crude oil density gradually becomes lighter,carbon isotope of whole oil gradually becomes heavier,and tricyclic terpenes/hopanes and Ts/Tm grad⁃ually increase.Biodegradation results in the degradation of n-alkanes in type IV crude oil,enrichment of isopar⁃kanes and cycloalkanes,decrease of wax content and freezing point,and increase of density.Because the source rock type of the second member of Fengcheng Formation is better than that of the third member of Fengcheng Formation,and the content of algae in organic matter is relatively more,the isoparkanes content in type I crude oil is higher,resulting in lower wax content and freezing point than that of type II crude oil.The physical proper⁃ties of type III crude oil are between those of type I crude oil and type II crude oil due to mixing.