准噶尔盆地烃源岩与原油地球化学特征

Geochemical Features of Source Rocks and Crude Oil in the Junggar Basin,Northwest China

准噶尔盆地是中国西部典型的多旋回叠合盆地,发育石炭系、二叠系、三叠系、侏罗系、白垩系和古近系6套烃源岩,广泛分布于盆地不同地区。石炭系海陆交互相烃源岩分布于盆地大部分地区,泥岩有机质丰度中等,以Ⅱ型有机质为主,碳质泥岩和煤有机质丰度高、类型差。二叠系湖相烃源岩分布广、厚度大,有机质丰度高、类型好,干酪根碳同位素组成轻,是盆地中最主要的烃源岩。三叠系湖相烃源岩在盆地东部有机质丰度较高、以Ⅱ型有机质为主。侏罗系为煤系烃源岩,有机质丰度高,但以Ⅱ_2型和Ⅲ型有机质为主,干酪根碳同位素组成重。白垩系和古近系湖相烃源岩主要分布于盆地南部,有机质丰度中等,但以Ⅰ、Ⅱ型有机质为主。石炭系烃源岩目前主要处于高—过成熟阶段,二叠系、三叠系烃源岩主要处于成熟—高成熟阶段,侏罗系烃源岩在大部分地区未成熟—低成熟,只在南部与东部坳陷达到成熟—高成熟,白垩系和古近系烃源岩目前也只在南部坳陷处于低成熟—成熟演化阶段。准噶尔盆地目前发现了6大类典型原油,其地球化学特征存在很大差异。第一类原油碳同位素组成特别重(δ^(13)C>-26‰);第二类原油碳同位素组成轻(δ^(13)C<-29‰)、含有丰富的胡萝卜烷、类异戊二烯烷烃、三环萜烷和伽马蜡烷,甾烷以C_(28)、C_(29)为主,基本没有重排甾烷;第三类原油碳同位素轻(δ^(13)C<-29‰)、重排甾烷、Ts、C_(29)Ts及C_(30)重排藿烷异常丰富;第四类原油碳同位素组成重(δ^(13)C-28‰^-26‰),Pr/Ph大于2.5,三环萜烷含量低且以C_(19)、C_(20)为主,藿烷系列丰富,伽马蜡烷极低,甾烷以C_(29)规则甾烷及重排甾烷为主;第五类原油碳同位素组成轻(δ^(13)C<-29‰)、Pr/Ph<1.0、伽马蜡烷丰富且有两个异构体、Ts、C_(29)Ts、C_(30)重排藿烷、C_(27)~C_(29)异胆甾烷及C_(30)甲基甾烷十分丰富;第六类原油主要为中低成熟油,碳同位素组成重(δ^(13)C-28‰^-26‰),C_(27)、C_(28)、C_(29)甾烷呈Ⅴ型分布,甲藻甾烷异常丰富。第二类原油广泛分布于盆地不同区域,其他类型原油均只分布于盆地局部地区。西北缘地区以第二类原油为主,可分为3个亚类;腹部地区以第二类原油为主,可分为4个亚类,还有少量第四类原油;东部地区有前4类典型原油,此外还有混合原油;南缘地区目前发现有第二、第四、第五及第六类4种典型原油,也有少量混合原油。

The Junggar Basin, a typical polycyclic superimposed basin in northwestern China, develops six sets of source rocks （Carboniferous, Permian, Triassic, Jurassic, Cretaceous, and Paleogene） which are widely spread. The Carboniferous marine-terrestrial facies source rocks are distributed in most areas of the basin. The mudstone is composed mainly of Type Ⅱ organic matter and has a medium abundance. The carbonaceous mudstone and coal have a high abundance but low-quality organic matter. The Permian lacustrine source rocks are widely spread, large in thickness, abundant in organic matter and high-quality in types. The kerogen has light carbon isotope and is the main source rock in the basin. The Triassic lacustrine source rocks in the eastern part o{ the basin, mainly composed of Type Ⅱ organic matter, have a relative high abundance in organic matter. The Jurassic coal measures, mainly composed of Type Ⅱ 2 and Type Ⅲ organic matter, are organic-rich source rock. The kerogen has heavy carbon isotope. The Cretaceous and Paleogene lacustrine source rocks, distributed mainly in the southern part of the basin, have a medium abundance of organic matter, which are mainly composed of Type I and Type Ⅱ organic matter. The Carboniferous source rocks are now mainly at high- and over-mature stages. The Permian and Triassic source rocks are now mainly at mature and high-mature stages. The Jurassic source rocks in most areas are at immature and low-mature stages and only those in the southern and eastern depressions are at mature and high-mature stages. As for the Cretaceous and Paleogene source rocks, only those in the southern depressions are now at low-mature and mature stages. To date, six groups of typical crude oils have been identified in the Junggar Basin and their geochemical features are quite different. The group I crude oil has a very heavy carbon isotope composition （δ13C〉-26‰）. The group Ⅱ crude oil has a light carbon isotope composition （δ13C〈-29‰）. It contains abundant carotenoids, isoprenoids, tricyclic terpane and gammacerane. Steranes are dominated by C28 and C29 and there is nearly no diasteranes. The group Ⅱ crude oil has a light carbon isotope composition （δ13C〈-29‰） and is abundant in diasteranes, Ts, C29Ts and C30 diahopane. The group IV crude oil has a heavy carbon isotope composition （δ13C -28‰ ～-26‰） and Pr/Ph〉2.5. The tricyclic terpanes are at low levels and is mainly composed of C19 and C20. Hopanes are extremely abundant, but gammacerane is at an extremely level. Steranes are composed mainly of C29regular steranes and diasteranes. The group V crude oil has a light carbon isotope composition （δ13C〈-29‰） and Pr/Ph%1. 0. The gammacerane is abundant and has two isomers. The Ts, C29Ts, C30 diahopane, C27～C29 isocholestanes and C30methyl steranes are also very abundant. The group VI crude oil with low- and medium-mature oil has a heavy carbon isotope composition （δ13C -28‰～- 26‰）. The C27, C28 and C29 steranes are distributed in V-shape. The C30 methyl steranes and dinosteranes are extremely abundant. The group Ⅱ crude oil is distributed widely in different regions of the basin while the other groups of crude oils are limited only to some local areas of the basin. The northwestern margin of the basin is dominated by the second group of crude oil, which can be divided into three subgroups. The central region of the basin is characterized mainly by the second group of crude oil, which can be divided into four subgroups. It also has a small amount of the fourth group oil. The eastern region contains the first four groups of typical crude oils and mixed crude oil. Four groups of typical crude oils （i. e. Groups Ⅱ , Ⅳ, Ⅴ and Ⅵ） and a minor amount of mixed crude oils have been found in the southern margin.