准噶尔盆地南缘油气生成与分布规律——烃源岩地球化学特征与生烃史

准噶尔盆地南缘发育二叠系、三叠系、侏罗系、白垩系和古近系5套可能的烃源岩,并在很多构造圈闭发现了不同物理化学性质的油气。长期以来对该地区有效烃源岩及所发现油气的来源存在很大争议。通过对南缘地区24条地面剖面及28口探井烃源岩岩心系统采样分析研究认为,不仅二叠系与侏罗系是南缘地区重要的烃源岩,三叠系是可能的烃源岩,白垩系与古近系也是非常重要的烃源岩。二叠系烃源岩有机质丰度很高、类型好,以I、II型有机质为主;三叠系与侏罗系烃源岩有机质丰度变化大,且类型较差,以II、III型有机质为主;白垩系和古近系烃源岩有机质丰度中等,但有机质类型好,以I、II型有机质为主。5套烃源岩目前成熟度差异较大,二叠系、三叠系、侏罗系烃源岩处于低成熟-高、过成熟阶段,白垩系烃源岩处于未成熟-高成熟阶段,古近系烃源岩处于未成熟-成熟演化阶段。5套烃源岩大量生烃时期明显不同:中二叠统烃源岩主要在晚侏罗世-古近纪,侏罗系在晚白垩世-新近纪;白垩系从始新世延续现今,在上新世初达到生油高峰;古近系中新世末期进入生油门限开始生油,目前仍未达生油高峰。白垩系在南缘中部地区为有效生烃源岩,古近系在南缘西部地区是有效的生油源岩。

Geochemcial features of source rocks in the southern margin,Junggar Basin,Northwestern China

There are five sets of possible source rocks developed in the southern margin, Junggar Basin, i.e., Permian, Triassic, Jurassic, Cretaceous and Paleogene source rocks. Meanwhile, hydrocarbons with different physical and chemical properties are discovered in many structural traps. However, there always exists a great dispute on effective source rocks and source of the oil and gas discovered in research area. Based on systematic sampling analysis for cores of the source rocks from 24 field sections and 28 wells, this study found the important Permian and Jurassic source rocks, possible Triassic source rock, as well as the all-important Cretaceous and Paleogene source rocks. Permian source rock possesses high abundance and good type of organic matter, dominated by type I and II; Triassic and Jurassic source rocks have a great change in organic matter abundance, dominated by the poorer organic types of Ⅱand Ⅲ; the Cretaceous and Paleogene source rocks have medium organic matter abundance, dominated by the good organic type of I and II. These source rocks have a significant difference in maturity: Permian, Triassic and Jurassic source rocks have been developing from the low-mature, to high- and post-mature stage; Cretaceous source rock has entered in immature-high mature stage, while the Paleogene source rock in immature-mature stage. There is also an obvious difference in the period when source rocks begin to generate a large amount of hydrocarbon. The major hydrocarbon generation stages of Middle Permian and Jurassic source rocks are Late Jurassic-Paleogene and Late Cretaceous-Neogene respectively. Cretaceous source rock has been generating hydrocarbon continuously from the Eocene until now, and reached a peak in the Early Pliocene; Paleogene source rock entered into oil generation threshold in the Late Miocene, but has not reached the oil generation peak so far. The effective Cretaceous source rock is distributed in the middle of southern margin, whereas the effective Paleogene source rock is developed in the west of southern margin.