鄂尔多斯盆地西南部西峰地区三叠系延长组烃源岩检出丰富的两环烷烃，主要为 C12~C14和C15、C16两组两环烷烃。其相对丰度表现出3种源岩模式：①以低碳数两环烷烃为主，出现于长71和长81段非烃源岩；②两组两环烷烃都很丰富，出现于长73段富有机质烃源岩；③以高碳数两环烷烃为主，主要发现于长73段烃源岩，也见于长72和长81段。尽管长7段热演化程度基本一致，但补身烷异构化指数变化明显，表明补身烷重排不仅受热演化的影响，而且受有机质来源和沉积环境的控制。延长组烃源岩具有明显的高补身烷优势，反映了烃源岩的还原性沉积环境。烃源岩高丰度C15、C16两环烷烃的检出则指示该地区晚三叠世发育淡水湖泊。葡萄藻不仅是该地区中生界石油的重要母质来源，而且可能是这些两环类标志物的直接生源。%A lot of bicyclic alkanes are detected from source rocks of Triassic Yanchang Formation in Xifeng area of southwestern Ordos Basin.They are two groups of bicyclic alkanes mainly including C12-C14 and C15 ,C16 .The relative abundances of the two groups have three kinds of source rock patterns:①there are mainly bicyclic alkanes with lower carbon number,which are detected from non-source rocks of Chang-71 and Chang-81 sections;②the two groups both are very rich,and are detected from organic-rich source rocks of Chang-73 section;③there are mainly bicyclic alkanes with higher carbon number,which are detected from source rocks of Chang-73 , Chang-72 and Chang-81 sections.Although the thermal evolutions of Chang-7 section are similar,the change of drimane isomerization index is significant,so that drimane rearrangement is effected not only by the thermal evolution,but also by the source of organic matter and the depositional environment.The source rocks of Yanchang Formation have an obvious advantage of homodrimane, so that the depositional environment is reductive. C15 , C16 bicyclic alkanes of source rocks with high abundance are detected in the area,indicating that freshwater lake develops in Late Triassic. Botryococcus braunii is an important parent material of Mesozoic oil in the area,and also may be the direct biological sources of these bicyclic biomarkers.
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