鄂尔多斯盆地南缘奥陶系露头沥青地球化学特征与意义

Geochemical characteristics and significance of bitumen of Ordovician outcrops at the southern margin of Ordos Basin

鄂尔多斯盆地南缘露头沥青赋存于下奥陶统亮甲山组灰岩裂缝中,裂缝近垂直于层面,沥青高约75 m,宽约1~44 cm,下宽上窄。其沥青质的碳同位素为-28‰,沥青等效反射率分布在1.829%~1.965%之间;沥青和上奥陶统背锅山组烃源岩均具有较低的Pr/Ph值、较高的C24四环萜烷的生物标志物,沥青的C29甾烷优势和低伽马蜡烷指数可能揭示其成烃母质形成于微咸水的低等海洋浮游生物的环境。沥青和背锅山组烃源岩规则甾烷组成均有＂V＂型特征,C24四环萜烷/C26三环萜烷、Tm/Ts值萜烷生物标志物参数值接近,表示他们具有亲缘关系。南缘奥陶系沥青地球化学特征及成因研究,对评价南缘下古生界碳酸盐岩的勘探潜力具有重要的理论和生产价值。

Outcrop bitumen occurs in the limestone cracks in the Lower Ordovician Liangjiashan Formation at the southern margin of Ordos Basin. The cracks are nearly vertical. The bitumen zone is about 75 m high and 1-44 cm wide,and is wide at the bottom and narrow on the top. The carbon isotope value of the bitumen is-28‰. Equivalent bitumen vitrinite reflectance ranges from 1. 829%-1. 965%. The bitumen and the source rocks in the Upper Ordovician Beiguoshan Formation have low Pr/Ph values and high C24 tetracyclic terpane biomarker concentrations. Bitumen samples with C29 sterane predominance and low gammacerane index reveal that hydrocarbon parent materials were formed in a low-brackish water environment from marine plankton. The composition of steroids is ＂V＂type,and the biomar-kers such as C24 tetracyclic terpanes/C26 tricyclic terpanes and Tm/Ts are similar,indicating a genetic relationship. The geochemical characteristics and significance of bitumen of the Ordovician outcrops at the southern margin of Ordos Basin have helpful for the evaluation of the Lower Paleozoic carbonate source rocks.