渤海湾盆地牛庄洼陷南斜坡油砂烃来源分析

对渤海湾盆地东营凹陷牛庄洼陷南斜坡不同埋深油砂及其相邻烃源岩的分析表明,埋深<2700m的油砂烃化学组成与相邻沙四段页岩抽提物有显著差异,两者不具相关性,油砂中的烃类主要为深部的运移烃。油砂烃具有常规油的气相色谱特征,而未熟-低熟页岩在高碳数部位有一明显的甾萜类等高分子量化合物未分辨鼓包,且正构烷烃奇偶优势明显;油砂烃含高丰度的"地质型"甾烷,而未熟-低熟页岩中C29甾烷αββ20S含量甚微,但热稳定性低的化合物5β(H)甾烷及13α(H),14α(H)-三环萜烷含量较高;油砂烃几乎不含甲藻甾烷,而未熟-低熟页岩中含丰富的此类化合物。由深至浅,油砂烃成熟度依次降低,伽玛蜡烷相对含量增高,升藿烷"翘尾"现象变得明显。油砂烃的成熟度梯度及烃类组成的规律性变化不仅可以指示油气运移的大致方向,而且揭示出不同演化阶段烃源岩的生烃特征有别和/或油气运移过程中未熟-低熟烃源岩沥青的混合作用。

ORIGIN OF HYDROCARBONS IN OIL SAND IN THE SOUTH SLOPE OF NIUZHUANG SAG,THE BOHAIWAN BASIN, EAST CHINA

Correlation of the hydrocarbons in the oil sand and the adjacent Es4 shale extracts from the south slope of Niuzhuang Sag,Bohai Bay Basin shows obvious differences between the oil sand with buried depth less than 2 700 m and the immediately adjacent Es4 shales which was originally considered as the main oil source rocks in the south slope,suggesting the hydrocarbons in shallow oil sand samples are hydrocarbons migrating from the deep seated source rocks.Hydrocarbons in the oil sand samples have normal GC fingerprints,while the adjacent shale extracts with buried depth less than 2 700 m are characteristic of high odd-even predominance of normal alkane,and a distinct bulge composed of undistinguished compounds of steroids and terpanoids was observed in the behind section of the GC fingerprint with high retain time.The low-matured Es4 shales with buried depth less than 2700m also have distinct features including low concentration of geological stereochemical configuration(i.e.20R20S and ααααββ) and such the high content of compounds with low thermal stability as 5-β steranes,13α(H)14α (H)-tricycle terpanes.In addition,little amount of dinosteranes were discovered in all of the oil sand samples while dinosteranes are the main constitution in Es4 shale above normal oil window,further indicating no possibility of the algae-rich Es4 interval with buried depth less than 2 700 m to be the source rocks of the oils in the area.With the buried depth decreasing,the maturity of hydrocarbons in oil sand samples and the level of C₃₅-/C₃₄-hopane increased and the concentration of Gamacerance decreased.These can not only indicate the direction of oil migration,but also reveal the different chemical compositions of hydrocarbons generated by the source rocks with different thermal stages and/or mixing of immature oils or bitumen with normal oils.