原油中C_(26)-C_(28)三芳甾烷相对含量计算方法及其应用

Determining the relative abundance of C_(26)-C_(28) triaromatic steroids in crude oils and its application in petroleum geochemistry

三芳甾烷(TAS)是规则甾烷芳构化产物,其中C_(26)-C_(28)三芳甾烷系列稳定分布于原油和烃源岩有机质中。在常规色谱-质谱分析得到的芳烃馏分(m/z 231)质量色谱图上,由于C2620R和C2720S TAS异构体共流出而不能确定3种不同碳数三芳甾烷的相对含量,无法建立类似于C27-C29规则甾烷的相对含量三角图,从而使三芳甾烷在油气地球化学中的应用受到限制。以塔里木盆地台盆区古生界代表性原油样品为例,建立了三芳甾烷相对含量三角图,并用北部湾盆地福山凹陷古近系原油进行了验证。结果表明:该区存在2种不同来源的原油,可以划分为2个不同的原油族群,而来自福山凹陷古近系的原油具有不同的C_(26)-C_(28)三芳甾烷组成特征,原油族群的划分也得到其他分子地球化学证据的支持。在高成熟的轻质油和凝析油中,规则甾烷系列由于浓度低而很难由常规的色谱-质谱分析技术进行检测和鉴定,三芳甾烷由于相对较高的热稳定性而稳定存在,因此可代替规则甾烷用于油-油对比和原油族群划分。

Triaromatic steroids（ TAS） may be formed by the aromatization of regular steranes. C26-C28 TAS widely occur in crude oils and source rocks. The C2620 R and C2720 S TAS typically co-elute with each other in m/z 231 mass chromatograms of aromatic fractions. Therefore,the relative abundance of C26,C27 and C29TAS cannot be determined by routine GC-MS analysis. A ternary diagram similar to that of C27-C29-C29 regular steranes cannot be established to classify oil families,which leads to the limited use of TAS in petroleum geochemistry. This paper proposed a simple method to estimate the relative abundance of homologues and isomers of TAS on the basis of the distribution patterns,origin and thermal evolution of TAS isomers. Taking representative Paleozoic oil samples from the tectonic region of the Tarim Basin as an example,this paper distinguished two petroleum populations on the ternary diagram of C26-C27-C29 triaromatic steroids determined by routine GC-MS analysis. The oils from the Fushan Sag in the Beibuwan Basin（ South China Sea） have different distribution patterns of TAS. The oil family classification is also consistent with that by other molecular markers. For their relatively higher thermal stabilities,triaromatic steroids are generally present in significant abundance in oils and condensates with high maturity. Therefore,this ternary diagram of aromatic steroids is a potential geochemical indicator for oil-to-oil correlation and oil family classification as a substitute for that of regular steranes.