在珠江口盆地珠一坳陷原油中检测到4个C24四环萜烷, 其中已知的3个化合物分别为10β(H)-脱-A-奥利烷(峰A)、10β(H)-脱-A-羽扇烷(峰B1)和17, 21-开环藿烷(峰C), 化合物B的确切结构目前仍然未知.该化合物分子离子为m/z 330, 表明它是一个C24四环萜烷, 基峰为m/z 191, 这与五环三萜烷分子结构中D/E环的断裂有关, 特征碎片m/z 287的出现证实其分子结构中存在一个异丙基, 可见它可能是某个五环三萜烷脱A环后的产物.与羽扇烷类标志物一样, C30藿烷的E环上也存在一个异丙基, 而脱-A-羽扇烷B1峰的确认排除了化合物 B 与羽扇烷类标志物有关的可能性.鉴于在所分析的煤成油中化合物 B 的含量极低, 而湖相原油中较为丰富, 且其相对丰度负相关于陆源三萜类而正相关于细菌成因的C30藿烷, 故将其定性鉴定为C24脱-A-藿烷.由于不同C24四环萜烷的相对丰度随Pr/Ph比值呈现不同的变化, 尤其是与细菌有关的化合物C和化合物B与Pr/Ph 比值间展现出完全不同的相关性, 这一现象可能表明它们的形成受特定的地球化学条件控制.研究区湖相烃源岩处于相对还原的、有利于厌氧细菌繁衍的环境, 而煤系烃源岩则处于偏氧化的、有利于喜氧细菌活动的环境, 这可能是导致地质样品中具有明显不同 C24四环萜烷的分布与组成特征的原因所在, 这一特征的揭示在研究区油源研究中具有重要的环境区分意义.%Four C24 tetracyclic terpanes have been detected in crude oils from the Zhu 1 Depression, three of them are identified as 10β(H)-des-A-oleanane (A), 10β(H)-des-A-lupane (B1), and 17, 21-secohopane (C), respectively. However, compound B is an unknown biomarker. Its mass spectrum indicates that it is a C24 tetracyclic terpane because of a molecular ion at m/z 330. In addition, its base peak is at m/z 191, which is derived from the D/E ring cleavage in pentacyclic triterpane, and a key fragment is at m/z 287, indicating an isopropyl group in its structure. Like lupanoids, there is an isopropyl group in the E ring of C30hopane, therefore, this compound is likely derived from degradation of C30 hopane or its precursor. The relative abundance of compound B is negatively correlated with terrigenous triterpanes and their degraded compounds, such as oleanane, 10β(H)-des-A-oleanane, 10β(H)-des-A-lupane, and bicadinane, but positively correlated with C30αβ-hopane, indicating bacteria input in crude oils. These relationships imply that compound B has a microorganism source and is tentatively identified as C24des-A-hopane. In addition, compound B is abundant in lacustrine oils and source rocks rich in C304-methylsteranes, but very low or almost absent in coaly oils and carboniferous mudstone having higher Pr/Ph ratios (>6.0) and predominant C29 diasteranes, regular steranes, and other terrigenous triterpanes. This implies that compound B, and its relative abundance, have an important role in indicating origin and source of crude oils in the study area. It is well-known that lacustrine source rocks are deposited under relatively reducing environments, advantageous to the growth of anaerobic microbes, but the coal-containing sequence formed under relatively oxic conditions, with strong aerobic microbe activity, which resulted in the different distributions and compositions of C24 tetracyclic terpanes in geological samples. Therefore, the different correlations between compound B, C and Pr/Ph ratios suggest that their formation requires different geochemical conditions, such as anaerobic or aerobic and related microbial populations.
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