我国南方过成熟地质样品中分子地球化学的特殊性研究

In the post-mature geological samples including source rocks and solid bitumen (Ro>2.0%) from the Lower Paleozoic marine sequences in South China, there are some unusual phenomenon to be observed. Firstly, the bimodal distribution of normal alkanes is very common in these post-mature geological samples. The known results show that the bimodal distribution for normal alkanes can be only observes in relatively lower mature geological samples, and usually accompany with carbon number predominance. However, with increasing maturity and C-C bond breakage, this phenomenon mentioned above should gradually weaken or disappear. Obviously, it is very difficult to be explained by the known geochemical knowledge. Secondly, relatively abundant 25-norhopanes can be detected in these post-mature geological samples. 25-norhopanes are mainly derived from severe biodegradation of crude oils in reservoirs and used as an indicator to evaluate the biodegradation extent for crude oils. In addition, this kind of biomarkers are occasionally detected in lower mature source rocks, and derived from microbial activity during early diagenesis. But it is common that those processes happen under lower temperature and pressure geological conditions. And then, what are reasons to result in occurrence of 25-norhopanes in post-mature geological samples? Strongly thermal degradation may be another factor or mechanism to result in generation of this kind of biomarker？It is worthy to be studied. Thirdly, the conclusion about that the distributions and compositions of triaromatic sterioids in geological samples are not affected by highly thermal evolution and sedimentary environment at post-mature stage, and can be used as an effective indicator for oil-source correlation is reliable or not. However, some known analytical results do not support this conclusion. Therefore, it is theoretically and practically significant to study those specific geochemical phenomenon mentioned above in post-mature geological samples from highly thermal evolution area.

我国南方高演化地区的地质样品中存在一些特殊的地球化学现象，它们包括正构烷烃系列的双峰态分布和普遍存在25-降藿烷系列。按照已有的知识，正构烷烃系列的双峰态分布一般出现在低演化阶段,并且会随着热演化程度的升高而逐渐减弱并消失，显然这一现象不应该出现在过成熟的地质样品中。而25-降藿烷系列是指示原油遭受严重生物降解作用的标志，低温低压条件下的细菌改造是这类生物标志物形成的主要机理。高演化地质样品中出现25-降藿烷系列不符合现有认知。三芳甾烷系列具有高热稳定性，但它们是否不受沉积环境和成熟度的影响而可以作为高演化地区油源对比的有效指标值得研究。因为已有实际分析资料显示这类标志物的分布也受成熟度影响。因此，针对高演化地区地质样品中的这些“特殊”地球化学现象开展系统研究，不但可以丰富现有的基础理论,同时对解决实际问题具有实用价值，可见这一研究无论在理论上还是应用上均具重要意义。

通过对高演化地区不同层位、不同性质地质样品（烃源岩和固体沥青）中饱和烃和芳烃馏分的系统剖析，发现高演化地质样品具有特殊的分子地球化学特征。大多数地质样品中，其正构烷烃系列呈现前、后两个峰群的双峰态分布，仅少数样品为前单峰型分布。分析结果显示分布特征不同的正构烷烃系列具有相近的单体烃碳同位素组成，其δ13C值介于-28‰~-31‰之间，表明它们具有相同的来源。不同层位不同性质的高演化地质样品中，其常用的甾、萜烷标志物包括甾烷系列、萜烷系列和三芳甾类及三芳甲藻甾烷的分布与组成特征存在明显的趋同现象，此时它们失去了其原有的地球化学意义而不再具有实用价值。此外，25-降藿烷系列普遍存在于高演化地质样品中，且它们的含量稳定，但在生物降解原油中，其含量随降解程度而变化，由此揭示出高演化作用也是此类标志物的重要机理，这与它们较藿烷系列具有更高的热稳定性有关。在高演化烃源岩中首次检测到烷基单金刚烷系列~烷基五金刚烷系列的复杂组合，且相关金刚烷类成熟度参数明显偏高，这是高演化的可靠标志。而在成熟-凝析油气阶段烃源岩中，仅存在烷基单金刚烷、烷基单金刚烷和烷基双金刚烷系列的简单组合，相关成熟度参数明显偏低。值得注意的是研究区大多数过成熟烃源岩中此类具有高稳定性的化合物缺失，这一现象揭示出尽管金刚烷类化合物具有高的热稳定性，但它也是有限度的，超过这一限度它们也会遭受裂解破坏。烃源岩热模拟实验可以再现自然演化剖面上高演化地质样品中甾、萜烷分布与组成特征具有趋同性这一特殊地球化学现象，由此表明它是一种客观存在，在类似地质背景下使用时应谨慎。