原油中金刚烷同系物同分异构体丰度差异及影响机制——以塔里木盆地轮古地区为例

Abundance difference and influence mechanism of different diamondoid isomers in crude oil:Taking Lungu area of Tarim Basin as an example

塔里木盆地轮古地区原油主要受到了气洗与热裂解这2种次生作用影响,使得原油具有较高丰度的金刚烷组分。采用GC-MS-MS技术定量表征全油中的金刚烷系列化合物,分析了不同成熟度油样中各金刚烷及其同系物同分异构体的相对丰度,发现无论是金刚烷同分异构体还是金刚烷同系物之间,均表现出以下特征:成熟度较低的油样中仲碳位取代的金刚烷占优,而成熟度较高的原油中叔碳位取代的金刚烷较高。这表明原油中金刚烷同系物异构体的丰度差异可以作为区分油样成熟度的定性指标。从化学机理方面应用价键理论分析认为,仲碳位取代金刚烷异构体中1个sp^(3)杂化轨道被C—H单键占据,叔碳位取代金刚烷异构体4个sp^(3)杂化轨道均被C—C单键占据,所以叔碳位取代金刚烷的原子轨道重合度高,键能大,可能是金刚烷系列中甲基取代在叔碳位比在仲碳位的热稳定性更高的本质原因。同时随着金刚烷碳数增加,化合物饱和蒸气压递减的物理性质也解释了气洗作用对金刚烷系列化合物差异化富集的现象。

Crude oil from the Lungu area of the Tarim Basin is mainly affected by gas washing and thermal cracking,which makes the oil enriched in diamondoids.This study applied GC-MS-MS on the whole oil samples for quantitative analysis of diamondoids.It was found that,either among the diamondoid isomers or the homologs,the oils at relatively low thermal maturity show the dominant abundance of secondary-substituted diamondoid isomers,while oils with higher thermal maturity show the dominance of tertiary-substituted isomers.The abundance difference of diamondoid structural isomers from the oils with a range of thermal maturity can be potentially used as a qualitative scale for oil maturity,especially at high thermal stage.The valence bond theory is used to figure out the chemical mechanism behind the aforementioned phenomena.Compared with that one sp^(3)hybrid orbital is occupied by C-H single bond in the secondary-substituted isomer,the four sp^(3)hybrid orbitals in the tertiary-substituted diamondoid isomer are occupied by C-C single bond making the whole molecule more stable.The coincidence degree of the atomic orbits in the tertiary-substituted isomers is higher with a greater bond dissociation energy than the secondary-substituted,which could be attributed to the observation that the thermal stability of diamondoid isomer with methyl substitution at bridgehead(tertiary-substituted)is higher than that of secondary-substituted methyl isomer.At the same time,the physical trait of decreasing saturated vapor pressure of compounds with increasing diamondoid carbon number explains the phenomena of differential enrichment of diamondoid series compounds by gas washing.