25-降藿烷的成因识别

Genetic Identification of 25-Norhopanes

藿烷系列化合物中的25-降藿烷系列广泛存在于严重生物降解原油中,具有特殊的地球化学意义。对不同热演化阶段的烃源岩抽提物分析后发现,低热演化阶段的烃源岩抽提物中未检测出25-降藿烷,但是在高热演化程度的烃源岩抽提物中均检测有高丰度的25-降藿烷,认为这与热演化程度有关,并非生物降解所致。四川盆地高温裂解形成的沥青抽提物中普遍存在25-降藿烷系列化合物,其分布特征与高热演化阶段的烃源岩抽提物饱和烃质量色谱图极为相似,而且均已达到平衡状态,藿烷参数比值也在相同区域,推断该沥青中的25-降藿烷也是受热演化程度影响。为了查明热演化相关的25-降藿烷与生物降解成因中25-降藿烷系列化合物的分布特征差异,对比研究了两者的分布特征,发现两种成因的25-降藿烷的质量色谱图及藿烷参数比值存在较大差异。

25-norhopanes in hopane series compounds exist widely in severely biodegraded oils and are generally considered to be formeddue to effect of natural biodegradation on the compounds, which has a special geochemical significance. The analysis of extracts fromsource rocks in different thermal evolution stages indicates that 25-norhopanes are not contained in such extracts in low thermal evolutionstage rather than high thermal evolution stage in which there exists high abundance of 25-norhopanes in them. This paper suggests that thisresult is related to the thermal evolution degree, not just resulted from biodegradation. In Sichuan basin, the extracts from bitumen formedby high temperature pyrolysis all contain 25-norhopanes, and the distribution features of them are very similar to the saturated hydrocarbonGC-MS diagrams for source rocks in high thermal evolution stage, and reach equilibrium state. Also, the hopane ratio parameter ranges inthe same region, it can be deduced that the 25-norhopanes existed in the bitumen are effected by thermal evolution degrees. In order tomake clear the differences between 25-norhopanes occurrence related with the thermal evolution degree and resulted from degradation, the25-norhopanes from biodegraded crude oil are compared with that from thermal evolution in distribution, it is verified that there exist largedifferences in GC-MS and hopane ratio parameter.