黔北坳陷高演化烃源岩中正构烷烃单体烃碳同位素组成

借助于色谱-质谱和单体烃碳同位素分析技术，对黔北坳陷高演化海相烃源岩和黔南坳陷虎47井原油中各类生物标志物和正构烷烃的单体烃碳同位素组成进行了系统分析。不同层位烃源岩中常用的甾、萜烷的分布与组成呈现明显趋同现象，因而失去了其原有的地球化学意义和实用性。虎47井原油中三环萜烷系列和孕甾烷系列丰富，但C_27-35藿烷系列和C_27-29甾烷系列的丰度极低，显示生油窗晚期的产物特征。绝大多数烃源岩中nC_16-28正构烷烃系列呈现以nC₂₀或nC₂₁为低谷的前、后2个峰群的分布模式，但其单体烃碳同位素组成则十分接近，它们的δ13C值介于-28‰~-31‰之间，呈负偏态变化，即随碳数增加其δ13C值变小，且这一特征不受正构烷烃系列分布特征的影响，表明高演化烃源岩中特殊的正构烷烃系列分布模式是客观地球化学现象。虎47井原油中正构烷烃的单体烃碳同位素组成也呈负偏态变化，但其δ13C值较高演化烃源岩中的轻约3‰，这可能与有机质热演化过程中的碳同位素分馏效应有关。因此，在利用正构烷烃的单体烃碳同位素组成开展油源对比时，所用样品的成熟度应该匹配。 

Carbon isotope compositions of individual alkanes in highly mature source rocks from Northern Guizhou Depression

Straight chain alkanes and various biomarkers such as steranes and terpanes and compound-specific carbon isotope compositions of individual n-alkanes in the saturate fractions of extracts from highly mature source rocks and crude oil from the Hu 47 well in the Northern and Southern Guizhou depressions were analyzed using GC-MS and GC-CSIRMS. The distribution and composition of regular steranes and terpanes in those source rocks are very similar and have lost their original geochemical significance. However, crude oil from the well Hu 47 contains abundant low molecular biomarkers such as tricyclic terpanes and pregnanes while C_27-35 hopanes and C_27-29 steranes are depleted, suggesting that it is a condensate with high maturity. There are bimodal distributions for n-alkanes with a minimum at n-C₂₀ or n-C₂₁ in most source rocks. Normally, this phenomenon should not appear in highly mature source rocks. However, for both bimodal and unimodal distributions of n-alkanes in those source rocks, the carbon isotope compositions of individual n-alkanes are similar. The δ13C values are between -28‰ and -31‰ and decrease with increasing n-alkane carbon number, suggesting that bimodal or unimodal n-alkanes have a similar source in highly mature source rocks. There is an obvious negative tendency in carbon isotope compositions of individual compounds with increasing carbon number of individual n-alkanes in light oil from the well Hu 47, but their δ13C values are lighter by about 3‰ compared with those in highly mature source rocks. This could be related to the fractionation of carbon isotope during the thermal evolution of organic matter. Therefore, when carbon isotope compositions of individual n-alkanes are used in oil-source correlation, it is important to avoid a large maturity difference between the crude oils and source rocks.