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.